gawk - pattern scanning and processing language
gawk [ POSIX or GNU style options ] -f program-file [ -- ] file ... gawk [ POSIX or GNU style options ] [ -- ] program-text file ...
Gawk is the GNU Project's implementation of the AWK proュ gramming language. It conforms to the definition of the language in the POSIX 1003.2 Command Language And Utiliュ ties Standard. This version in turn is based on the description in The AWK Programming Language, by Aho, Kernighan, and Weinberger, with the additional features found in the System V Release 4 version of UNIX awk. Gawk also provides more recent Bell Labs awk extensions, and some GNU-specific extensions. The command line consists of options to gawk itself, the AWK program text (if not supplied via the -f or --file options), and values to be made available in the ARGC and ARGV pre-defined AWK variables.
Gawk options may be either the traditional POSIX one letュ ter options, or the GNU style long options. POSIX options start with a single ``-'', while long options start with ``--''. Long options are provided for both GNU-specific features and for POSIX mandated features. Following the POSIX standard, gawk-specific options are supplied via arguments to the -W option. Multiple -W options may be supplied Each -W option has a corresponding long option, as detailed below. Arguments to long options are either joined with the option by an = sign, with no intervening spaces, or they may be provided in the next command line argument. Long options may be abbreviated, as long as the abbreviation remains unique.
Gawk accepts the following options. -F fs --field-separator fs Use fs for the input field separator (the value of the FS predefined variable). -v var=val --assign var=val Assign the value val, to the variable var, before execution of the program begins. Such variable -f program-file --file program-file Read the AWK program source from the file program- file, instead of from the first command line arguュ ment. Multiple -f (or --file) options may be used. -mf NNN -mr NNN Set various memory limits to the value NNN. The f flag sets the maximum number of fields, and the r flag sets the maximum record size. These two flags and the -m option are from the Bell Labs research version of UNIX awk. They are ignored by gawk, since gawk has no pre-defined limits. -W traditional -W compat --traditional --compat Run in compatibility mode. In compatibility mode, gawk behaves identically to UNIX awk; none of the GNU-specific extensions are recognized. The use of --traditional is preferred over the other forms of this option. See GNU EXTENSIONS, below, for more information. -W copyleft -W copyright --copyleft --copyright Print the short version of the GNU copyright inforュ mation message on the standard output, and exits successfully. -W help -W usage --help --usage Print a relatively short summary of the available options on the standard output. (Per the GNU Codュ ing Standards, these options cause an immediate, successful exit.) -W lint --lint Provide warnings about constructs that are dubious or non-portable to other AWK implementations. -W lint-old --lint-old Provide warnings about constructs that are not portable to the original version of Unix awk. --posix This turns on compatibility mode, with the followュ ing additional restrictions: キ \x escape sequences are not recognized. キ Only space and tab act as field separators when FS is set to a single space, newline does not. キ The synonym func for the keyword function is not recognized. キ The operators ** and **= cannot be used in place of ^ and ^=. キ The fflush() function is not available. -W re-interval --re-interval Enable the use of interval expressions in regular expression matching (see Regular Expressions, below). Interval expressions were not traditionュ ally available in the AWK language. The POSIX stanュ dard added them, to make awk and egrep consistent with each other. However, their use is likely to break old AWK programs, so gawk only provides them if they are requested with this option, or when --posix is specified. -W source program-text --source program-text Use program-text as AWK program source code. This option allows the easy intermixing of library funcュ tions (used via the -f and --file options) with source code entered on the command line. It is intended primarily for medium to large AWK programs used in shell scripts. -W version --version Print version information for this particular copy of gawk on the standard output. This is useful mainly for knowing if the current copy of gawk on your system is up to date with respect to whatever the Free Software Foundation is distributing. This is also useful when reporting bugs. (Per the GNU Coding Standards, these options cause an immediate, successful exit.) -- Signal the end of options. This is useful to allow further arguments to the AWK program itself to start with a ``-''. This is mainly for consistency In compatibility mode, any other options are flagged as illegal, but are otherwise ignored. In normal operation, as long as program text has been supplied, unknown options are passed on to the AWK program in the ARGV array for processing. This is particularly useful for running AWK programs via the ``#!'' executable interpreter mechanism.
An AWK program consists of a sequence of pattern-action
statements and optional function definitions.
pattern { action statements }
function name(parameter list) { statements }
Gawk first reads the program source from the program-
file(s) if specified, from arguments to --source, or from
the first non-option argument on the command line. The -f
and --source options may be used multiple times on the
command line. Gawk will read the program text as if all
the program-files and command line source texts had been
concatenated together. This is useful for building
libraries of AWK functions, without having to include them
in each new AWK program that uses them. It also provides
the ability to mix library functions with command line
programs.
The environment variable AWKPATH specifies a search path
to use when finding source files named with the -f option.
If this variable does not exist, the default path is
".:/usr/local/share/awk". (The actual directory may vary,
depending upon how gawk was built and installed.) If a
file name given to the -f option contains a ``/'' characュ
ter, no path search is performed.
Gawk executes AWK programs in the following order. First,
all variable assignments specified via the -v option are
performed. Next, gawk compiles the program into an interュ
nal form. Then, gawk executes the code in the BEGIN
block(s) (if any), and then proceeds to read each file
named in the ARGV array. If there are no files named on
the command line, gawk reads the standard input.
If a filename on the command line has the form var=val it
is treated as a variable assignment. The variable var will
be assigned the value val. (This happens after any BEGIN
block(s) have been run.) Command line variable assignment
is most useful for dynamically assigning values to the
variables AWK uses to control how input is broken into
fields and records. It is also useful for controlling
state if multiple passes are needed over a single data
file.
(""), gawk skips over it.
For each record in the input, gawk tests to see if it
matches any pattern in the AWK program. For each pattern
that the record matches, the associated action is exeュ
cuted. The patterns are tested in the order they occur in
the program.
Finally, after all the input is exhausted, gawk executes
the code in the END block(s) (if any).
AWK variables are dynamic; they come into existence when they are first used. Their values are either floating- point numbers or strings, or both, depending upon how they are used. AWK also has one dimensional arrays; arrays with multiple dimensions may be simulated. Several pre-defined variables are set as a program runs; these will be described as needed and summarized below. Records Normally, records are separated by newline characters. You can control how records are separated by assigning values to the built-in variable RS. If RS is any single characュ ter, that character separates records. Otherwise, RS is a regular expression. Text in the input that matches this regular expression will separate the record. However, in compatibility mode, only the first character of its string value is used for separating records. If RS is set to the null string, then records are separated by blank lines. When RS is set to the null string, the newline character always acts as a field separator, in addition to whatever value FS may have. Fields As each input record is read, gawk splits the record into fields, using the value of the FS variable as the field separator. If FS is a single character, fields are sepaュ rated by that character. If FS is the null string, then each individual character becomes a separate field. Othュ erwise, FS is expected to be a full regular expression. In the special case that FS is a single space, fields are separated by runs of spaces and/or tabs and/or newlines. (But see the discussion of --posix, below). Note that the value of IGNORECASE (see below) will also affect how fields are split when FS is a regular expression, and how records are separated when RS is a regular expression. If the FIELDWIDTHS variable is set to a space separated list of numbers, each field is expected to have fixed width, and gawk will split up the record using the speciュ fied widths. The value of FS is ignored. Assigning a new Each field in the input record may be referenced by its position, 1ドル, 2ドル, and so on. 0ドル is the whole record. The value of a field may be assigned to as well. Fields need not be referenced by constants: n = 5 print $n prints the fifth field in the input record. The variable NF is set to the total number of fields in the input record. References to non-existent fields (i.e. fields after $NF) produce the null-string. However, assigning to a non-exisュ tent field (e.g., $(NF+2) = 5) will increase the value of NF, create any intervening fields with the null string as their value, and cause the value of 0ドル to be recomputed, with the fields being separated by the value of OFS. Refュ erences to negative numbered fields cause a fatal error. Decrementing NF causes the values of fields past the new value to be lost, and the value of 0ドル to be recomputed, with the fields being separated by the value of OFS. Built-in Variables Gawk's built-in variables are: ARGC The number of command line arguments (does not include options to gawk, or the program source). ARGIND The index in ARGV of the current file being processed. ARGV Array of command line arguments. The array is indexed from 0 to ARGC - 1. Dynamically changing the contents of ARGV can control the files used for data. CONVFMT The conversion format for numbers, "%.6g", by default. ENVIRON An array containing the values of the current environment. The array is indexed by the environment variables, each element being the value of that variable (e.g., ENVIRON["HOME"] might be /home/arnold). Changing this array does not affect the environment seen by proュ grams which gawk spawns via redirection or the system() function. (This may change in a future version of gawk.) rection for getline, during a read for getュ line, or during a close(), then ERRNO will contain a string describing the error. FIELDWIDTHS A white-space separated list of fieldwidths. When set, gawk parses the input into fields of fixed width, instead of using the value of the FS variable as the field separator. The fixed field width facility is still experimental; the semantics may change as gawk evolves over time. FILENAME The name of the current input file. If no files are specified on the command line, the value of FILENAME is ``-''. However, FILENAME is undefined inside the BEGIN block. FNR The input record number in the current input file. FS The input field separator, a space by default. See Fields, above. IGNORECASE Controls the case-sensitivity of all regular expression and string operations. If IGNOREュ CASE has a non-zero value, then string comparュ isons and pattern matching in rules, field splitting with FS, record separating with RS, regular expression matching with ~ and !~, and the gensub(), gsub(), index(), match(), split(), and sub() pre-defined functions will all ignore case when doing regular expression operations. Thus, if IGNORECASE is not equal to zero, /aB/ matches all of the strings "ab", "aB", "Ab", and "AB". As with all AWK variュ ables, the initial value of IGNORECASE is zero, so all regular expression and string operations are normally case-sensitive. Under Unix, the full ISO 8859-1 Latin-1 character set is used when ignoring case. NOTE: In verュ sions of gawk prior to 3.0, IGNORECASE only affected regular expression operations. It now affects string comparisons as well. NF The number of fields in the current input record. NR The total number of input records seen so far. OFMT The output format for numbers, "%.6g", by default. ORS The output record separator, by default a newュ line. RS The input record separator, by default a newュ line. RT The record terminator. Gawk sets RT to the input text that matched the character or reguュ lar expression specified by RS. RSTART The index of the first character matched by match(); 0 if no match. RLENGTH The length of the string matched by match(); -1 if no match. SUBSEP The character used to separate multiple subュ scripts in array elements, by default "034円". Arrays Arrays are subscripted with an expression between square brackets ([ and ]). If the expression is an expression list (expr, expr ...) then the array subscript is a string consisting of the concatenation of the (string) value of each expression, separated by the value of the SUBSEP variable. This facility is used to simulate multiュ ply dimensioned arrays. For example: i = "A"; j = "B"; k = "C" x[i, j, k] = "hello, world\n" assigns the string "hello, world\n" to the element of the array x which is indexed by the string "A034円B034円C". All arrays in AWK are associative, i.e. indexed by string valュ ues. The special operator in may be used in an if or while statement to see if an array has an index consisting of a particular value. if (val in array) print array[val] If the array has multiple subscripts, use (i, j) in array. The in construct may also be used in a for loop to iterate over all the elements of an array. An element may be deleted from an array using the delete statement. The delete statement may also be used to delete the entire contents of an array, just by specifying Variables and fields may be (floating point) numbers, or strings, or both. How the value of a variable is interュ preted depends upon its context. If used in a numeric expression, it will be treated as a number, if used as a string it will be treated as a string. To force a variable to be treated as a number, add 0 to it; to force it to be treated as a string, concatenate it with the null string. When a string must be converted to a number, the converュ sion is accomplished using atof(3). A number is converted to a string by using the value of CONVFMT as a format string for sprintf(3), with the numeric value of the variュ able as the argument. However, even though all numbers in AWK are floating-point, integral values are always conュ verted as integers. Thus, given CONVFMT = "%2.2f" a = 12 b = a "" the variable b has a string value of "12" and not "12.00". Gawk performs comparisons as follows: If two variables are numeric, they are compared numerically. If one value is numeric and the other has a string value that is a ``numeric string,'' then comparisons are also done numeriュ cally. Otherwise, the numeric value is converted to a string and a string comparison is performed. Two strings are compared, of course, as strings. According to the POSIX standard, even if two strings are numeric strings, a numeric comparison is performed. However, this is clearly incorrect, and gawk does not do this. Note that string constants, such as "57", are not numeric strings, they are string constants. The idea of ``numeric string'' only applies to fields, getline input, FILENAME, ARGV elements, ENVIRON elements and the elements of an array created by split() that are numeric strings. The basic idea is that user input, and only user input, that looks numeric, should be treated that way. Uninitialized variables have the numeric value 0 and the string value "" (the null, or empty, string).
AWK is a line oriented language. The pattern comes first,
and then the action. Action statements are enclosed in {
and }. Either the pattern may be missing, or the action
may be missing, but, of course, not both. If the pattern
is missing, the action will be executed for every single
which prints the entire record.
Comments begin with the ``#'' character, and continue
until the end of the line. Blank lines may be used to
separate statements. Normally, a statement ends with a
newline, however, this is not the case for lines ending in
a ``,'', {, ?, :, &&, or ||. Lines ending in do or else
also have their statements automatically continued on the
following line. In other cases, a line can be continued
by ending it with a ``\'', in which case the newline will
be ignored.
Multiple statements may be put on one line by separating
them with a ``;''. This applies to both the statements
within the action part of a pattern-action pair (the usual
case), and to the pattern-action statements themselves.
Patterns
AWK patterns may be one of the following:
BEGIN
END
/regular expression/
relational expression
pattern && pattern
pattern || pattern
pattern ? pattern : pattern
(pattern)
! pattern
pattern1, pattern2
BEGIN and END are two special kinds of patterns which are
not tested against the input. The action parts of all
BEGIN patterns are merged as if all the statements had
been written in a single BEGIN block. They are executed
before any of the input is read. Similarly, all the END
blocks are merged, and executed when all the input is
exhausted (or when an exit statement is executed). BEGIN
and END patterns cannot be combined with other patterns in
pattern expressions. BEGIN and END patterns cannot have
missing action parts.
For /regular expression/ patterns, the associated stateュ
ment is executed for each input record that matches the
regular expression. Regular expressions are the same as
those in egrep(1), and are summarized below.
A relational expression may use any of the operators
defined below in the section on actions. These generally
test whether certain fields match certain regular expresュ
sions.
and logical NOT, respectively, as in C. They do short-
circuit evaluation, also as in C, and are used for combinュ
ing more primitive pattern expressions. As in most lanュ
guages, parentheses may be used to change the order of
evaluation.
The ?: operator is like the same operator in C. If the
first pattern is true then the pattern used for testing is
the second pattern, otherwise it is the third. Only one of
the second and third patterns is evaluated.
The pattern1, pattern2 form of an expression is called a
range pattern. It matches all input records starting with
a record that matches pattern1, and continuing until a
record that matches pattern2, inclusive. It does not comュ
bine with any other sort of pattern expression.
Regular Expressions
Regular expressions are the extended kind found in egrep.
They are composed of characters as follows:
c matches the non-metacharacter c.
\c matches the literal character c.
. matches any character including newline.
^ matches the beginning of a string.
$ matches the end of a string.
[abc...] character list, matches any of the characters
abc....
[^abc...] negated character list, matches any character
except abc....
r1|r2 alternation: matches either r1 or r2.
r1r2 concatenation: matches r1, and then r2.
r+ matches one or more r's.
r* matches zero or more r's.
r? matches zero or one r's.
(r) grouping: matches r.
r{n}
r{n,}
r{n,m} One or two numbers inside braces denote an
n times. If there are two numbers separated by
a comma, r is repeated n to m times. If there
is one number followed by a comma, then r is
repeated at least n times.
Interval expressions are only available if
either --posix or --re-interval is specified on
the command line.
\y matches the empty string at either the beginュ
ning or the end of a word.
\B matches the empty string within a word.
\< matches the empty string at the beginning of a
word.
\> matches the empty string at the end of a word.
\w matches any word-constituent character (letter,
digit, or underscore).
\W matches any character that is not word-conュ
stituent.
\` matches the empty string at the beginning of a
buffer (string).
\' matches the empty string at the end of a
buffer.
The escape sequences that are valid in string constants
(see below) are also legal in regular expressions.
Character classes are a new feature introduced in the
POSIX standard. A character class is a special notation
for describing lists of characters that have a specific
attribute, but where the actual characters themselves can
vary from country to country and/or from character set to
character set. For example, the notion of what is an
alphabetic character differs in the USA and in France.
A character class is only valid in a regexp inside the
brackets of a character list. Character classes consist
of [:, a keyword denoting the class, and :]. Here are the
character classes defined by the POSIX standard.
[:alnum:]
Alphanumeric characters.
[:alpha:]
Alphabetic characters.
Space or tab characters.
[:cntrl:]
Control characters.
[:digit:]
Numeric characters.
[:graph:]
Characters that are both printable and visible. (A
space is printable, but not visible, while an a is
both.)
[:lower:]
Lower-case alphabetic characters.
[:print:]
Printable characters (characters that are not conュ
trol characters.)
[:punct:]
Punctuation characters (characters that are not
letter, digits, control characters, or space charュ
acters).
[:space:]
Space characters (such as space, tab, and formfeed,
to name a few).
[:upper:]
Upper-case alphabetic characters.
[:xdigit:]
Characters that are hexadecimal digits.
For example, before the POSIX standard, to match alphanuュ
meric characters, you would have had to write
/[A-Za-z0-9]/. If your character set had other alphabetic
characters in it, this would not match them. With the
POSIX character classes, you can write /[[:alnum:]]/, and
this will match all the alphabetic and numeric characters
in your character set.
Two additional special sequences can appear in character
lists. These apply to non-ASCII character sets, which can
have single symbols (called collating elements) that are
represented with more than one character, as well as sevュ
eral characters that are equivalent for collating, or
sorting, purposes. (E.g., in French, a plain ``e'' and a
grave-accented e` are equivalent.)
Collating Symbols
is a collating element, then [[.ch.]] is a regexp
that matches this collating element, while [ch] is
a regexp that matches either c or h.
Equivalence Classes
An equivalence class is a locale-specific name for
a list of characters that are equivalent. The name
is enclosed in [= and =]. For example, the name e
might be used to represent all of ``e,'' ``e`,''
and ``e`.'' In this case, [[=e]] is a regexp that
matches any of
.BR e ,
.BR eエ , or
.BR e` .
These features are very valuable in non-English speaking
locales. The library functions that gawk uses for regular
expression matching currently only recognize POSIX characュ
ter classes; they do not recognize collating symbols or
equivalence classes.
The \y, \B, \<, \>, \w, \W, \`, and \' operators are speュ
cific to gawk; they are extensions based on facilities in
the GNU regexp libraries.
The various command line options control how gawk interュ
prets characters in regexps.
No options
In the default case, gawk provide all the faciliュ
ties of POSIX regexps and the GNU regexp operators
described above. However, interval expressions are
not supported.
--posix
Only POSIX regexps are supported, the GNU operators
are not special. (E.g., \w matches a literal w).
Interval expressions are allowed.
--traditional
Traditional Unix awk regexps are matched. The GNU
operators are not special, interval expressions are
not available, and neither are the POSIX character
classes ([[:alnum:]] and so on). Characters
described by octal and hexadecimal escape sequences
are treated literally, even if they represent regュ
exp metacharacters.
--re-interval
Allow interval expressions in regexps, even if
--traditional has been provided.
Action statements are enclosed in braces, { and }. Action
statements consist of the usual assignment, conditional,
and looping statements found in most languages. The operaュ
tors, control statements, and input/output statements
available are patterned after those in C.
Operators
The operators in AWK, in order of decreasing precedence,
are
(...) Grouping
$ Field reference.
++ -- Increment and decrement, both prefix and postュ
fix.
^ Exponentiation (** may also be used, and **=
for the assignment operator).
+ - ! Unary plus, unary minus, and logical negation.
* / % Multiplication, division, and modulus.
+ - Addition and subtraction.
space String concatenation.
< >
<= >=
!= == The regular relational operators.
~ !~ Regular expression match, negated match.
NOTE: Do not use a constant regular expression
(/foo/) on the left-hand side of a ~ or !~.
Only use one on the right-hand side. The
expression /foo/ ~ exp has the same meaning as
((0ドル ~ /foo/) ~ exp). This is usually not
what was intended.
in Array membership.
&& Logical AND.
|| Logical OR.
?: The C conditional expression. This has the
form expr1 ? expr2 : expr3. If expr1 is true,
the value of the expression is expr2, otherュ
wise it is expr3. Only one of expr2 and expr3
is evaluated.
*= /= %= ^= Assignment. Both absolute assignment (var =
value) and operator-assignment (the other
forms) are supported.
Control Statements
The control statements are as follows:
if (condition) statement [ else statement ]
while (condition) statement
do statement while (condition)
for (expr1; expr2; expr3) statement
for (var in array) statement
break
continue
delete array[index]
delete array
exit [ expression ]
{ statements }
I/O Statements
The input/output statements are as follows:
close(file) Close file (or pipe, see below).
getline Set 0ドル from next input record; set
NF, NR, FNR.
getline <file Set 0ドル from next record of file; set
NF.
getline var Set var from next input record; set
NR, FNR.
getline var <file Set var from next record of file.
next Stop processing the current input
record. The next input record is
read and processing starts over with
the first pattern in the AWK proュ
gram. If the end of the input data
is reached, the END block(s), if
any, are executed.
nextfile Stop processing the current input
file. The next input record read
comes from the next input file.
FILENAME and ARGIND are updated, FNR
is reset to 1, and processing starts
over with the first pattern in the
AWK program. If the end of the input
data is reached, the END block(s),
two words. While this usage is still
recognized, it generates a warning
message and will eventually be
removed.
print Prints the current record. The outュ
put record is terminated with the
value of the ORS variable.
print expr-list Prints expressions. Each expression
is separated by the value of the OFS
variable. The output record is terュ
minated with the value of the ORS
variable.
print expr-list >file Prints expressions on file. Each
expression is separated by the value
of the OFS variable. The output
record is terminated with the value
of the ORS variable.
printf fmt, expr-list Format and print.
printf fmt, expr-list >file
Format and print on file.
system(cmd-line) Execute the command cmd-line, and
return the exit status. (This may
not be available on non-POSIX sysュ
tems.)
fflush([file]) Flush any buffers associated with
the open output file or pipe file.
If file is missing, then standard
output is flushed. If file is the
null string, then all open output
files and pipes have their buffers
flushed.
Other input/output redirections are also allowed. For
print and printf, >>file appends output to the file, while
| command writes on a pipe. In a similar fashion, command
| getline pipes into getline. The getline command will
return 0 on end of file, and -1 on an error.
The printf Statement
The AWK versions of the printf statement and sprintf()
function (see below) accept the following conversion specュ
ification formats:
%c An ASCII character. If the argument used for %c is
numeric, it is treated as a character and printed.
printed.
%d
%i A decimal number (the integer part).
%e
%E A floating point number of the form
[-]d.dddddde[+-]dd. The %E format uses E instead
of e.
%f A floating point number of the form [-]ddd.dddddd.
%g
%G Use %e or %f conversion, whichever is shorter, with
nonsignificant zeros suppressed. The %G format
uses %E instead of %e.
%o An unsigned octal number (again, an integer).
%s A character string.
%x
%X An unsigned hexadecimal number (an integer). %X
format uses ABCDEF instead of abcdef.
%% A single % character; no argument is converted.
There are optional, additional parameters that may lie
between the % and the control letter:
- The expression should be left-justified within its
field.
space For numeric conversions, prefix positive values
with a space, and negative values with a minus
sign.
+ The plus sign, used before the width modifier (see
below), says to always supply a sign for numeric
conversions, even if the data to be formatted is
positive. The + overrides the space modifier.
# Use an ``alternate form'' for certain control letュ
ters. For %o, supply a leading zero. For %x, and
%X, supply a leading 0x or 0X for a nonzero result.
For %e, %E, and %f, the result will always contain
a decimal point. For %g, and %G, trailing zeros
are not removed from the result.
0 A leading 0 (zero) acts as a flag, that indicates
output should be padded with zeroes instead of
spaces. This applies even to non-numeric output
width The field should be padded to this width. The field
is normally padded with spaces. If the 0 flag has
been used, it is padded with zeroes.
.prec A number that specifies the precision to use when
printing. For the %e, %E, and %f formats, this
specifies the number of digits you want printed to
the right of the decimal point. For the %g, and %G
formats, it specifies the maximum number of signifュ
icant digits. For the %d, %o, %i, %u, %x, and %X
formats, it specifies the minimum number of digits
to print. For a string, it specifies the maximum
number of characters from the string that should be
printed.
The dynamic width and prec capabilities of the ANSI C
printf() routines are supported. A * in place of either
the width or prec specifications will cause their values
to be taken from the argument list to printf or sprintf().
Special File Names
When doing I/O redirection from either print or printf
into a file, or via getline from a file, gawk recognizes
certain special filenames internally. These filenames
allow access to open file descriptors inherited from
gawk's parent process (usually the shell). Other special
filenames provide access to information about the running
gawk process. The filenames are:
/dev/pid Reading this file returns the process ID of
the current process, in decimal, terminated
with a newline.
/dev/ppid Reading this file returns the parent process
ID of the current process, in decimal, termiュ
nated with a newline.
/dev/pgrpid Reading this file returns the process group ID
of the current process, in decimal, terminated
with a newline.
/dev/user Reading this file returns a single record terュ
minated with a newline. The fields are sepaュ
rated with spaces. 1ドル is the value of the
getuid(2) system call, 2ドル is the value of the
geteuid(2) system call, 3ドル is the value of the
getgid(2) system call, and 4ドル is the value of
the getegid(2) system call. If there are any
additional fields, they are the group IDs
returned by getgroups(2). Multiple groups may
not be supported on all systems.
/dev/stdout The standard output.
/dev/stderr The standard error output.
/dev/fd/n The file associated with the open file
descriptor n.
These are particularly useful for error messages. For
example:
print "You blew it!" > "/dev/stderr"
whereas you would otherwise have to use
print "You blew it!" | "cat 1>&2"
These file names may also be used on the command line to
name data files.
Numeric Functions
AWK has the following pre-defined arithmetic functions:
atan2(y, x) returns the arctangent of y/x in radians.
cos(expr) returns the cosine of expr, which is in
radians.
exp(expr) the exponential function.
int(expr) truncates to integer.
log(expr) the natural logarithm function.
rand() returns a random number between 0 and 1.
sin(expr) returns the sine of expr, which is in radiュ
ans.
sqrt(expr) the square root function.
srand([expr]) uses expr as a new seed for the random numュ
ber generator. If no expr is provided, the
time of day will be used. The return value
is the previous seed for the random number
generator.
String Functions
Gawk has the following pre-defined string functions:
r. If h is a string beginning
with g or G, then replace all
matches of r with s. Otherwise, h
is a number indicating which match
of r to replace. If no t is supュ
plied, 0ドル is used instead. Within
the replacement text s, the
sequence \n, where n is a digit
from 1 to 9, may be used to indiュ
cate just the text that matched
the n'th parenthesized subexpresュ
sion. The sequence 0円 represents
the entire matched text, as does
the character &. Unlike sub() and
gsub(), the modified string is
returned as the result of the
function, and the original target
string is not changed.
gsub(r, s [, t]) for each substring matching the
regular expression r in the string
t, substitute the string s, and
return the number of substituュ
tions. If t is not supplied, use
0ドル. An & in the replacement text
is replaced with the text that was
actually matched. Use \& to get a
literal &. See AWK Language Proュ
gramming for a fuller discussion
of the rules for &'s and backュ
slashes in the replacement text of
sub(), gsub(), and gensub().
index(s, t) returns the index of the string t
in the string s, or 0 if t is not
present.
length([s]) returns the length of the string
s, or the length of 0ドル if s is not
supplied.
match(s, r) returns the position in s where
the regular expression r occurs,
or 0 if r is not present, and sets
the values of RSTART and RLENGTH.
split(s, a [, r]) splits the string s into the array
a on the regular expression r, and
returns the number of fields. If r
is omitted, FS is used instead.
The array a is cleared first.
Splitting behaves identically to
and returns the resulting string.
sub(r, s [, t]) just like gsub(), but only the
first matching substring is
replaced.
substr(s, i [, n]) returns the at most n-character
substring of s starting at i. If
n is omitted, the rest of s is
used.
tolower(str) returns a copy of the string str,
with all the upper-case characters
in str translated to their correュ
sponding lower-case counterparts.
Non-alphabetic characters are left
unchanged.
toupper(str) returns a copy of the string str,
with all the lower-case characters
in str translated to their correュ
sponding upper-case counterparts.
Non-alphabetic characters are left
unchanged.
Time Functions
Since one of the primary uses of AWK programs is processュ
ing log files that contain time stamp information, gawk
provides the following two functions for obtaining time
stamps and formatting them.
systime() returns the current time of day as the number of
seconds since the Epoch (Midnight UTC, January
1, 1970 on POSIX systems).
strftime([format [, timestamp]])
formats timestamp according to the specification
in format. The timestamp should be of the same
form as returned by systime(). If timestamp is
missing, the current time of day is used. If
format is missing, a default format equivalent
to the output of date(1) will be used. See the
specification for the strftime() function in
ANSI C for the format conversions that are guarュ
anteed to be available. A public-domain version
of strftime(3) and a man page for it come with
gawk; if that version was used to build gawk,
then all of the conversions described in that
man page are available to gawk.
String Constants
tain escape sequences are recognized, as in C. These are:
\\ A literal backslash.
\a The ``alert'' character; usually the ASCII BEL charュ
acter.
\b backspace.
\f form-feed.
\n newline.
\r carriage return.
\t horizontal tab.
\v vertical tab.
\xhex digits
The character represented by the string of hexadeciュ
mal digits following the \x. As in ANSI C, all folュ
lowing hexadecimal digits are considered part of the
escape sequence. (This feature should tell us someュ
thing about language design by committee.) E.g.,
"\x1B" is the ASCII ESC (escape) character.
\ddd The character represented by the 1-, 2-, or 3-digit
sequence of octal digits. E.g. "033円" is the ASCII
ESC (escape) character.
\c The literal character c.
The escape sequences may also be used inside constant regュ
ular expressions (e.g., /[ \t\f\n\r\v]/ matches whitespace
characters).
In compatibility mode, the characters represented by octal
and hexadecimal escape sequences are treated literally
when used in regexp constants. Thus, /a52円b/ is equivalent
to /a\*b/.
Functions in AWK are defined as follows:
function name(parameter list) { statements }
Functions are executed when they are called from within
expressions in either patterns or actions. Actual parameュ
ters supplied in the function call are used to instantiate
the formal parameters declared in the function. Arrays
Since functions were not originally part of the AWK lanュ
guage, the provision for local variables is rather clumsy:
They are declared as extra parameters in the parameter
list. The convention is to separate local variables from
real parameters by extra spaces in the parameter list. For
example:
function f(p, q, a, b) # a & b are local
{
.....
}
/abc/ { ... ; f(1, 2) ; ... }
The left parenthesis in a function call is required to
immediately follow the function name, without any interュ
vening white space. This is to avoid a syntactic ambiguュ
ity with the concatenation operator. This restriction
does not apply to the built-in functions listed above.
Functions may call each other and may be recursive. Funcュ
tion parameters used as local variables are initialized to
the null string and the number zero upon function invocaュ
tion.
Use return expr to return a value from a function. The
return value is undefined if no value is provided, or if
the function returns by ``falling off'' the end.
If --lint has been provided, gawk will warn about calls to
undefined functions at parse time, instead of at run time.
Calling an undefined function at run time is a fatal
error.
The word func may be used in place of function.
Print and sort the login names of all users:
BEGIN { FS = ":" }
{ print 1ドル | "sort" }
Count lines in a file:
{ nlines++ }
END { print nlines }
Precede each line by its number in the file:
{ print FNR, 0ドル }
egrep(1) , getpid(2) , getppid(2) , getpgrp(2) , getuid(2) , geteuid(2) , getgid(2) , getegid(2) , getgroups(2) The AWK Programming Language, Alfred V. Aho, Brian W. Kernighan, Peter J. Weinberger, Addison-Wesley, 1988. ISBN 0-201-07981-X. AWK Language Programming, Edition 1.0, published by the Free Software Foundation, 1995.
A primary goal for gawk is compatibility with the POSIX standard, as well as with the latest version of UNIX awk. To this end, gawk incorporates the following user visible features which are not described in the AWK book, but are part of the Bell Labs version of awk, and are in the POSIX standard. The -v option for assigning variables before program exeュ cution starts is new. The book indicates that command line variable assignment happens when awk would otherwise open the argument as a file, which is after the BEGIN block is executed. However, in earlier implementations, when such an assignment appeared before any file names, the assignment would happen before the BEGIN block was run. Applications came to depend on this ``feature.'' When awk was changed to match its documentation, this option was added to accommodate applications that depended upon the old behavior. (This feature was agreed upon by both the AT&T and GNU developers.) The -W option for implementation specific features is from the POSIX standard. When processing arguments, gawk uses the special option ``--'' to signal the end of arguments. In compatibility mode, it will warn about, but otherwise ignore, undefined options. In normal operation, such arguments are passed on to the AWK program for it to process. The AWK book does not define the return value of srand(). The POSIX standard has it return the seed it was using, to allow keeping track of random number sequences. Therefore srand() in gawk also returns its current seed. Other new features are: The use of multiple -f options (from MKS awk); the ENVIRON array; the \a, and \v escape sequences (done originally in gawk and fed back into AT&T's); the tolower() and toupper() built-in functions (from AT&T); and the ANSI C conversion specifications in Gawk has a number of extensions to POSIX awk. They are described in this section. All the extensions described here can be disabled by invoking gawk with the --tradiュ tional option. The following features of gawk are not available in POSIX awk. キ The \x escape sequence. (Disabled with --posix.) キ The fflush() function. (Disabled with --posix.) キ The systime(), strftime(), and gensub() funcュ tions. キ The special file names available for I/O redirecュ tion are not recognized. キ The ARGIND, ERRNO, and RT variables are not speュ cial. キ The IGNORECASE variable and its side-effects are not available. キ The FIELDWIDTHS variable and fixed-width field splitting. キ The use of RS as a regular expression. キ The ability to split out individual characters using the null string as the value of FS, and as the third argument to split(). キ No path search is performed for files named via the -f option. Therefore the AWKPATH environment variable is not special. キ The use of nextfile to abandon processing of the current input file. キ The use of delete array to delete the entire conュ tents of an array. The AWK book does not define the return value of the close() function. Gawk's close() returns the value from fclose(3) , or pclose(3) , when closing a file or pipe, respectively. When gawk is invoked with the --traditional option, if the fs argument to the -F option is ``t'', then FS will be set to the tab character. Note that typing gawk -F\t ... simply causes the shell to quote the ``t,'', and does not ior also does not occur if --posix has been specified. To really get a tab character as the field separator, it is best to use quotes: gawk -F'\t' ....
There are two features of historical AWK implementations that gawk supports. First, it is possible to call the length() built-in function not only with no argument, but even without parentheses! Thus, a = length # Holy Algol 60, Batman! is the same as either of a = length() a = length(0ドル) This feature is marked as ``deprecated'' in the POSIX standard, and gawk will issue a warning about its use if --lint is specified on the command line. The other feature is the use of either the continue or the break statements outside the body of a while, for, or do loop. Traditional AWK implementations have treated such usage as equivalent to the next statement. Gawk will supュ port this usage if --traditional has been specified.
If POSIXLY_CORRECT exists in the environment, then gawk behaves exactly as if --posix had been specified on the command line. If --lint has been specified, gawk will issue a warning message to this effect. The AWKPATH environment variable can be used to provide a list of directories that gawk will search when looking for files named via the -f and --file options.
The -F option is not necessary given the command line variable assignment feature; it remains only for backwards compatibility. If your system actually has support for /dev/fd and the associated /dev/stdin, /dev/stdout, and /dev/stderr files, you may get different output from gawk than you would get on a system without those files. When gawk interprets these files internally, it synchronizes output to the standard output with output to /dev/stdout, while on a system with those files, the output is actually to differュ ent open files. Caveat Emptor. Syntactically invalid single character programs tend to diagnose in the completely general case, and the effort to do so really is not worth it.
This man page documents gawk, version 3.0.4.
The original version of UNIX awk was designed and impleュ mented by Alfred Aho, Peter Weinberger, and Brian Kernighan of AT&T Bell Labs. Brian Kernighan continues to maintain and enhance it. Paul Rubin and Jay Fenlason, of the Free Software Foundaュ tion, wrote gawk, to be compatible with the original verュ sion of awk distributed in Seventh Edition UNIX. John Woods contributed a number of bug fixes. David Trueman, with contributions from Arnold Robbins, made gawk compatiュ ble with the new version of UNIX awk. Arnold Robbins is the current maintainer. The initial DOS port was done by Conrad Kwok and Scott Garfinkle. Scott Deifik is the current DOS maintainer. Pat Rankin did the port to VMS, and Michal Jaegermann did the port to the Atari ST. The port to OS/2 was done by Kai Uwe Rommel, with contributions and help from Darrel Hankerson. Fred Fish supplied support for the Amiga.
If you find a bug in gawk, please send electronic mail to bug-gnu-utils@gnu.org, with a carbon copy to arnold@gnu.org. Please include your operating system and its revision, the version of gawk, what C compiler you used to compile it, and a test program and data that are as small as possible for reproducing the problem. Before sending a bug report, please do two things. First, verify that you have the latest version of gawk. Many bugs (usually subtle ones) are fixed at each release, and if yours is out of date, the problem may already have been solved. Second, please read this man page and the referュ ence manual carefully to be sure that what you think is a bug really is, instead of just a quirk in the language. Whatever you do, do NOT post a bug report in comp.lang.awk. While the gawk developers occasionally read this newsgroup, posting bug reports there is an unreュ liable way to report bugs. Instead, please use the elecュ tronic mail addresses given above.
Brian Kernighan of Bell Labs provided valuable assistance during testing and debugging. We thank him. Copyright ゥ) 1996,97,98,99 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual page provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified verュ sions of this manual page under the conditions for verbaュ tim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual page into another language, under the above conditions for modified versions, except that this permisュ sion notice may be stated in a translation approved by the Foundation.
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