GNU awk
DOC DATE
June 2000
NAME
awk, Gawk - Aho, Weinberger and Kernighan's pattern processing language
WHAT IT IS
Gawk
is the GNU Project's implementation of the AWK programming language, which
is the command named awk in cLIeNUX. awk is a scripting
language or batch interpreter. It is not a line-wise interactive
interpreter or shell like sh, Forth or BASIC. awk takes it's
current program as a whole and converts it as a whole into an internal
form before doing anything. awk provides strings, floating point
numbers, and subscripted-by-strings arrays that I will hereinafter call
lists. At the highest level program flow in awk is controlled by
pattern-action constructs similar to ed, which ease causing an
awk program to traverse a file, but which are somewhat confusing.
Within an action construct, however, awk is a pleasingly generic
procedural language.
Generic stuff in awk includes unix-style string literals and
concatenation like "\n\n\n\t\tHuh?\n\n\n" "Oh.\n", a nice little
set of floating point math functions, A BASIC-like set of string parsing
functions, the % interger remainder operator, and subroutines
similar to the mis-named sh "functions". Variables and lists have no
required prefix, and may be handled as strings or numbers, depending on
context. That is, all regular variables are stored as strings. There are
no required "variable declarations".
Not quite as generic, there is implicit support for a "current line".
Data read into awk is implicitly split into records and fields
based on the FS and RS delimiter variables and each
field is assigned a number, and $ is the field reference
operator. The defaults are for lines of text as records. By default then,
$1 represents the first "word" of the current input line.
awk also provides the ~ operator for comparing a string
to a regular expression and providing a result flag. File input-output can
be controlled within an awk program much like in the unix shell.
The in operator checks a list for an item name, or can loop over
list items via the for (VAR in list) construct.
AWK, it appears to me, is the main predecessor of Perl, and is present in
cLIeNUX Core while Perl is not. Awk is much smaller than Perl, and to my
tastes, has a cleaner syntax. The massive facilities of Perl exist
elsewhere in cLIeNUX, and can be accessed from an awk program,
via such things as netcat, as one example (for sockets). Perl is,
I believe, a bit faster than awk, but both are terribly slow compared to
compiled C, or even a relatively fast Forth like GForth. This slowness
comes at a benefit, however. An awk-adept person can get handy,
unusual little programs working very rapidly with startlingly little code.
As soon as Bash or sh starts to look confining, look into awk.
INVOCATION
synopsis
awk [ options ] -f program-file [ -- ] [file...]
awk [ options ] [ -- ] program-text [file...]
examples
The command line consists of options to awk itself, which are
listed toward the end of this seedoc, 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
variable and list.
If the awk program is to be taken from standard input, it is
usually the case that it must be excluded from interpretation by the
shell. Thus the prevalent syntax for an awk quickie is... (with
awk's answer hi-lited)
:;awk '
:;:;BEGIN { print "Dare I say it? HELLO WORLD!" }'
Dare I say it? HELLO WORLD!
:;
The right-quotes protect the awk program from the shell. The above
example works at the shell prompt, and that format will be used for
subsequent examples. This technique will also allow an awk
program to be included within a pipe sequence. The above example uses a
BEGIN "pattern" and affiliated {...}-delimited action,
the print keyword, and a string literal. A BEGIN pattern
doesn't scan an input file, so nothing in this example looks for an input
file, so this program quits without waiting for more input. Note that the
shell waited for the second ', and that therefor awk
didn't do anything until the program was input completely, did it's thing,
and then exited.
An awk program that doesn't look for an input data file can thus
easily take arbitrary commandline arguments, since the program will not
attempt to interpret them as files.
:;awk ' BEGIN { print ARGV[3] } ' /dev/null /GPL woof /dev/null
woof
:;
Here we used the built-in ARGV list. When awk itself
generates a list the names of the list items (the subscripts) will be
integers. Note that the arguments in this case can be shell-expanded. Can
you guess what the above would return if we print ARGV[0]?
Flow of Execution (abbreviated)
First, all variable assignments specified via the
-v VAR=val
option construct are performed.
Next, awk compiles the program into an internal form.
Then, awk executes the code in the BEGIN block(s) (if
any).
Then we proceed to read each file named in the ARGV list
IF there are pattern-actions other than BEGIN or END.
If there are no files named on the command line, and if there are
pattern-actions other than BEGIN or END, awk
reads the standard input.
Line-wise Patterns
For each record in the data input file(s),
awk
tests to see if it matches any pattern
in the AWK program (besides BEGIN and END).
For each pattern that the record matches, the associated action
is executed. The patterns are tested in the order they occur in the program.
Finally, after all the input is exhausted,
awk executes the code in the END
block(s) (if any).
VARIABLES, RECORDS AND FIELDS
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 lists;
lists with multiple dimensions may be simulated with "N-dimensional
subscripts". 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 character, 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,
awk
splits the record into
fields,
using the value of the
FS
variable as the field separator.
If
FS
is a single character, fields are separated by that character.
If
FS
is the null string, then each individual character becomes a
separate field.
Otherwise,
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
awk
will split up the record using the specified widths. The value of
FS
is ignored.
Assigning a new value to
FS
overrides the use of
FIELDWIDTHS,
and restores the default behavior.
Each field in the input record may be referenced by its position,
$1,
$2,
and so on.
$0
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-existent 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
to be recomputed, with the fields being separated by the value of
OFS.
References 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
to be recomputed, with the fields being separated by the value of
OFS.
Built-in Variables
awk's
built-in variables are:
- ARGC
-
The number of command line arguments (does not include options to
awk,
or the program source).
- ARGIND
-
The index in
ARGV
of the current file being processed.
- ARGV
-
list of command line arguments. The list items are named from
0 to
the value of 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
-
A list containing the values of the current environment. The list is
indexed (subscripted) by the environment variables, each element being the
value of that variable (e.g., ENVIRON["HOME"] might be
/home/arnold).
Changing this list does not affect the environment seen by programs which
awk
spawns via redirection or the
system()
function.
(This may change in a future version of
awk.)
- ERRNO
-
If a system error occurs either doing a redirection for
getline,
during a read for
getline,
or during a
close(),
then
ERRNO
will contain
a string describing the error.
- FIELDWIDTHS
-
A white-space separated list of fieldwidths. When set,
awk
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
awk
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
IGNORECASE
has a non-zero value, then string comparisons 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.
- 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.
- OFS
-
The output field separator, a space by default.
- ORS
-
The output record separator, by default a newline.
- RS
-
The input record separator, by default a newline.
- RT
-
The record terminator.
awk
sets
RT
to the input text that matched the character or regular 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 subscripts in list
elements, by default "\034".
LISTS
Lists are subscripted with an expression between square brackets
([ and ]).
If the expression is an expression list
(expr, expr ...)
then the list 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 multiply dimensioned
lists. 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
list
x
which is indexed by the string "A\034B\034C".
The special operator
in
may be used in an
if
or
while
statement to see if a list has an item name consisting of a particular
value.
-
if (val in list)
print list[val]
If the list has multiple subscripts, use
(i, j) in list.
The
in
construct may also be used in a
for
loop to iterate over all the items in a list.
An element may be deleted from a list using the
delete
statement.
The
delete
statement may also be used to delete the entire contents of a list,
just by specifying the list name without a subscript.
Variable Typing And Conversion
Variables and fields
may be (floating point) numbers, or strings, or both. How the
value of a variable is interpreted 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 conversion 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 variable as the argument.
However, even though all numbers in AWK are floating-point,
integral values are
always
converted as integers. Thus, given
-
CONVFMT = "%2.2f"
a = 12
b = a ""
the variable
b
has a string value of "12" and not "12.00".
awk
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
numerically. 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 awk does not do this. (This
is from the GNU manpage. I can't make any sense of it. It looks like
a thinko that should read "...a string comparison is performed. Rick. )
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 a list created by
split()
that are strings representing integers.
Uninitialized variables have the numeric value 0 and the string value
"" (the null, or empty, string).
PATTERNS AND ACTIONS
In every statement, 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 record of input. A missing action is equivalent to -
{ print }
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.
A newline in awk is analagous to ; in other languages; it terminates a
statement. 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 statement 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 expressions.
The
&&,
||,
and
!
operators are logical AND, logical OR, and logical NOT, respectively, as in C.
They do short-circuit evaluation, also as in C, and are used for combining
more primitive pattern expressions. As in most languages, 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 combine 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
interval expression.
If there is one number in the braces, the preceding regexp
r
is repeated
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 beginning 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-constituent.
- \`
-
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.
- [:blank:]
-
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 control characters.)
- [:punct:]
-
Punctuation characters (characters that are not letter, digits,
control characters, or space characters).
- [: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 alphanumeric
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 several 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
-
A collating symbols is a multi-character collating element enclosed in
[.
and
.].
For example, if
ch
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' ,
or
.BR e` .
These features are very valuable in non-English speaking locales.
The library functions that
awk
uses for regular expression matching
currently only recognize POSIX character classes; they do not recognize
collating symbols or equivalence classes.
The
\y,
\B,
\<,
\>,
\w,
\W,
\`,
and
\'
operators are specific to
awk;
they are extensions based on facilities in the GNU regexp libraries.
The various command line options
control how
awk
interprets characters in regexps.
- No options
-
In the default case,
awk
provide all the facilities 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 regexp metacharacters.
- --re-interval
-
Allow interval expressions in regexps, even if
--traditional
has been provided.
Actions
Action statements are enclosed in braces,
{
and
}.
Action statements consist of the usual assignment, conditional, and looping
statements found in most languages. The operators, 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 postfix.
- ^
-
Exponentiation (** may also be used, and **= for
the assignment operator).
- + - !
-
Unary plus, unary minus, and logical negation.
- * / %
-
Multiplication, division, and modulus (integer remainder).
- + -
-
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 ~ /foo/) ~ exp).
This is usually
not
what was intended.
- in
-
list membership, by item name.
- &&
-
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,
otherwise 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 list) statement
break
continue
delete list[item name]
delete list
exit [ expression ]
{ statements }
I/O Statements
The input/output statements are as follows:
- close(file)
-
Close file (or pipe, see below).
- getline
-
Set
$0
from next input record; set
NF,
NR,
FNR.
- getline <file
-
Set
$0
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 program. 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), if any, are executed.
NOTE:
Earlier versions of awk used
next file,
as two words. While this usage is still recognized, it generates a
warning message and will eventually be removed.
- print
-
Prints the current record.
The output 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 terminated 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 systems.)
- 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 specification formats:
- %c
-
An ASCII character.
If the argument used for
%c
is numeric, it is treated as a character and printed.
Otherwise, the argument is assumed to be a string, and the only first
character of that string is 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 letters.
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 formats.
This flag only has an effect when the field width is wider than the
value to be printed.
- 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 significant 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,
awk
recognizes certain special filenames internally. These filenames
allow access to open file descriptors inherited from
awk's
parent process (usually the shell).
Other special filenames provide access to information about the running
awk
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, terminated 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 terminated with a newline.
The fields are separated with spaces.
$1
is the value of the
getuid(2)
system call,
$2
is the value of the
geteuid(2)
system call,
$3
is the value of the
getgid(2)
system call, and
$4
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/stdin
-
The standard input.
- /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 radians.
- sqrt(expr)
-
the square root function.
- srand([expr])
-
uses
expr
as a new seed for the random number 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
awk
has the following pre-defined string functions:
- gensub(r, s, h [, t])
-
search the target string
t
for matches of the regular expression
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 supplied,
$0
is used instead.
Within the replacement text
s,
the sequence
\n,
where
n
is a digit from 1 to 9, may be used to indicate just the text that
matched the
n'th
parenthesized subexpression. The sequence
\0
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 substitutions.
If
t
is not supplied, use
$0.
An
&
in the replacement text is replaced with the text that was actually matched.
Use
\&
to get a literal
&.
See
AWK Language Programming
for a fuller discussion of the rules for
&'s
and backslashes 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
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 list
a
on the regular expression
r,
and returns the number of fields. If
r
is omitted,
FS
is used instead.
The list
a
is cleared first.
Splitting behaves identically to field splitting, described above.
- sprintf(fmt, expr-list)
-
prints
expr-list
according to
fmt,
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 corresponding 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 corresponding upper-case counterparts.
Non-alphabetic characters are left unchanged.
Time Functions
Since one of the primary uses of AWK programs is processing log files
that contain time stamp information,
awk
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
guaranteed to be available.
A public-domain version of
strftime(3)
and a man page for it come with
awk;
if that version was used to build
awk,
then all of the conversions described in that man page are available to
awk.
String Constants
String constants in AWK are sequences of characters enclosed
between double quotes ("). Within strings, certain
escape sequences
are recognized, as in C. These are:
- \\
-
A literal backslash.
- \a
-
The ``alert'' character; usually the ASCII BEL character.
- \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 hexadecimal digits following
the
\x.
As in ANSI C, all following hexadecimal digits are considered part of
the escape sequence.
(This feature should tell us something 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" is the ASCII ESC (escape) character.
- \c
-
The literal character
c.
The escape sequences may also be used inside constant regular 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,
/a\52b/
is equivalent to
/a\*b/.
SUBROUTINES
Subroutines in AWK are defined as follows:
-
function name(parameter list) { statements }
"Function" is a misnomer for awk subroutines, and many other uses of the
term in unix. It is a defined named
segment of program that may be invoked from anywhere in the program by
name. Once named/declared/defined, an awk subroutine may be used in
patterns or actions. Defining a subroutine is semantically and
syntactically independant of program flow, and subroutine declarations may
be above the pattern/action code.
The values the subroutine operates on are named for use inside the code
of the subroutine when it is defined. The items that a particular
invocation of a subroutine operates on are given to it when it is invoked,
in the argument list. The declaration defines the names used to refer to a
subroutine's arguments within the code of a particular instance of using
the subroutine. What values the subroutine gets for particular named
arguments is subject to what are called scoping issues.
Lists named in the argument list are accessed by the function "by name".
That is, the function can change the contents of the list itself. The
subroutine can thus change the list in ways that are external to the
subroutine, i.e. other parts of the program will be effected. This is
"call by reference", or "global scope". This is also called
"side-effects", and is why C and awk "functions" aren't functions in the
formal sense.
Regular variables passed to a use of a subroutine in it's argument list
are converted to a local copy of thier value. The subroutine doesn't change
the value of that variable as seen by any other part of the program. This
is also called "call by value" or "local scope".
Since functions were not originally part of the AWK language, 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 intervening white space.
This is to avoid a syntactic ambiguity with the concatenation operator.
This restriction does not apply to the built-in functions listed above.
Functions may call each other and may call themselves, which is
"recursion". Function parameters used as local variables are initialized
to the null string and the number zero upon function invocation.
If
--lint
has been provided,
awk
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.
EXAMPLES
Print and sort the login names of all users:
BEGIN { FS = ":" }
{ print $1 | "sort" }
Count lines in a file:
{ nlines++ }
END { print nlines }
Precede each line by its number in the file:
{ print FNR, $0 }
Concatenate and line number (a variation on a theme):
{ print NR, $0 }
SEE ALSO
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.
POSIX COMPATIBILITY
A primary goal for
awk
is compatibility with the POSIX standard, as well as with the
latest version of UNIX
awk.
To this end,
awk
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 execution 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,
awk
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
awk
also returns its current seed.
Other new features are:
The use of multiple
-f
options (from MKS
awk);
the
ENVIRON
list; the
\a,
and
\v
escape sequences (done originally in
awk
and fed back into AT&T's); the
tolower()
and
toupper()
built-in functions (from AT&T); and the ANSI C conversion specifications in
printf
(done first in AT&T's version).
GNU EXTENSIONS
awk
has a number of extensions to POSIX
awk.
They are described in this section. All the extensions described here
can be disabled by
invoking
awk
with the
--traditional
option.
The following features of
awk
are not available in
POSIX
awk.
-
- *
-
The
\x
escape sequence.
(Disabled with
--posix.)
- *
-
The
fflush()
function.
(Disabled with
--posix.)
- *
-
The
systime(),
strftime(),
and
gensub()
functions.
- *
-
The special file names available for I/O redirection are not recognized.
- *
-
The
ARGIND,
ERRNO,
and
RT
variables are not special.
- *
-
The
IGNORECASE
variable and its side-effects are not available.
- *
-
The
FIELDWIDTHS
variable and fixed-width field splitting.
- *
-
The use of
RS
as a regular expression.
- *
-
The ability to split out individual characters using the null string
as the value of
FS,
and as the third argument to
split().
- *
-
No path search is performed for files named via the
-f
option. Therefore the
AWKPATH
environment variable is not special.
- *
-
The use of
nextfile
to abandon processing of the current input file.
- *
-
The use of
delete list
to delete the entire contents of a list.
gawk's
close()
returns the value from
fclose(3),
or
pclose(3),
when closing a file or pipe, respectively.
When
awk
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
awk -F\t ...
simply causes the shell to quote the ``t,'', and does not pass
``\t'' to the
-F
option.
Since this is a rather ugly special case, it is not the default behavior.
This behavior 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:
awk -F'\t' ....
HISTORICAL FEATURES
There are two features of historical AWK implementations that
awk
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)
This feature is marked as ``deprecated'' in the POSIX standard, and
awk
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.
awk
will support this usage if
--traditional
has been specified.
OPTION FORMAT
awk
options may be either the traditional POSIX one
letter 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,
awk-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.
OPTIONS
awk
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 values are available to the
BEGIN
block of an AWK program.
-
-
-f program-file
-
-
--file program-file
Read the AWK program source from the file
program-file,
instead of from the first command line argument.
Multiple
-f
(or
--file)
options may be used.
-
-
-mf NNN
-
-
-mr NNN
Set various memory limits to the value
NNN.
-
-
-W traditional
-
-
-W compat
-
-
--traditional
-
-
--compat
Run in
compatibility
mode. In compatibility mode,
awk
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 information 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.
-
-
-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.
-
-
-W posix
-
-
--posix
This turns on
compatibility
mode, with the following additional restrictions:
-
- *
-
\x
escape sequences are not recognized.
- *
-
Only space and tab act as field separators when
FS
is set to a single space, newline does not.
- *
-
The synonym
func
for the keyword
function
is not recognized.
- *
-
The operators
**
and
**=
cannot be used in place of
^
and
^=.
- *
-
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 traditionally available in the
AWK language. The POSIX standard added them, to make
awk
and
egrep
consistent with each other.
However, their use is likely
to break old AWK programs, so
awk
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 functions (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
awk
on the standard output.
- --
-
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 with the argument parsing convention used
by most other POSIX programs.
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
list for processing.
ENVIRONMENT VARIABLES
If
POSIXLY_CORRECT
exists in the environment, then
awk
behaves exactly as if
--posix
had been specified on the command line.
If
--lint
has been specified,
awk
will issue a warning message to this effect.
The
AWKPATH
environment variable can be used to provide a list of directories that
awk
will search when looking for files named via the
-f
and
--file
options.
BUGS
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
awk
than you would get on a system without those files. When
awk
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 different
open files.
Caveat Emptor.
Syntactically invalid single character programs tend to overflow
the parse stack, generating a rather unhelpful message. Such programs
are surprisingly difficult to diagnose in the completely general case,
and the effort to do so really is not worth it.
VERSION INFORMATION
This man page documents
awk,
version 3.0.2.
AUTHORS
The original version of UNIX
awk
was designed and implemented 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 Foundation, wrote
GNU awk,
to be compatible with the original version of
awk
distributed in Seventh Edition UNIX.
John Woods contributed a number of bug fixes.
David Trueman, with contributions
from Arnold Robbins, made
awk
compatible with the new version of UNIX
awk.
Arnold Robbins is the current maintainer.
BUG REPORTS
If you find a bug in
GNU awk,
please send electronic mail to
bug-gnu-utils@prep.ai.mit.edu,
with
a carbon copy to
arnold@gnu.ai.mit.edu.
Please include your operating system and its revision, the version of
awk,
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
awk.
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 reference 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
awk
developers occasionally read this newsgroup, posting bug reports there
is an unreliable way to report bugs. Instead, please use the electronic mail
addresses given above.
ACKNOWLEDGEMENTS
Brian Kernighan of Bell Labs
provided valuable assistance during testing and debugging.
We (GNU) thank him. Kernighan is the K in K&R. Weinberger invented the filesystem
abstraction layer known as VFS in the Linux implementation of it. I'm sure Aho
is a kook too. What can you say about those Bell Labs boys? They also invented
MERT, which rtlinux is based on.
COPYING PERMISSIONS
Copyright ©) 1996 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 versions of this
manual page under the conditions for verbatim 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 permission notice may be stated in
a translation approved by the Foundation.