awk命令
命令用途
awk 是一个强大的文本处理工具，广泛用于模式扫描和处理任务。它能够逐行读取输入，匹配模式并执行相应的动作。awk 被设计用于在输入数据中根据特定模式进行搜索、筛选和处理，常用于日志分析、数据处理等场景。
常用用法示例
打印文件中的每一行
1
awk '{print}' filename
该命令将文件 filename 中的每一行打印出来。
打印特定字段
1
awk '{print $1}' filename
该命令打印文件中每一行的第一个字段（以空格或制表符为分隔符）。
根据条件筛选并打印匹配的行
1
awk '$1 == "John" {print $0}' filename
该命令筛选出第一列为 “John” 的行，并打印整个行。
计算文件中第二列的和
1
awk '{sum += $2} END {print sum}' filename
该命令计算文件中第二列所有数值的总和。
以逗号分隔的文件中打印特定字段
1
awk -F, '{print $3}' filename
该命令将字段分隔符设置为逗号，并打印每行的第三个字段。
打印匹配多个条件的行
1
awk '$1 == "John" && $2 > 50 {print $0}' filename
该命令筛选出第一列为 “John” 且第二列大于50的行。
打印每行的行号
1
awk '{print NR, $0}' filename
该命令打印文件中每一行的行号和内容。
替换字段中的内容
1
awk '{gsub(/apple/, "orange"); print $0}' filename
该命令将每行中的 “apple” 替换为 “orange” 并打印结果。
常用参数选项
-F sepstring
设置输入字段分隔符。
awk -F”:” ‘{print $1}’ filename
将字段分隔符设置为冒号，打印文件中每行的第一个字段。-f progfile
指定包含 awk 程序的文件。
awk -f program.awk filename
从 program.awk 文件中读取 awk 程序，并应用于文件 filename。-v assignment
在执行 awk 程序前先进行变量赋值。
awk -v var=10 ‘{print var}’ filename
为 var 变量赋值为 10，并打印该变量的值。-v FS=sepstring
设置输入记录的字段分隔符。
awk -v FS=”;” ‘{print $1}’ filename
将字段分隔符设置为分号，并打印每行的第一个字段。BEGIN
在处理输入文件之前执行的代码块。
awk ‘BEGIN {print “Start Processing”} {print $1}’ filename
在处理文件之前，先打印 “Start Processing”。END
在处理完输入文件之后执行的代码块。
awk ‘END {print “Processing Completed”}’ filename
在处理完所有行后打印 “Processing Completed”。-F,
指定逗号作为字段分隔符。
awk -F, ‘{print $2}’ filename
将字段分隔符设置为逗号，并打印第二个字段。NR
表示当前行号。
awk ‘{print NR, $0}’ filename
打印每行的行号及其内容。
原厂文档
PROLOG
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
NAME
awk — pattern scanning and processing language
SYNOPSIS
awk [-F sepstring] [-v assignment]... program [argument...]
awk [-F sepstring] -f progfile [-f progfile]... [-v assignment]...
[argument...]
DESCRIPTION
The awk utility shall execute programs written in the awk
programming language, which is specialized for textual data
manipulation. An awk program is a sequence of patterns and
corresponding actions. When input is read that matches a pattern,
the action associated with that pattern is carried out.
Input shall be interpreted as a sequence of records. By default, a
record is a line, less its terminating <newline>, but this can be
changed by using the RS built-in variable. Each record of input
shall be matched in turn against each pattern in the program. For
each pattern matched, the associated action shall be executed.
The awk utility shall interpret each input record as a sequence of
fields where, by default, a field is a string of non-<blank>
non-<newline> characters. This default <blank> and <newline> field
delimiter can be changed by using the FS built-in variable or the
-F sepstring option. The awk utility shall denote the first field
in a record $1, the second $2, and so on. The symbol $0 shall
refer to the entire record; setting any other field causes the re-
evaluation of $0. Assigning to $0 shall reset the values of all
other fields and the NF built-in variable.
OPTIONS
The awk utility shall conform to the Base Definitions volume of
POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
-F sepstring
Define the input field separator. This option shall be
equivalent to:
-v FS=sepstring
except that if -F sepstring and -v FS=sepstring are both
used, it is unspecified whether the FS assignment
resulting from -F sepstring is processed in command line
order or is processed after the last -v FS=sepstring.
See the description of the FS built-in variable, and how
it is used, in the EXTENDED DESCRIPTION section.
-f progfile
Specify the pathname of the file progfile containing an
awk program. A pathname of '-' shall denote the standard
input. If multiple instances of this option are
specified, the concatenation of the files specified as
progfile in the order specified shall be the awk
program. The awk program can alternatively be specified
in the command line as a single argument.
-v assignment
The application shall ensure that the assignment
argument is in the same form as an assignment operand.
The specified variable assignment shall occur prior to
executing the awk program, including the actions
associated with BEGIN patterns (if any). Multiple
occurrences of this option can be specified.
OPERANDS
The following operands shall be supported:
program If no -f option is specified, the first operand to awk
shall be the text of the awk program. The application
shall supply the program operand as a single argument to
awk. If the text does not end in a <newline>, awk shall
interpret the text as if it did.
argument Either of the following two types of argument can be
intermixed:
file A pathname of a file that contains the input
to be read, which is matched against the set
of patterns in the program. If no file
operands are specified, or if a file operand
is '-', the standard input shall be used.
assignment
An operand that begins with an <underscore> or
alphabetic character from the portable
character set (see the table in the Base
Definitions volume of POSIX.1‐2017, Section
6.1, Portable Character Set), followed by a
sequence of underscores, digits, and
alphabetics from the portable character set,
followed by the '=' character, shall specify a
variable assignment rather than a pathname.
The characters before the '=' represent the
name of an awk variable; if that name is an
awk reserved word (see Grammar) the behavior
is undefined. The characters following the
<equals-sign> shall be interpreted as if they
appeared in the awk program preceded and
followed by a double-quote ('"') character, as
a STRING token (see Grammar), except that if
the last character is an unescaped
<backslash>, it shall be interpreted as a
literal <backslash> rather than as the first
character of the sequence "\"". The variable
shall be assigned the value of that STRING
token and, if appropriate, shall be considered
a numeric string (see Expressions in awk), the
variable shall also be assigned its numeric
value. Each such variable assignment shall
occur just prior to the processing of the
following file, if any. Thus, an assignment
before the first file argument shall be
executed after the BEGIN actions (if any),
while an assignment after the last file
argument shall occur before the END actions
(if any). If there are no file arguments,
assignments shall be executed before
processing the standard input.
STDIN
The standard input shall be used only if no file operands are
specified, or if a file operand is '-', or if a progfile option-
argument is '-'; see the INPUT FILES section. If the awk program
contains no actions and no patterns, but is otherwise a valid awk
program, standard input and any file operands shall not be read
and awk shall exit with a return status of zero.
INPUT FILES
Input files to the awk program from any of the following sources
shall be text files:
* Any file operands or their equivalents, achieved by modifying
the awk variables ARGV and ARGC
* Standard input in the absence of any file operands
* Arguments to the getline function
Whether the variable RS is set to a value other than a <newline>
or not, for these files, implementations shall support records
terminated with the specified separator up to {LINE_MAX} bytes and
may support longer records.
If -f progfile is specified, the application shall ensure that the
files named by each of the progfile option-arguments are text
files and their concatenation, in the same order as they appear in
the arguments, is an awk program.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of
awk:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base
Definitions volume of POSIX.1‐2017, Section 8.2,
Internationalization Variables for the precedence of
internationalization variables used to determine the
values of locale categories.)
LC_ALL If set to a non-empty string value, override the values
of all the other internationalization variables.
LC_COLLATE
Determine the locale for the behavior of ranges,
equivalence classes, and multi-character collating
elements within regular expressions and in comparisons
of string values.
LC_CTYPE Determine the locale for the interpretation of sequences
of bytes of text data as characters (for example,
single-byte as opposed to multi-byte characters in
arguments and input files), the behavior of character
classes within regular expressions, the identification
of characters as letters, and the mapping of uppercase
and lowercase characters for the toupper and tolower
functions.
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
LC_NUMERIC
Determine the radix character used when interpreting
numeric input, performing conversions between numeric
and string values, and formatting numeric output.
Regardless of locale, the <period> character (the
decimal-point character of the POSIX locale) is the
decimal-point character recognized in processing awk
programs (including assignments in command line
arguments).
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
PATH Determine the search path when looking for commands
executed by system(expr), or input and output pipes; see
the Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables.
In addition, all environment variables shall be visible via the
awk variable ENVIRON.
ASYNCHRONOUS EVENTS
Default.
STDOUT
The nature of the output files depends on the awk program.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
The nature of the output files depends on the awk program.
EXTENDED DESCRIPTION
Overall Program Structure
An awk program is composed of pairs of the form:
pattern { action }
Either the pattern or the action (including the enclosing brace
characters) can be omitted.
A missing pattern shall match any record of input, and a missing
action shall be equivalent to:
{ print }
Execution of the awk program shall start by first executing the
actions associated with all BEGIN patterns in the order they occur
in the program. Then each file operand (or standard input if no
files were specified) shall be processed in turn by reading data
from the file until a record separator is seen (<newline> by
default). Before the first reference to a field in the record is
evaluated, the record shall be split into fields, according to the
rules in Regular Expressions, using the value of FS that was
current at the time the record was read. Each pattern in the
program then shall be evaluated in the order of occurrence, and
the action associated with each pattern that matches the current
record executed. The action for a matching pattern shall be
executed before evaluating subsequent patterns. Finally, the
actions associated with all END patterns shall be executed in the
order they occur in the program.
Expressions in awk
Expressions describe computations used in patterns and actions.
In the following table, valid expression operations are given in
groups from highest precedence first to lowest precedence last,
with equal-precedence operators grouped between horizontal lines.
In expression evaluation, where the grammar is formally ambiguous,
higher precedence operators shall be evaluated before lower
precedence operators. In this table expr, expr1, expr2, and expr3
represent any expression, while lvalue represents any entity that
can be assigned to (that is, on the left side of an assignment
operator). The precise syntax of expressions is given in Grammar.
Table 4-1: Expressions in Decreasing Precedence in awk
┌──────────────────────┬─────────────────────────┬────────────────┬──────────────┐
│ Syntax │ Name │ Type of Result │Associativity │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ ( expr ) │Grouping │Type of expr │N/A │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ $expr │Field reference │String │N/A │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ lvalue ++ │Post-increment │Numeric │N/A │
│ lvalue – │Post-decrement │Numeric │N/A │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ ++ lvalue │Pre-increment │Numeric │N/A │
│ – lvalue │Pre-decrement │Numeric │N/A │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr ^ expr │Exponentiation │Numeric │Right │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ ! expr │Logical not │Numeric │N/A │
│ + expr │Unary plus │Numeric │N/A │
│ - expr │Unary minus │Numeric │N/A │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr * expr │Multiplication │Numeric │Left │
│ expr / expr │Division │Numeric │Left │
│ expr % expr │Modulus │Numeric │Left │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr + expr │Addition │Numeric │Left │
│ expr - expr │Subtraction │Numeric │Left │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr expr │String concatenation │String │Left │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr < expr │Less than │Numeric │None │
│ expr <= expr │Less than or equal to │Numeric │None │
│ expr != expr │Not equal to │Numeric │None │
│ expr == expr │Equal to │Numeric │None │
│ expr > expr │Greater than │Numeric │None │
│ expr >= expr │Greater than or equal to │Numeric │None │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr ~ expr │ERE match │Numeric │None │
│ expr !~ expr │ERE non-match │Numeric │None │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr in array │Array membership │Numeric │Left │
│ ( index ) in array │Multi-dimension array │Numeric │Left │
│ │membership │ │ │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr && expr │Logical AND │Numeric │Left │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr || expr │Logical OR │Numeric │Left │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ expr1 ? expr2 : expr3│Conditional expression │Type of selected│Right │
│ │ │expr2 or expr3 │ │
├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
│ lvalue ^= expr │Exponentiation assignment│Numeric │Right │
│ lvalue %= expr │Modulus assignment │Numeric │Right │
│ lvalue *= expr │Multiplication assignment│Numeric │Right │
│ lvalue /= expr │Division assignment │Numeric │Right │
│ lvalue += expr │Addition assignment │Numeric │Right │
│ lvalue -= expr │Subtraction assignment │Numeric │Right │
│ lvalue = expr │Assignment │Type of expr │Right │
└──────────────────────┴─────────────────────────┴────────────────┴──────────────┘
Each expression shall have either a string value, a numeric value,
or both. Except as stated for specific contexts, the value of an
expression shall be implicitly converted to the type needed for
the context in which it is used. A string value shall be converted
to a numeric value either by the equivalent of the following calls
to functions defined by the ISO C standard:
setlocale(LC_NUMERIC, "");
numeric_value = atof(string_value);
or by converting the initial portion of the string to type double
representation as follows:
The input string is decomposed into two parts: an initial,
possibly empty, sequence of white-space characters (as
specified by isspace()) and a subject sequence interpreted
as a floating-point constant.
The expected form of the subject sequence is an optional
'+' or '-' sign, then a non-empty sequence of digits
optionally containing a <period>, then an optional exponent
part. An exponent part consists of 'e' or 'E', followed by
an optional sign, followed by one or more decimal digits.
The sequence starting with the first digit or the <period>
(whichever occurs first) is interpreted as a floating
constant of the C language, and if neither an exponent part
nor a <period> appears, a <period> is assumed to follow the
last digit in the string. If the subject sequence begins
with a <hyphen-minus>, the value resulting from the
conversion is negated.
A numeric value that is exactly equal to the value of an integer
(see Section 1.1.2, Concepts Derived from the ISO C Standard)
shall be converted to a string by the equivalent of a call to the
sprintf function (see String Functions) with the string "%d" as
the fmt argument and the numeric value being converted as the
first and only expr argument. Any other numeric value shall be
converted to a string by the equivalent of a call to the sprintf
function with the value of the variable CONVFMT as the fmt
argument and the numeric value being converted as the first and
only expr argument. The result of the conversion is unspecified if
the value of CONVFMT is not a floating-point format specification.
This volume of POSIX.1‐2017 specifies no explicit conversions
between numbers and strings. An application can force an
expression to be treated as a number by adding zero to it, or can
force it to be treated as a string by concatenating the null
string ("") to it.
A string value shall be considered a numeric string if it comes
from one of the following:
1. Field variables
2. Input from the getline() function
3. FILENAME
4. ARGV array elements
5. ENVIRON array elements
6. Array elements created by the split() function
7. A command line variable assignment
8. Variable assignment from another numeric string variable
and an implementation-dependent condition corresponding to either
case (a) or (b) below is met.
a. After the equivalent of the following calls to functions
defined by the ISO C standard, string_value_end would differ
from string_value, and any characters before the terminating
null character in string_value_end would be <blank>
characters:
char *string_value_end;
setlocale(LC_NUMERIC, "");
numeric_value = strtod (string_value, &string_value_end);
b. After all the following conversions have been applied, the
resulting string would lexically be recognized as a NUMBER
token as described by the lexical conventions in Grammar:
-- All leading and trailing <blank> characters are discarded.
-- If the first non-<blank> is '+' or '-', it is discarded.
-- Each occurrence of the decimal point character from the
current locale is changed to a <period>.
In case (a) the numeric value of the numeric string shall be the
value that would be returned by the strtod() call. In case (b) if
the first non-<blank> is '-', the numeric value of the numeric
string shall be the negation of the numeric value of the
recognized NUMBER token; otherwise, the numeric value of the
numeric string shall be the numeric value of the recognized NUMBER
token. Whether or not a string is a numeric string shall be
relevant only in contexts where that term is used in this section.
When an expression is used in a Boolean context, if it has a
numeric value, a value of zero shall be treated as false and any
other value shall be treated as true. Otherwise, a string value of
the null string shall be treated as false and any other value
shall be treated as true. A Boolean context shall be one of the
following:
* The first subexpression of a conditional expression
* An expression operated on by logical NOT, logical AND, or
logical OR
* The second expression of a for statement
* The expression of an if statement
* The expression of the while clause in either a while or
do...while statement
* An expression used as a pattern (as in Overall Program
Structure)
All arithmetic shall follow the semantics of floating-point
arithmetic as specified by the ISO C standard (see Section 1.1.2,
Concepts Derived from the ISO C Standard).
The value of the expression:
expr1 ^ expr2
shall be equivalent to the value returned by the ISO C standard
function call:
pow(expr1, expr2)
The expression:
lvalue ^= expr
shall be equivalent to the ISO C standard expression:
lvalue = pow(lvalue, expr)
except that lvalue shall be evaluated only once. The value of the
expression:
expr1 % expr2
shall be equivalent to the value returned by the ISO C standard
function call:
fmod(expr1, expr2)
The expression:
lvalue %= expr
shall be equivalent to the ISO C standard expression:
lvalue = fmod(lvalue, expr)
except that lvalue shall be evaluated only once.
Variables and fields shall be set by the assignment statement:
lvalue = expression
and the type of expression shall determine the resulting variable
type. The assignment includes the arithmetic assignments ("+=",
"-=", "*=", "/=", "%=", "^=", "++", "--") all of which shall
produce a numeric result. The left-hand side of an assignment and
the target of increment and decrement operators can be one of a
variable, an array with index, or a field selector.
The awk language supplies arrays that are used for storing numbers
or strings. Arrays need not be declared. They shall initially be
empty, and their sizes shall change dynamically. The subscripts,
or element identifiers, are strings, providing a type of
associative array capability. An array name followed by a
subscript within square brackets can be used as an lvalue and thus
as an expression, as described in the grammar; see Grammar.
Unsubscripted array names can be used in only the following
contexts:
* A parameter in a function definition or function call
* The NAME token following any use of the keyword in as
specified in the grammar (see Grammar); if the name used in
this context is not an array name, the behavior is undefined
A valid array index shall consist of one or more <comma>-separated
expressions, similar to the way in which multi-dimensional arrays
are indexed in some programming languages. Because awk arrays are
really one-dimensional, such a <comma>-separated list shall be
converted to a single string by concatenating the string values of
the separate expressions, each separated from the other by the
value of the SUBSEP variable. Thus, the following two index
operations shall be equivalent:
var[expr1, expr2, ... exprn]
var[expr1 SUBSEP expr2 SUBSEP ... SUBSEP exprn]
The application shall ensure that a multi-dimensioned index used
with the in operator is parenthesized. The in operator, which
tests for the existence of a particular array element, shall not
cause that element to exist. Any other reference to a nonexistent
array element shall automatically create it.
Comparisons (with the '<', "<=", "!=", "==", '>', and ">="
operators) shall be made numerically if both operands are numeric,
if one is numeric and the other has a string value that is a
numeric string, or if one is numeric and the other has the
uninitialized value. Otherwise, operands shall be converted to
strings as required and a string comparison shall be made as
follows:
* For the "!=" and "==" operators, the strings should be
compared to check if they are identical but may be compared
using the locale-specific collation sequence to check if they
collate equally.
* For the other operators, the strings shall be compared using
the locale-specific collation sequence.
The value of the comparison expression shall be 1 if the relation
is true, or 0 if the relation is false.
Variables and Special Variables
Variables can be used in an awk program by referencing them. With
the exception of function parameters (see User-Defined Functions),
they are not explicitly declared. Function parameter names shall
be local to the function; all other variable names shall be
global. The same name shall not be used as both a function
parameter name and as the name of a function or a special awk
variable. The same name shall not be used both as a variable name
with global scope and as the name of a function. The same name
shall not be used within the same scope both as a scalar variable
and as an array. Uninitialized variables, including scalar
variables, array elements, and field variables, shall have an
uninitialized value. An uninitialized value shall have both a
numeric value of zero and a string value of the empty string.
Evaluation of variables with an uninitialized value, to either
string or numeric, shall be determined by the context in which
they are used.
Field variables shall be designated by a '$' followed by a number
or numerical expression. The effect of the field number expression
evaluating to anything other than a non-negative integer is
unspecified; uninitialized variables or string values need not be
converted to numeric values in this context. New field variables
can be created by assigning a value to them. References to
nonexistent fields (that is, fields after $NF), shall evaluate to
the uninitialized value. Such references shall not create new
fields. However, assigning to a nonexistent field (for example,
$(NF+2)=5) shall increase the value of NF; create any intervening
fields with the uninitialized value; and cause the value of $0 to
be recomputed, with the fields being separated by the value of
OFS. Each field variable shall have a string value or an
uninitialized value when created. Field variables shall have the
uninitialized value when created from $0 using FS and the variable
does not contain any characters. If appropriate, the field
variable shall be considered a numeric string (see Expressions in
awk).
Implementations shall support the following other special
variables that are set by awk:
ARGC The number of elements in the ARGV array.
ARGV An array of command line arguments, excluding options
and the program argument, numbered from zero to ARGC-1.
The arguments in ARGV can be modified or added to; ARGC
can be altered. As each input file ends, awk shall treat
the next non-null element of ARGV, up to the current
value of ARGC-1, inclusive, as the name of the next
input file. Thus, setting an element of ARGV to null
means that it shall not be treated as an input file. The
name '-' indicates the standard input. If an argument
matches the format of an assignment operand, this
argument shall be treated as an assignment rather than a
file argument.
CONVFMT The printf format for converting numbers to strings
(except for output statements, where OFMT is used);
"%.6g" by default.
ENVIRON An array representing the value of the environment, as
described in the exec functions defined in the System
Interfaces volume of POSIX.1‐2017. The indices of the
array shall be strings consisting of the names of the
environment variables, and the value of each array
element shall be a string consisting of the value of
that variable. If appropriate, the environment variable
shall be considered a numeric string (see Expressions in
awk); the array element shall also have its numeric
value.
In all cases where the behavior of awk is affected by
environment variables (including the environment of any
commands that awk executes via the system function or
via pipeline redirections with the print statement, the
printf statement, or the getline function), the
environment used shall be the environment at the time
awk began executing; it is implementation-defined
whether any modification of ENVIRON affects this
environment.
FILENAME A pathname of the current input file. Inside a BEGIN
action the value is undefined. Inside an END action the
value shall be the name of the last input file
processed.
FNR The ordinal number of the current record in the current
file. Inside a BEGIN action the value shall be zero.
Inside an END action the value shall be the number of
the last record processed in the last file processed.
FS Input field separator regular expression; a <space> by
default.
NF The number of fields in the current record. Inside a
BEGIN action, the use of NF is undefined unless a
getline function without a var argument is executed
previously. Inside an END action, NF shall retain the
value it had for the last record read, unless a
subsequent, redirected, getline function without a var
argument is performed prior to entering the END action.
NR The ordinal number of the current record from the start
of input. Inside a BEGIN action the value shall be
zero. Inside an END action the value shall be the number
of the last record processed.
OFMT The printf format for converting numbers to strings in
output statements (see Output Statements); "%.6g" by
default. The result of the conversion is unspecified if
the value of OFMT is not a floating-point format
specification.
OFS The print statement output field separator; <space> by
default.
ORS The print statement output record separator; a <newline>
by default.
RLENGTH The length of the string matched by the match function.
RS The first character of the string value of RS shall be
the input record separator; a <newline> by default. If
RS contains more than one character, the results are
unspecified. If RS is null, then records are separated
by sequences consisting of a <newline> plus one or more
blank lines, leading or trailing blank lines shall not
result in empty records at the beginning or end of the
input, and a <newline> shall always be a field
separator, no matter what the value of FS is.
RSTART The starting position of the string matched by the match
function, numbering from 1. This shall always be
equivalent to the return value of the match function.
SUBSEP The subscript separator string for multi-dimensional
arrays; the default value is implementation-defined.
Regular Expressions
The awk utility shall make use of the extended regular expression
notation (see the Base Definitions volume of POSIX.1‐2017, Section
9.4, Extended Regular Expressions) except that it shall allow the
use of C-language conventions for escaping special characters
within the EREs, as specified in the table in the Base Definitions
volume of POSIX.1‐2017, Chapter 5, File Format Notation (‘\‘,
‘\a’, ‘\b’, ‘\f’, ‘\n’, ‘\r’, ‘\t’, ‘\v’) and the following table;
these escape sequences shall be recognized both inside and outside
bracket expressions. Note that records need not be separated by
characters, so even the “\n” sequence is valid in awk EREs. Using
a
the following table.
Table 4-2: Escape Sequences in awk
┌──────────┬────────────────────────────────────┬────────────────────────────────────┐
│ Escape │ │ │
│ Sequence │ Description │ Meaning │
├──────────┼────────────────────────────────────┼────────────────────────────────────┤
│ " │
├──────────┼────────────────────────────────────┼────────────────────────────────────┤
│ / │
├──────────┼────────────────────────────────────┼────────────────────────────────────┤
│ \ddd │ A
│ │ by the longest sequence of one, │ represented by the one, two, or │
│ │ two, or three octal-digit │ three-digit octal integer. Multi- │
│ │ characters (01234567). If all of │ byte characters require multiple, │
│ │ the digits are 0 (that is, │ concatenated escape sequences of │
│ │ representation of the NUL │ this type, including the leading │
│ │ character), the behavior is │
│ │ undefined. │ │
├──────────┼────────────────────────────────────┼────────────────────────────────────┤
│ \c │ A
│ │ by any character not described in │ │
│ │ this table or in the table in the │ │
│ │ Base Definitions volume of │ │
│ │ POSIX.1‐2017, Chapter 5, File │ │
│ │ Format Notation (‘\‘, ‘\a’, ‘\b’, │ │
│ │ ‘\f’, ‘\n’, ‘\r’, ‘\t’, ‘\v’). │ │
└──────────┴────────────────────────────────────┴────────────────────────────────────┘
A regular expression can be matched against a specific field or
string by using one of the two regular expression matching
operators, '~' and "!~". These operators shall interpret their
right-hand operand as a regular expression and their left-hand
operand as a string. If the regular expression matches the string,
the '~' expression shall evaluate to a value of 1, and the "!~"
expression shall evaluate to a value of 0. (The regular expression
matching operation is as defined by the term matched in the Base
Definitions volume of POSIX.1‐2017, Section 9.1, Regular
Expression Definitions, where a match occurs on any part of the
string unless the regular expression is limited with the
<circumflex> or <dollar-sign> special characters.) If the regular
expression does not match the string, the '~' expression shall
evaluate to a value of 0, and the "!~" expression shall evaluate
to a value of 1. If the right-hand operand is any expression other
than the lexical token ERE, the string value of the expression
shall be interpreted as an extended regular expression, including
the escape conventions described above. Note that these same
escape conventions shall also be applied in determining the value
of a string literal (the lexical token STRING), and thus shall be
applied a second time when a string literal is used in this
context.
When an ERE token appears as an expression in any context other
than as the right-hand of the '~' or "!~" operator or as one of
the built-in function arguments described below, the value of the
resulting expression shall be the equivalent of:
$0 ~ /ere/
The ere argument to the gsub, match, sub functions, and the fs
argument to the split function (see String Functions) shall be
interpreted as extended regular expressions. These can be either
ERE tokens or arbitrary expressions, and shall be interpreted in
the same manner as the right-hand side of the '~' or "!~"
operator.
An extended regular expression can be used to separate fields by
assigning a string containing the expression to the built-in
variable FS, either directly or as a consequence of using the -F
sepstring option. The default value of the FS variable shall be a
single <space>. The following describes FS behavior:
1. If FS is a null string, the behavior is unspecified.
2. If FS is a single character:
a. If FS is <space>, skip leading and trailing <blank> and
<newline> characters; fields shall be delimited by sets of
one or more <blank> or <newline> characters.
b. Otherwise, if FS is any other character c, fields shall be
delimited by each single occurrence of c.
3. Otherwise, the string value of FS shall be considered to be an
extended regular expression. Each occurrence of a sequence
matching the extended regular expression shall delimit fields.
Except for the '~' and "!~" operators, and in the gsub, match,
split, and sub built-in functions, ERE matching shall be based on
input records; that is, record separator characters (the first
character of the value of the variable RS, <newline> by default)
cannot be embedded in the expression, and no expression shall
match the record separator character. If the record separator is
not <newline>, <newline> characters embedded in the expression can
be matched. For the '~' and "!~" operators, and in those four
built-in functions, ERE matching shall be based on text strings;
that is, any character (including <newline> and the record
separator) can be embedded in the pattern, and an appropriate
pattern shall match any character. However, in all awk ERE
matching, the use of one or more NUL characters in the pattern,
input record, or text string produces undefined results.
Patterns
A pattern is any valid expression, a range specified by two
expressions separated by a comma, or one of the two special
patterns BEGIN or END.
Special Patterns
The awk utility shall recognize two special patterns, BEGIN and
END. Each BEGIN pattern shall be matched once and its associated
action executed before the first record of input is read—except
possibly by use of the getline function (see Input/Output and
General Functions) in a prior BEGIN action—and before command line
assignment is done. Each END pattern shall be matched once and its
associated action executed after the last record of input has been
read. These two patterns shall have associated actions.
BEGIN and END shall not combine with other patterns. Multiple
BEGIN and END patterns shall be allowed. The actions associated
with the BEGIN patterns shall be executed in the order specified
in the program, as are the END actions. An END pattern can precede
a BEGIN pattern in a program.
If an awk program consists of only actions with the pattern BEGIN,
and the BEGIN action contains no getline function, awk shall exit
without reading its input when the last statement in the last
BEGIN action is executed. If an awk program consists of only
actions with the pattern END or only actions with the patterns
BEGIN and END, the input shall be read before the statements in
the END actions are executed.
Expression Patterns
An expression pattern shall be evaluated as if it were an
expression in a Boolean context. If the result is true, the
pattern shall be considered to match, and the associated action
(if any) shall be executed. If the result is false, the action
shall not be executed.
Pattern Ranges
A pattern range consists of two expressions separated by a comma;
in this case, the action shall be performed for all records
between a match of the first expression and the following match of
the second expression, inclusive. At this point, the pattern range
can be repeated starting at input records subsequent to the end of
the matched range.
Actions
An action is a sequence of statements as shown in the grammar in
Grammar. Any single statement can be replaced by a statement list
enclosed in curly braces. The application shall ensure that
statements in a statement list are separated by
executed sequentially in the order that they appear.
The expression acting as the conditional in an if statement shall
be evaluated and if it is non-zero or non-null, the following
statement shall be executed; otherwise, if else is present, the
statement following the else shall be executed.
The if, while, do...while, for, break, and continue statements are
based on the ISO C standard (see Section 1.1.2, Concepts Derived
from the ISO C Standard), except that the Boolean expressions
shall be treated as described in Expressions in awk, and except in
the case of:
for (variable in array)
which shall iterate, assigning each index of array to variable in
an unspecified order. The results of adding new elements to array
within such a for loop are undefined. If a break or continue
statement occurs outside of a loop, the behavior is undefined.
The delete statement shall remove an individual array element.
Thus, the following code deletes an entire array:
for (index in array)
delete array[index]
The next statement shall cause all further processing of the
current input record to be abandoned. The behavior is undefined if
a next statement appears or is invoked in a BEGIN or END action.
The exit statement shall invoke all END actions in the order in
which they occur in the program source and then terminate the
program without reading further input. An exit statement inside an
END action shall terminate the program without further execution
of END actions. If an expression is specified in an exit
statement, its numeric value shall be the exit status of awk,
unless subsequent errors are encountered or a subsequent exit
statement with an expression is executed.
Output Statements
Both print and printf statements shall write to standard output by
default. The output shall be written to the location specified by
output_redirection if one is supplied, as follows:
> expression
>> expression
| expression
In all cases, the expression shall be evaluated to produce a
string that is used as a pathname into which to write (for '>' or
">>") or as a command to be executed (for '|'). Using the first
two forms, if the file of that name is not currently open, it
shall be opened, creating it if necessary and using the first
form, truncating the file. The output then shall be appended to
the file. As long as the file remains open, subsequent calls in
which expression evaluates to the same string value shall simply
append output to the file. The file remains open until the close
function (see Input/Output and General Functions) is called with
an expression that evaluates to the same string value.
The third form shall write output onto a stream piped to the input
of a command. The stream shall be created if no stream is
currently open with the value of expression as its command name.
The stream created shall be equivalent to one created by a call to
the popen() function defined in the System Interfaces volume of
POSIX.1‐2017 with the value of expression as the command argument
and a value of w as the mode argument. As long as the stream
remains open, subsequent calls in which expression evaluates to
the same string value shall write output to the existing stream.
The stream shall remain open until the close function (see
Input/Output and General Functions) is called with an expression
that evaluates to the same string value. At that time, the stream
shall be closed as if by a call to the pclose() function defined
in the System Interfaces volume of POSIX.1‐2017.
As described in detail by the grammar in Grammar, these output
statements shall take a <comma>-separated list of expressions
referred to in the grammar by the non-terminal symbols expr_list,
print_expr_list, or print_expr_list_opt. This list is referred to
here as the expression list, and each member is referred to as an
expression argument.
The print statement shall write the value of each expression
argument onto the indicated output stream separated by the current
output field separator (see variable OFS above), and terminated by
the output record separator (see variable ORS above). All
expression arguments shall be taken as strings, being converted if
necessary; this conversion shall be as described in Expressions in
awk, with the exception that the printf format in OFMT shall be
used instead of the value in CONVFMT. An empty expression list
shall stand for the whole input record ($0).
The printf statement shall produce output based on a notation
similar to the File Format Notation used to describe file formats
in this volume of POSIX.1‐2017 (see the Base Definitions volume of
POSIX.1‐2017, Chapter 5, File Format Notation). Output shall be
produced as specified with the first expression argument as the
string format and subsequent expression arguments as the strings
arg1 to argn, inclusive, with the following exceptions:
1. The format shall be an actual character string rather than a
graphical representation. Therefore, it cannot contain empty
character positions. The <space> in the format string, in any
context other than a flag of a conversion specification, shall
be treated as an ordinary character that is copied to the
output.
2. If the character set contains a '' character and that
character appears in the format string, it shall be treated as
an ordinary character that is copied to the output.
3. The escape sequences beginning with a <backslash> character
shall be treated as sequences of ordinary characters that are
copied to the output. Note that these same sequences shall be
interpreted lexically by awk when they appear in literal
strings, but they shall not be treated specially by the printf
statement.
4. A field width or precision can be specified as the '*'
character instead of a digit string. In this case the next
argument from the expression list shall be fetched and its
numeric value taken as the field width or precision.
5. The implementation shall not precede or follow output from the
d or u conversion specifier characters with <blank> characters
not specified by the format string.
6. The implementation shall not precede output from the o
conversion specifier character with leading zeros not
specified by the format string.
7. For the c conversion specifier character: if the argument has
a numeric value, the character whose encoding is that value
shall be output. If the value is zero or is not the encoding
of any character in the character set, the behavior is
undefined. If the argument does not have a numeric value, the
first character of the string value shall be output; if the
string does not contain any characters, the behavior is
undefined.
8. For each conversion specification that consumes an argument,
the next expression argument shall be evaluated. With the
exception of the c conversion specifier character, the value
shall be converted (according to the rules specified in
Expressions in awk) to the appropriate type for the conversion
specification.
9. If there are insufficient expression arguments to satisfy all
the conversion specifications in the format string, the
behavior is undefined.
10. If any character sequence in the format string begins with a
'%' character, but does not form a valid conversion
specification, the behavior is unspecified.
Both print and printf can output at least {LINE_MAX} bytes.
Functions
The awk language has a variety of built-in functions: arithmetic,
string, input/output, and general.
Arithmetic Functions
The arithmetic functions, except for int, shall be based on the
ISO C standard (see Section 1.1.2, Concepts Derived from the ISO C
Standard). The behavior is undefined in cases where the ISO C
standard specifies that an error be returned or that the behavior
is undefined. Although the grammar (see Grammar) permits built-in
functions to appear with no arguments or parentheses, unless the
argument or parentheses are indicated as optional in the following
list (by displaying them within the “[]” brackets), such use is
undefined.
atan2(y,x)
Return arctangent of y/x in radians in the range [-π,π].
cos(x) Return cosine of x, where x is in radians.
sin(x) Return sine of x, where x is in radians.
exp(x) Return the exponential function of x.
log(x) Return the natural logarithm of x.
sqrt(x) Return the square root of x.
int(x) Return the argument truncated to an integer. Truncation
shall be toward 0 when x>0.
rand() Return a random number n, such that 0≤n<1.
srand([expr])
Set the seed value for rand to expr or use the time of
day if expr is omitted. The previous seed value shall be
returned.
String Functions
The string functions in the following list shall be supported.
Although the grammar (see Grammar) permits built-in functions to
appear with no arguments or parentheses, unless the argument or
parentheses are indicated as optional in the following list (by
displaying them within the “[]” brackets), such use is undefined.
gsub(ere, repl[, in])
Behave like sub (see below), except that it shall
replace all occurrences of the regular expression (like
the ed utility global substitute) in $0 or in the in
argument, when specified.
index(s, t)
Return the position, in characters, numbering from 1, in
string s where string t first occurs, or zero if it does
not occur at all.
length[([s])]
Return the length, in characters, of its argument taken
as a string, or of the whole record, $0, if there is no
argument.
match(s, ere)
Return the position, in characters, numbering from 1, in
string s where the extended regular expression ere
occurs, or zero if it does not occur at all. RSTART
shall be set to the starting position (which is the same
as the returned value), zero if no match is found;
RLENGTH shall be set to the length of the matched
string, -1 if no match is found.
split(s, a[, fs ])
Split the string s into array elements a[1], a[2], ...,
a[n], and return n. All elements of the array shall be
deleted before the split is performed. The separation
shall be done with the ERE fs or with the field
separator FS if fs is not given. Each array element
shall have a string value when created and, if
appropriate, the array element shall be considered a
numeric string (see Expressions in awk). The effect of
a null string as the value of fs is unspecified.
sprintf(fmt, expr, expr, ...)
Format the expressions according to the printf format
given by fmt and return the resulting string.
sub(ere, repl[, in ])
Substitute the string repl in place of the first
instance of the extended regular expression ERE in
string in and return the number of substitutions. An
<ampersand> ('&') appearing in the string repl shall be
replaced by the string from in that matches the ERE. An
<ampersand> preceded with a <backslash> shall be
interpreted as the literal <ampersand> character. An
occurrence of two consecutive <backslash> characters
shall be interpreted as just a single literal
<backslash> character. Any other occurrence of a
<backslash> (for example, preceding any other character)
shall be treated as a literal <backslash> character.
Note that if repl is a string literal (the lexical token
STRING; see Grammar), the handling of the <ampersand>
character occurs after any lexical processing, including
any lexical <backslash>-escape sequence processing. If
in is specified and it is not an lvalue (see Expressions
in awk), the behavior is undefined. If in is omitted,
awk shall use the current record ($0) in its place.
substr(s, m[, n ])
Return the at most n-character substring of s that
begins at position m, numbering from 1. If n is omitted,
or if n specifies more characters than are left in the
string, the length of the substring shall be limited by
the length of the string s.
tolower(s)
Return a string based on the string s. Each character
in s that is an uppercase letter specified to have a
tolower mapping by the LC_CTYPE category of the current
locale shall be replaced in the returned string by the
lowercase letter specified by the mapping. Other
characters in s shall be unchanged in the returned
string.
toupper(s)
Return a string based on the string s. Each character
in s that is a lowercase letter specified to have a
toupper mapping by the LC_CTYPE category of the current
locale is replaced in the returned string by the
uppercase letter specified by the mapping. Other
characters in s are unchanged in the returned string.
All of the preceding functions that take ERE as a parameter expect
a pattern or a string valued expression that is a regular
expression as defined in Regular Expressions.
Input/Output and General Functions
The input/output and general functions are:
close(expression)
Close the file or pipe opened by a print or printf
statement or a call to getline with the same string-
valued expression. The limit on the number of open
expression arguments is implementation-defined. If the
close was successful, the function shall return zero;
otherwise, it shall return non-zero.
expression | getline [var]
Read a record of input from a stream piped from the
output of a command. The stream shall be created if no
stream is currently open with the value of expression as
its command name. The stream created shall be equivalent
to one created by a call to the popen() function with
the value of expression as the command argument and a
value of r as the mode argument. As long as the stream
remains open, subsequent calls in which expression
evaluates to the same string value shall read subsequent
records from the stream. The stream shall remain open
until the close function is called with an expression
that evaluates to the same string value. At that time,
the stream shall be closed as if by a call to the
pclose() function. If var is omitted, $0 and NF shall be
set; otherwise, var shall be set and, if appropriate, it
shall be considered a numeric string (see Expressions in
awk).
The getline operator can form ambiguous constructs when
there are unparenthesized operators (including
concatenate) to the left of the '|' (to the beginning of
the expression containing getline). In the context of
the '$' operator, '|' shall behave as if it had a lower
precedence than '$'. The result of evaluating other
operators is unspecified, and conforming applications
shall parenthesize properly all such usages.
getline Set $0 to the next input record from the current input
file. This form of getline shall set the NF, NR, and FNR
variables.
getline var
Set variable var to the next input record from the
current input file and, if appropriate, var shall be
considered a numeric string (see Expressions in awk).
This form of getline shall set the FNR and NR variables.
getline [var] < expression
Read the next record of input from a named file. The
expression shall be evaluated to produce a string that
is used as a pathname. If the file of that name is not
currently open, it shall be opened. As long as the
stream remains open, subsequent calls in which
expression evaluates to the same string value shall read
subsequent records from the file. The file shall remain
open until the close function is called with an
expression that evaluates to the same string value. If
var is omitted, $0 and NF shall be set; otherwise, var
shall be set and, if appropriate, it shall be considered
a numeric string (see Expressions in awk).
The getline operator can form ambiguous constructs when
there are unparenthesized binary operators (including
concatenate) to the right of the '<' (up to the end of
the expression containing the getline). The result of
evaluating such a construct is unspecified, and
conforming applications shall parenthesize properly all
such usages.
system(expression)
Execute the command given by expression in a manner
equivalent to the system() function defined in the
System Interfaces volume of POSIX.1‐2017 and return the
exit status of the command.
All forms of getline shall return 1 for successful input, zero for
end-of-file, and -1 for an error.
Where strings are used as the name of a file or pipeline, the
application shall ensure that the strings are textually identical.
The terminology ``same string value'' implies that ``equivalent
strings'', even those that differ only by <space> characters,
represent different files.
User-Defined Functions
The awk language also provides user-defined functions. Such
functions can be defined as:
function name([parameter, ...]) { statements }
A function can be referred to anywhere in an awk program; in
particular, its use can precede its definition. The scope of a
function is global.
Function parameters, if present, can be either scalars or arrays;
the behavior is undefined if an array name is passed as a
parameter that the function uses as a scalar, or if a scalar
expression is passed as a parameter that the function uses as an
array. Function parameters shall be passed by value if scalar and
by reference if array name.
The number of parameters in the function definition need not match
the number of parameters in the function call. Excess formal
parameters can be used as local variables. If fewer arguments are
supplied in a function call than are in the function definition,
the extra parameters that are used in the function body as scalars
shall evaluate to the uninitialized value until they are otherwise
initialized, and the extra parameters that are used in the
function body as arrays shall be treated as uninitialized arrays
where each element evaluates to the uninitialized value until
otherwise initialized.
When invoking a function, no white space can be placed between the
function name and the opening parenthesis. Function calls can be
nested and recursive calls can be made upon functions. Upon return
from any nested or recursive function call, the values of all of
the calling function's parameters shall be unchanged, except for
array parameters passed by reference. The return statement can be
used to return a value. If a return statement appears outside of a
function definition, the behavior is undefined.
In the function definition, <newline> characters shall be optional
before the opening brace and after the closing brace. Function
definitions can appear anywhere in the program where a pattern-
action pair is allowed.
Grammar
The grammar in this section and the lexical conventions in the
following section shall together describe the syntax for awk
programs. The general conventions for this style of grammar are
described in Section 1.3, Grammar Conventions. A valid program
can be represented as the non-terminal symbol program in the
grammar. This formal syntax shall take precedence over the
preceding text syntax description.
%token NAME NUMBER STRING ERE
%token FUNC_NAME /* Name followed by '(' without white space. */
/* Keywords */
%token Begin End
/* 'BEGIN' 'END' */
%token Break Continue Delete Do Else
/* 'break' 'continue' 'delete' 'do' 'else' */
%token Exit For Function If In
/* 'exit' 'for' 'function' 'if' 'in' */
%token Next Print Printf Return While
/* 'next' 'print' 'printf' 'return' 'while' */
/* Reserved function names */
%token BUILTIN_FUNC_NAME
/* One token for the following:
* atan2 cos sin exp log sqrt int rand srand
* gsub index length match split sprintf sub
* substr tolower toupper close system
*/
%token GETLINE
/* Syntactically different from other built-ins. */
/* Two-character tokens. */
%token ADD_ASSIGN SUB_ASSIGN MUL_ASSIGN DIV_ASSIGN MOD_ASSIGN POW_ASSIGN
/* '+=' '-=' '*=' '/=' '%=' '^=' */
%token OR AND NO_MATCH EQ LE GE NE INCR DECR APPEND
/* '||' '&&' '!~' '==' '<=' '>=' '!=' '++' '--' '>>' */
/* One-character tokens. */
%token '{' '}' '(' ')' '[' ']' ',' ';' NEWLINE
%token '+' '-' '*' '%' '^' '!' '>' '<' '|' '?' ':' '~' '$' '='
%start program
%%
program : item_list
| item_list item
;
item_list : /* empty */
| item_list item terminator
;
item : action
| pattern action
| normal_pattern
| Function NAME '(' param_list_opt ')'
newline_opt action
| Function FUNC_NAME '(' param_list_opt ')'
newline_opt action
;
param_list_opt : /* empty */
| param_list
;
param_list : NAME
| param_list ',' NAME
;
pattern : normal_pattern
| special_pattern
;
normal_pattern : expr
| expr ',' newline_opt expr
;
special_pattern : Begin
| End
;
action : '{' newline_opt '}'
| '{' newline_opt terminated_statement_list '}'
| '{' newline_opt unterminated_statement_list '}'
;
terminator : terminator NEWLINE
| ';'
| NEWLINE
;
terminated_statement_list : terminated_statement
| terminated_statement_list terminated_statement
;
unterminated_statement_list : unterminated_statement
| terminated_statement_list unterminated_statement
;
terminated_statement : action newline_opt
| If '(' expr ')' newline_opt terminated_statement
| If '(' expr ')' newline_opt terminated_statement
Else newline_opt terminated_statement
| While '(' expr ')' newline_opt terminated_statement
| For '(' simple_statement_opt ';'
expr_opt ';' simple_statement_opt ')' newline_opt
terminated_statement
| For '(' NAME In NAME ')' newline_opt
terminated_statement
| ';' newline_opt
| terminatable_statement NEWLINE newline_opt
| terminatable_statement ';' newline_opt
;
unterminated_statement : terminatable_statement
| If '(' expr ')' newline_opt unterminated_statement
| If '(' expr ')' newline_opt terminated_statement
Else newline_opt unterminated_statement
| While '(' expr ')' newline_opt unterminated_statement
| For '(' simple_statement_opt ';'
expr_opt ';' simple_statement_opt ')' newline_opt
unterminated_statement
| For '(' NAME In NAME ')' newline_opt
unterminated_statement
;
terminatable_statement : simple_statement
| Break
| Continue
| Next
| Exit expr_opt
| Return expr_opt
| Do newline_opt terminated_statement While '(' expr ')'
;
simple_statement_opt : /* empty */
| simple_statement
;
simple_statement : Delete NAME '[' expr_list ']'
| expr
| print_statement
;
print_statement : simple_print_statement
| simple_print_statement output_redirection
;
simple_print_statement : Print print_expr_list_opt
| Print '(' multiple_expr_list ')'
| Printf print_expr_list
| Printf '(' multiple_expr_list ')'
;
output_redirection : '>' expr
| APPEND expr
| '|' expr
;
expr_list_opt : /* empty */
| expr_list
;
expr_list : expr
| multiple_expr_list
;
multiple_expr_list : expr ',' newline_opt expr
| multiple_expr_list ',' newline_opt expr
;
expr_opt : /* empty */
| expr
;
expr : unary_expr
| non_unary_expr
;
unary_expr : '+' expr
| '-' expr
| unary_expr '^' expr
| unary_expr '*' expr
| unary_expr '/' expr
| unary_expr '%' expr
| unary_expr '+' expr
| unary_expr '-' expr
| unary_expr non_unary_expr
| unary_expr '<' expr
| unary_expr LE expr
| unary_expr NE expr
| unary_expr EQ expr
| unary_expr '>' expr
| unary_expr GE expr
| unary_expr '~' expr
| unary_expr NO_MATCH expr
| unary_expr In NAME
| unary_expr AND newline_opt expr
| unary_expr OR newline_opt expr
| unary_expr '?' expr ':' expr
| unary_input_function
;
non_unary_expr : '(' expr ')'
| '!' expr
| non_unary_expr '^' expr
| non_unary_expr '*' expr
| non_unary_expr '/' expr
| non_unary_expr '%' expr
| non_unary_expr '+' expr
| non_unary_expr '-' expr
| non_unary_expr non_unary_expr
| non_unary_expr '<' expr
| non_unary_expr LE expr
| non_unary_expr NE expr
| non_unary_expr EQ expr
| non_unary_expr '>' expr
| non_unary_expr GE expr
| non_unary_expr '~' expr
| non_unary_expr NO_MATCH expr
| non_unary_expr In NAME
| '(' multiple_expr_list ')' In NAME
| non_unary_expr AND newline_opt expr
| non_unary_expr OR newline_opt expr
| non_unary_expr '?' expr ':' expr
| NUMBER
| STRING
| lvalue
| ERE
| lvalue INCR
| lvalue DECR
| INCR lvalue
| DECR lvalue
| lvalue POW_ASSIGN expr
| lvalue MOD_ASSIGN expr
| lvalue MUL_ASSIGN expr
| lvalue DIV_ASSIGN expr
| lvalue ADD_ASSIGN expr
| lvalue SUB_ASSIGN expr
| lvalue '=' expr
| FUNC_NAME '(' expr_list_opt ')'
/* no white space allowed before '(' */
| BUILTIN_FUNC_NAME '(' expr_list_opt ')'
| BUILTIN_FUNC_NAME
| non_unary_input_function
;
print_expr_list_opt : /* empty */
| print_expr_list
;
print_expr_list : print_expr
| print_expr_list ',' newline_opt print_expr
;
print_expr : unary_print_expr
| non_unary_print_expr
;
unary_print_expr : '+' print_expr
| '-' print_expr
| unary_print_expr '^' print_expr
| unary_print_expr '*' print_expr
| unary_print_expr '/' print_expr
| unary_print_expr '%' print_expr
| unary_print_expr '+' print_expr
| unary_print_expr '-' print_expr
| unary_print_expr non_unary_print_expr
| unary_print_expr '~' print_expr
| unary_print_expr NO_MATCH print_expr
| unary_print_expr In NAME
| unary_print_expr AND newline_opt print_expr
| unary_print_expr OR newline_opt print_expr
| unary_print_expr '?' print_expr ':' print_expr
;
non_unary_print_expr : '(' expr ')'
| '!' print_expr
| non_unary_print_expr '^' print_expr
| non_unary_print_expr '*' print_expr
| non_unary_print_expr '/' print_expr
| non_unary_print_expr '%' print_expr
| non_unary_print_expr '+' print_expr
| non_unary_print_expr '-' print_expr
| non_unary_print_expr non_unary_print_expr
| non_unary_print_expr '~' print_expr
| non_unary_print_expr NO_MATCH print_expr
| non_unary_print_expr In NAME
| '(' multiple_expr_list ')' In NAME
| non_unary_print_expr AND newline_opt print_expr
| non_unary_print_expr OR newline_opt print_expr
| non_unary_print_expr '?' print_expr ':' print_expr
| NUMBER
| STRING
| lvalue
| ERE
| lvalue INCR
| lvalue DECR
| INCR lvalue
| DECR lvalue
| lvalue POW_ASSIGN print_expr
| lvalue MOD_ASSIGN print_expr
| lvalue MUL_ASSIGN print_expr
| lvalue DIV_ASSIGN print_expr
| lvalue ADD_ASSIGN print_expr
| lvalue SUB_ASSIGN print_expr
| lvalue '=' print_expr
| FUNC_NAME '(' expr_list_opt ')'
/* no white space allowed before '(' */
| BUILTIN_FUNC_NAME '(' expr_list_opt ')'
| BUILTIN_FUNC_NAME
;
lvalue : NAME
| NAME '[' expr_list ']'
| '$' expr
;
non_unary_input_function : simple_get
| simple_get '<' expr
| non_unary_expr '|' simple_get
;
unary_input_function : unary_expr '|' simple_get
;
simple_get : GETLINE
| GETLINE lvalue
;
newline_opt : /* empty */
| newline_opt NEWLINE
;
This grammar has several ambiguities that shall be resolved as
follows:
* Operator precedence and associativity shall be as described in
Table 4-1, Expressions in Decreasing Precedence in awk.
* In case of ambiguity, an else shall be associated with the
most immediately preceding if that would satisfy the grammar.
* In some contexts, a <slash> ('/') that is used to surround an
ERE could also be the division operator. This shall be
resolved in such a way that wherever the division operator
could appear, a <slash> is assumed to be the division
operator. (There is no unary division operator.)
Each expression in an awk program shall conform to the precedence
and associativity rules, even when this is not needed to resolve
an ambiguity. For example, because '$' has higher precedence than
'++', the string "$x++--" is not a valid awk expression, even
though it is unambiguously parsed by the grammar as "$(x++)--".
One convention that might not be obvious from the formal grammar
is where <newline> characters are acceptable. There are several
obvious placements such as terminating a statement, and a
<backslash> can be used to escape <newline> characters between any
lexical tokens. In addition, <newline> characters without
<backslash> characters can follow a comma, an open brace, logical
AND operator ("&&"), logical OR operator ("||"), the do keyword,
the else keyword, and the closing parenthesis of an if, for, or
while statement. For example:
{ print $1,
$2 }
Lexical Conventions
The lexical conventions for awk programs, with respect to the
preceding grammar, shall be as follows:
1. Except as noted, awk shall recognize the longest possible
token or delimiter beginning at a given point.
2. A comment shall consist of any characters beginning with the
<number-sign> character and terminated by, but excluding the
next occurrence of, a <newline>. Comments shall have no
effect, except to delimit lexical tokens.
3. The <newline> shall be recognized as the token NEWLINE.
4. A <backslash> character immediately followed by a <newline>
shall have no effect.
5. The token STRING shall represent a string constant. A string
constant shall begin with the character '"'. Within a string
constant, a <backslash> character shall be considered to begin
an escape sequence as specified in the table in the Base
Definitions volume of POSIX.1‐2017, Chapter 5, File Format
Notation ('\\', '\a', '\b', '\f', '\n', '\r', '\t', '\v'). In
addition, the escape sequences in Table 4-2, Escape Sequences
in awk shall be recognized. A <newline> shall not occur within
a string constant. A string constant shall be terminated by
the first unescaped occurrence of the character '"' after the
one that begins the string constant. The value of the string
shall be the sequence of all unescaped characters and values
of escape sequences between, but not including, the two
delimiting '"' characters.
6. The token ERE represents an extended regular expression
constant. An ERE constant shall begin with the <slash>
character. Within an ERE constant, a <backslash> character
shall be considered to begin an escape sequence as specified
in the table in the Base Definitions volume of POSIX.1‐2017,
Chapter 5, File Format Notation. In addition, the escape
sequences in Table 4-2, Escape Sequences in awk shall be
recognized. The application shall ensure that a <newline> does
not occur within an ERE constant. An ERE constant shall be
terminated by the first unescaped occurrence of the <slash>
character after the one that begins the ERE constant. The
extended regular expression represented by the ERE constant
shall be the sequence of all unescaped characters and values
of escape sequences between, but not including, the two
delimiting <slash> characters.
7. A <blank> shall have no effect, except to delimit lexical
tokens or within STRING or ERE tokens.
8. The token NUMBER shall represent a numeric constant. Its form
and numeric value shall either be equivalent to the decimal-
floating-constant token as specified by the ISO C standard, or
it shall be a sequence of decimal digits and shall be
evaluated as an integer constant in decimal. In addition,
implementations may accept numeric constants with the form and
numeric value equivalent to the hexadecimal-constant and
hexadecimal-floating-constant tokens as specified by the ISO C
standard.
If the value is too large or too small to be representable
(see Section 1.1.2, Concepts Derived from the ISO C Standard),
the behavior is undefined.
9. A sequence of underscores, digits, and alphabetics from the
portable character set (see the Base Definitions volume of
POSIX.1‐2017, Section 6.1, Portable Character Set), beginning
with an <underscore> or alphabetic character, shall be
considered a word.
10. The following words are keywords that shall be recognized as
individual tokens; the name of the token is the same as the
keyword:
BEGIN delete END function in printf
break do exit getline next return
continue else for if print while
11. The following words are names of built-in functions and shall
be recognized as the token BUILTIN_FUNC_NAME:
atan2 gsub log split sub toupper
close index match sprintf substr
cos int rand sqrt system
exp length sin srand tolower
The above-listed keywords and names of built-in functions are
considered reserved words.
12. The token NAME shall consist of a word that is not a keyword
or a name of a built-in function and is not followed
immediately (without any delimiters) by the '(' character.
13. The token FUNC_NAME shall consist of a word that is not a
keyword or a name of a built-in function, followed immediately
(without any delimiters) by the '(' character. The '('
character shall not be included as part of the token.
14. The following two-character sequences shall be recognized as
the named tokens:
┌────────────┬──────────┬────────────┬──────────┐
│ Token Name │ Sequence │ Token Name │ Sequence │
├────────────┼──────────┼────────────┼──────────┤
│ ADD_ASSIGN │ += │ NO_MATCH │ !~ │
│ SUB_ASSIGN │ -= │ EQ │ == │
│ MUL_ASSIGN │ *= │ LE │ <= │
│ DIV_ASSIGN │ /= │ GE │ >= │
│ MOD_ASSIGN │ %= │ NE │ != │
│ POW_ASSIGN │ ^= │ INCR │ ++ │
│ OR │ || │ DECR │ -- │
│ AND │ && │ APPEND │ >> │
└────────────┴──────────┴────────────┴──────────┘
15. The following single characters shall be recognized as tokens
whose names are the character:
<newline> { } ( ) [ ] , ; + - * % ^ ! > < | ? : ~ $ =
There is a lexical ambiguity between the token ERE and the tokens
'/' and DIV_ASSIGN. When an input sequence begins with a <slash>
character in any syntactic context where the token '/' or
DIV_ASSIGN could appear as the next token in a valid program, the
longer of those two tokens that can be recognized shall be
recognized. In any other syntactic context where the token ERE
could appear as the next token in a valid program, the token ERE
shall be recognized.
EXIT STATUS
The following exit values shall be returned:
0 All input files were processed successfully.
>0 An error occurred.
The exit status can be altered within the program by using an exit
expression.
CONSEQUENCES OF ERRORS
If any file operand is specified and the named file cannot be
accessed, awk shall write a diagnostic message to standard error
and terminate without any further action.
If the program specified by either the program operand or a
progfile operand is not a valid awk program (as specified in the
EXTENDED DESCRIPTION section), the behavior is undefined.
The following sections are informative.
APPLICATION USAGE
The index, length, match, and substr functions should not be
confused with similar functions in the ISO C standard; the awk
versions deal with characters, while the ISO C standard deals with
bytes.
Because the concatenation operation is represented by adjacent
expressions rather than an explicit operator, it is often
necessary to use parentheses to enforce the proper evaluation
precedence.
When using awk to process pathnames, it is recommended that
LC_ALL, or at least LC_CTYPE and LC_COLLATE, are set to POSIX or C
in the environment, since pathnames can contain byte sequences
that do not form valid characters in some locales, in which case
the utility's behavior would be undefined. In the POSIX locale
each byte is a valid single-byte character, and therefore this
problem is avoided.
On implementations where the "==" operator checks if strings
collate equally, applications needing to check whether strings are
identical can use:
length(a) == length(b) && index(a,b) == 1
On implementations where the "==" operator checks if strings are
identical, applications needing to check whether strings collate
equally can use:
a <= b && a >= b
EXAMPLES
The awk program specified in the command line is most easily
specified within single-quotes (for example, 'program') for
applications using sh, because awk programs commonly contain
characters that are special to the shell, including double-quotes.
In the cases where an awk program contains single-quote
characters, it is usually easiest to specify most of the program
as strings within single-quotes concatenated by the shell with
quoted single-quote characters. For example:
awk '/'\''/ { print "quote:", $0 }'
prints all lines from the standard input containing a single-quote
character, prefixed with quote:.
The following are examples of simple awk programs:
1. Write to the standard output all input lines for which field 3
is greater than 5:
$3 > 5
2. Write every tenth line:
(NR % 10) == 0
3. Write any line with a substring matching the regular
expression:
/(G|D)(2[0-9][[:alpha:]]*)/
4. Print any line with a substring containing a 'G' or 'D',
followed by a sequence of digits and characters. This example
uses character classes digit and alpha to match language-
independent digit and alphabetic characters respectively:
/(G|D)([[:digit:][:alpha:]]*)/
5. Write any line in which the second field matches the regular
expression and the fourth field does not:
$2 ~ /xyz/ && $4 !~ /xyz/
6. Write any line in which the second field contains a
<backslash>:
$2 ~ /\\/
7. Write any line in which the second field contains a
<backslash>. Note that <backslash>-escapes are interpreted
twice; once in lexical processing of the string and once in
processing the regular expression:
$2 ~ "\\\\"
8. Write the second to the last and the last field in each line.
Separate the fields by a <colon>:
{OFS=":";print $(NF-1), $NF}
9. Write the line number and number of fields in each line. The
three strings representing the line number, the <colon>, and
the number of fields are concatenated and that string is
written to standard output:
{print NR ":" NF}
10. Write lines longer than 72 characters:
length($0) > 72
11. Write the first two fields in opposite order separated by OFS:
{ print $2, $1 }
12. Same, with input fields separated by a <comma> or <space> and
<tab> characters, or both:
BEGIN { FS = ",[ \t]*|[ \t]+" }
{ print $2, $1 }
13. Add up the first column, print sum, and average:
{s += $1 }
END {print "sum is ", s, " average is", s/NR}
14. Write fields in reverse order, one per line (many lines out
for each line in):
{ for (i = NF; i > 0; --i) print $i }
15. Write all lines between occurrences of the strings start and
stop:
/start/, /stop/
16. Write all lines whose first field is different from the
previous one:
$1 != prev { print; prev = $1 }
17. Simulate echo:
BEGIN {
for (i = 1; i < ARGC; ++i)
printf("%s%s", ARGV[i], i==ARGC-1?"\n":" ")
}
18. Write the path prefixes contained in the PATH environment
variable, one per line:
BEGIN {
n = split (ENVIRON["PATH"], path, ":")
for (i = 1; i <= n; ++i)
print path[i]
}
19. If there is a file named input containing page headers of the
form: Page #
and a file named program that contains:
/Page/ { $2 = n++; }
{ print }
then the command line:
awk -f program n=5 input
prints the file input, filling in page numbers starting at 5.
RATIONALE
This description is based on the new awk, ``nawk'', (see the
referenced The AWK Programming Language), which introduced a
number of new features to the historical awk:
1. New keywords: delete, do, function, return
2. New built-in functions: atan2, close, cos, gsub, match, rand,
sin, srand, sub, system
3. New predefined variables: FNR, ARGC, ARGV, RSTART, RLENGTH,
SUBSEP
4. New expression operators: ?, :, ,, ^
5. The FS variable and the third argument to split, now treated
as extended regular expressions.
6. The operator precedence, changed to more closely match the C
language. Two examples of code that operate differently are:
while ( n /= 10 > 1) ...
if (!"wk" ~ /bwk/) ...
Several features have been added based on newer implementations of
awk:
* Multiple instances of -f progfile are permitted.
* The new option -v assignment.
* The new predefined variable ENVIRON.
* New built-in functions toupper and tolower.
* More formatting capabilities are added to printf to match the
ISO C standard.
Earlier versions of this standard required implementations to
support multiple adjacent <semicolon>s, lines with one or more
<semicolon> before a rule (pattern-action pairs), and lines with
only <semicolon>(s). These are not required by this standard and
are considered poor programming practice, but can be accepted by
an implementation of awk as an extension.
The overall awk syntax has always been based on the C language,
with a few features from the shell command language and other
sources. Because of this, it is not completely compatible with any
other language, which has caused confusion for some users. It is
not the intent of the standard developers to address such issues.
A few relatively minor changes toward making the language more
compatible with the ISO C standard were made; most of these
changes are based on similar changes in recent implementations, as
described above. There remain several C-language conventions that
are not in awk. One of the notable ones is the <comma> operator,
which is commonly used to specify multiple expressions in the C
language for statement. Also, there are various places where awk
is more restrictive than the C language regarding the type of
expression that can be used in a given context. These limitations
are due to the different features that the awk language does
provide.
Regular expressions in awk have been extended somewhat from
historical implementations to make them a pure superset of
extended regular expressions, as defined by POSIX.1‐2008 (see the
Base Definitions volume of POSIX.1‐2017, Section 9.4, Extended
Regular Expressions). The main extensions are
internationalization features and interval expressions. Historical
implementations of awk have long supported <backslash>-escape
sequences as an extension to extended regular expressions, and
this extension has been retained despite inconsistency with other
utilities. The number of escape sequences recognized in both
extended regular expressions and strings has varied (generally
increasing with time) among implementations. The set specified by
POSIX.1‐2008 includes most sequences known to be supported by
popular implementations and by the ISO C standard. One sequence
that is not supported is hexadecimal value escapes beginning with
'\x'. This would allow values expressed in more than 9 bits to be
used within awk as in the ISO C standard. However, because this
syntax has a non-deterministic length, it does not permit the
subsequent character to be a hexadecimal digit. This limitation
can be dealt with in the C language by the use of lexical string
concatenation. In the awk language, concatenation could also be a
solution for strings, but not for extended regular expressions
(either lexical ERE tokens or strings used dynamically as regular
expressions). Because of this limitation, the feature has not been
added to POSIX.1‐2008.
When a string variable is used in a context where an extended
regular expression normally appears (where the lexical token ERE
is used in the grammar) the string does not contain the literal
<slash> characters.
Some versions of awk allow the form:
func name(args, ... ) { statements }
This has been deprecated by the authors of the language, who asked
that it not be specified.
Historical implementations of awk produce an error if a next
statement is executed in a BEGIN action, and cause awk to
terminate if a next statement is executed in an END action. This
behavior has not been documented, and it was not believed that it
was necessary to standardize it.
The specification of conversions between string and numeric values
is much more detailed than in the documentation of historical
implementations or in the referenced The AWK Programming Language.
Although most of the behavior is designed to be intuitive, the
details are necessary to ensure compatible behavior from different
implementations. This is especially important in relational
expressions since the types of the operands determine whether a
string or numeric comparison is performed. From the perspective of
an application developer, it is usually sufficient to expect
intuitive behavior and to force conversions (by adding zero or
concatenating a null string) when the type of an expression does
not obviously match what is needed. The intent has been to specify
historical practice in almost all cases. The one exception is
that, in historical implementations, variables and constants
maintain both string and numeric values after their original value
is converted by any use. This means that referencing a variable or
constant can have unexpected side-effects. For example, with
historical implementations the following program:
{
a = "+2"
b = 2
if (NR % 2)
c = a + b
if (a == b)
print "numeric comparison"
else
print "string comparison"
}
would perform a numeric comparison (and output numeric comparison)
for each odd-numbered line, but perform a string comparison (and
output string comparison) for each even-numbered line.
POSIX.1‐2008 ensures that comparisons will be numeric if
necessary. With historical implementations, the following program:
BEGIN {
OFMT = "%e"
print 3.14
OFMT = "%f"
print 3.14
}
would output "3.140000e+00" twice, because in the second print
statement the constant "3.14" would have a string value from the
previous conversion. POSIX.1‐2008 requires that the output of the
second print statement be "3.140000". The behavior of historical
implementations was seen as too unintuitive and unpredictable.
It was pointed out that with the rules contained in early drafts,
the following script would print nothing:
BEGIN {
y[1.5] = 1
OFMT = "%e"
print y[1.5]
}
Therefore, a new variable, CONVFMT, was introduced. The OFMT
variable is now restricted to affecting output conversions of
numbers to strings and CONVFMT is used for internal conversions,
such as comparisons or array indexing. The default value is the
same as that for OFMT, so unless a program changes CONVFMT (which
no historical program would do), it will receive the historical
behavior associated with internal string conversions.
The POSIX awk lexical and syntactic conventions are specified more
formally than in other sources. Again the intent has been to
specify historical practice. One convention that may not be
obvious from the formal grammar as in other verbal descriptions is
where <newline> characters are acceptable. There are several
obvious placements such as terminating a statement, and a
<backslash> can be used to escape <newline> characters between any
lexical tokens. In addition, <newline> characters without
<backslash> characters can follow a comma, an open brace, a
logical AND operator ("&&"), a logical OR operator ("||"), the do
keyword, the else keyword, and the closing parenthesis of an if,
for, or while statement. For example:
{ print $1,
$2 }
The requirement that awk add a trailing <newline> to the program
argument text is to simplify the grammar, making it match a text
file in form. There is no way for an application or test suite to
determine whether a literal <newline> is added or whether awk
simply acts as if it did.
POSIX.1‐2008 requires several changes from historical
implementations in order to support internationalization. Probably
the most subtle of these is the use of the decimal-point
character, defined by the LC_NUMERIC category of the locale, in
representations of floating-point numbers. This locale-specific
character is used in recognizing numeric input, in converting
between strings and numeric values, and in formatting output.
However, regardless of locale, the <period> character (the
decimal-point character of the POSIX locale) is the decimal-point
character recognized in processing awk programs (including
assignments in command line arguments). This is essentially the
same convention as the one used in the ISO C standard. The
difference is that the C language includes the setlocale()
function, which permits an application to modify its locale.
Because of this capability, a C application begins executing with
its locale set to the C locale, and only executes in the
environment-specified locale after an explicit call to
setlocale(). However, adding such an elaborate new feature to the
awk language was seen as inappropriate for POSIX.1‐2008. It is
possible to execute an awk program explicitly in any desired
locale by setting the environment in the shell.
The undefined behavior resulting from NULs in extended regular
expressions allows future extensions for the GNU gawk program to
process binary data.
The behavior in the case of invalid awk programs (including
lexical, syntactic, and semantic errors) is undefined because it
was considered overly limiting on implementations to specify. In
most cases such errors can be expected to produce a diagnostic and
a non-zero exit status. However, some implementations may choose
to extend the language in ways that make use of certain invalid
constructs. Other invalid constructs might be deemed worthy of a
warning, but otherwise cause some reasonable behavior. Still other
constructs may be very difficult to detect in some
implementations. Also, different implementations might detect a
given error during an initial parsing of the program (before
reading any input files) while others might detect it when
executing the program after reading some input. Implementors
should be aware that diagnosing errors as early as possible and
producing useful diagnostics can ease debugging of applications,
and thus make an implementation more usable.
The unspecified behavior from using multi-character RS values is
to allow possible future extensions based on extended regular
expressions used for record separators. Historical implementations
take the first character of the string and ignore the others.
Unspecified behavior when split(string,array,<null>) is used is to
allow a proposed future extension that would split up a string
into an array of individual characters.
In the context of the getline function, equally good arguments for
different precedences of the | and < operators can be made.
Historical practice has been that:
getline < "a" "b"
is parsed as:
( getline < "a" ) "b"
although many would argue that the intent was that the file ab
should be read. However:
getline < "x" + 1
parses as:
getline < ( "x" + 1 )
Similar problems occur with the | version of getline, particularly
in combination with $. For example:
$"echo hi" | getline
(This situation is particularly problematic when used in a print
statement, where the |getline part might be a redirection of the
print.)
Since in most cases such constructs are not (or at least should
not) be used (because they have a natural ambiguity for which
there is no conventional parsing), the meaning of these constructs
has been made explicitly unspecified. (The effect is that a
conforming application that runs into the problem must
parenthesize to resolve the ambiguity.) There appeared to be few
if any actual uses of such constructs.
Grammars can be written that would cause an error under these
circumstances. Where backwards-compatibility is not a large
consideration, implementors may wish to use such grammars.
Some historical implementations have allowed some built-in
functions to be called without an argument list, the result being
a default argument list chosen in some ``reasonable'' way. Use of
length as a synonym for length($0) is the only one of these forms
that is thought to be widely known or widely used; this particular
form is documented in various places (for example, most historical
awk reference pages, although not in the referenced The AWK
Programming Language) as legitimate practice. With this
exception, default argument lists have always been undocumented
and vaguely defined, and it is not at all clear how (or if) they
should be generalized to user-defined functions. They add no
useful functionality and preclude possible future extensions that
might need to name functions without calling them. Not
standardizing them seems the simplest course. The standard
developers considered that length merited special treatment,
however, since it has been documented in the past and sees
possibly substantial use in historical programs. Accordingly,
this usage has been made legitimate, but Issue 5 removed the
obsolescent marking for XSI-conforming implementations and many
otherwise conforming applications depend on this feature.
In sub and gsub, if repl is a string literal (the lexical token
STRING), then two consecutive <backslash> characters should be
used in the string to ensure a single <backslash> will precede the
<ampersand> when the resultant string is passed to the function.
(For example, to specify one literal <ampersand> in the
replacement string, use gsub(ERE, "\\&").)
Historically, the only special character in the repl argument of
sub and gsub string functions was the <ampersand> ('&') character
and preceding it with the <backslash> character was used to turn
off its special meaning.
The description in the ISO POSIX‐2:1993 standard introduced
behavior such that the <backslash> character was another special
character and it was unspecified whether there were any other
special characters. This description introduced several
portability problems, some of which are described below, and so it
has been replaced with the more historical description. Some of
the problems include:
* Historically, to create the replacement string, a script could
use gsub(ERE, "\\&"), but with the ISO POSIX‐2:1993 standard
wording, it was necessary to use gsub(ERE, "\\\\&"). The
<backslash> characters are doubled here because all string
literals are subject to lexical analysis, which would reduce
each pair of <backslash> characters to a single <backslash>
before being passed to gsub.
* Since it was unspecified what the special characters were, for
portable scripts to guarantee that characters are printed
literally, each character had to be preceded with a
<backslash>. (For example, a portable script had to use
gsub(ERE, "\\h\\i") to produce a replacement string of "hi".)
The description for comparisons in the ISO POSIX‐2:1993 standard
did not properly describe historical practice because of the way
numeric strings are compared as numbers. The current rules cause
the following code:
if (0 == "000")
print "strange, but true"
else
print "not true"
to do a numeric comparison, causing the if to succeed. It should
be intuitively obvious that this is incorrect behavior, and
indeed, no historical implementation of awk actually behaves this
way.
To fix this problem, the definition of numeric string was enhanced
to include only those values obtained from specific circumstances
(mostly external sources) where it is not possible to determine
unambiguously whether the value is intended to be a string or a
numeric.
Variables that are assigned to a numeric string shall also be
treated as a numeric string. (For example, the notion of a numeric
string can be propagated across assignments.) In comparisons, all
variables having the uninitialized value are to be treated as a
numeric operand evaluating to the numeric value zero.
Uninitialized variables include all types of variables including
scalars, array elements, and fields. The definition of an
uninitialized value in Variables and Special Variables is
necessary to describe the value placed on uninitialized variables
and on fields that are valid (for example, < $NF) but have no
characters in them and to describe how these variables are to be
used in comparisons. A valid field, such as $1, that has no
characters in it can be obtained from an input line of "\t\t" when
FS='\t'. Historically, the comparison ($1<10) was done
numerically after evaluating $1 to the value zero.
The phrase ``... also shall have the numeric value of the numeric
string'' was removed from several sections of the ISO POSIX‐2:1993
standard because is specifies an unnecessary implementation
detail. It is not necessary for POSIX.1‐2008 to specify that these
objects be assigned two different values. It is only necessary to
specify that these objects may evaluate to two different values
depending on context.
Historical implementations of awk did not parse hexadecimal
integer or floating constants like "0xa" and "0xap0". Due to an
oversight, the 2001 through 2004 editions of this standard
required support for hexadecimal floating constants. This was due
to the reference to atof(). This version of the standard allows
but does not require implementations to use atof() and includes a
description of how floating-point numbers are recognized as an
alternative to match historic behavior. The intent of this change
is to allow implementations to recognize floating-point constants
according to either the ISO/IEC 9899:1990 standard or
ISO/IEC 9899:1999 standard, and to allow (but not require)
implementations to recognize hexadecimal integer constants.
Historical implementations of awk did not support floating-point
infinities and NaNs in numeric strings; e.g., "-INF" and "NaN".
However, implementations that use the atof() or strtod() functions
to do the conversion picked up support for these values if they
used a ISO/IEC 9899:1999 standard version of the function instead
of a ISO/IEC 9899:1990 standard version. Due to an oversight, the
2001 through 2004 editions of this standard did not allow support
for infinities and NaNs, but in this revision support is allowed
(but not required). This is a silent change to the behavior of awk
programs; for example, in the POSIX locale the expression:
("-INF" + 0 < 0)
formerly had the value 0 because "-INF" converted to 0, but now it
may have the value 0 or 1.
FUTURE DIRECTIONS
A future version of this standard may require the "!=" and "=="
operators to perform string comparisons by checking if the strings
are identical (and not by checking if they collate equally).
SEE ALSO
Section 1.3, Grammar Conventions, grep(1p), lex(1p), sed(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 5, File
Format Notation, Section 6.1, Portable Character Set, Chapter 8,
Environment Variables, Chapter 9, Regular Expressions, Section
12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2017, atof(3p), exec(1p),
isspace(3p), popen(3p), setlocale(3p), strtod(3p)
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
本博客所有文章除特别声明外，均采用 CC BY-NC-SA 4.0 许可协议。转载请注明来源 Chan Revival Sky！
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