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RTR's Win95Pak: The GAWK Manual - gawk Summary
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This appendix provides a brief summary of the gawk command line and the
awk language. It is designed to serve as ``quick reference.'' It is
therefore terse, but complete.
The command line consists of options to gawk itself, the
awk program text (if not supplied via the -f option), and
values to be made available in the ARGC and ARGV
predefined awk variables:
awk [POSIX or GNU style options] -f source-file [--] file ...
awk [POSIX or GNU style options] [--] 'program' file ...
The options that gawk accepts are:
-F fs
--field-separator=fs
Use fs for the input field separator (the value of the FS
predefined variable).
-f program-file
--file=program-file
Read the awk program source from the file program-file, instead
of from the first command line argument.
-v var=val
--assign=var=val
Assign the variable var the value val before program execution
begins.
-W compat
--compat
Specifies compatibility mode, in which gawk extensions are turned
off.
-W copyleft
-W copyright
--copyleft
--copyright
Print the short version of the General Public License on the error
output. This option may disappear in a future version of gawk.
-W help
-W usage
--help
--usage
Print a relatively short summary of the available options on the error output.
-W lint
--lint
Give warnings about dubious or non-portable awk constructs.
-W posix
--posix
Specifies POSIX compatibility mode, in which gawk extensions
are turned off and additional restrictions apply.
-W source=program-text
--source=program-text
Use program-text as awk program source code. This option allows
mixing command line source code with source code from files, and is
particularly useful for mixing command line programs with library functions.
-W version
--version
Print version information for this particular copy of gawk on the error
output. This option may disappear in a future version of gawk.
--
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 conventions of POSIX.
Any other options are flagged as invalid, but are otherwise ignored.
An awk program consists of a sequence of pattern-action statements
and optional function definitions.
pattern { action statements }
function name(parameter list) { action statements }
gawk first reads the program source from the
program-file(s) if specified, or from the first non-option
argument on the command line. The -f option may be used multiple
times on the command line. gawk reads the program text from all
the program-file files, effectively concatenating them in the
order they are specified. This is useful for building libraries of
awk functions, without having to include them in each new
awk program that uses them. To use a library function in a file
from a program typed in on the command line, specify -f /dev/tty;
then type your program, and end it with a Control-d.
See section Invoking awk specifies a search path to use
when finding source files named with the -f option. The default
path, which is
.:/usr/lib/awk:/usr/local/lib/awk is used if AWKPATH is not set.
If a file name given to the -f option contains a / character,
no path search is performed.
See section The AWKPATH Environment Variable environment variable.
gawk compiles the program into an internal form, 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 ``file'' named on the command line has the form
var=val, it is treated as a variable assignment: the
variable var is assigned the value val.
If any of the files have a value that is the null string, that
element in the list is skipped.
For each line in the input, gawk tests to see if it matches any
pattern in the awk program. For each pattern that the line
matches, the associated action is executed.
awk variables are dynamic; they come into existence when they are
first used. Their values are either floating-point numbers or strings.
awk also has one-dimension arrays; multiple-dimensional arrays
may be simulated. There are several predefined variables that
awk sets as a program runs; these are summarized below.
As each input line is read, gawk splits the line 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. Otherwise, FS is expected to be a full regular
expression. In the special case that FS is a single blank,
fields are separated by runs of blanks and/or tabs. Note that the value
of IGNORECASE (see section Case-sensitivity in Matching)
also affects how fields are split when FS is a regular expression.
Each field in the input line may be referenced by its position, $1,
$2, and so on. $0 is the whole line. The value of a field may
be assigned to as well. Field numbers need not be constants:
n = 5
print $n
prints the fifth field in the input line. The variable NF is set to
the total number of fields in the input line.
References to nonexistent fields (i.e., fields after $NF) return
the null-string. However, assigning to a nonexistent field (e.g.,
$(NF+2) = 5) increases the value of NF, creates any
intervening fields with the null string as their value, and causes the
value of $0 to be recomputed, with the fields being separated by
the value of OFS.
See section Reading Input Files, for a full description of the
way awk defines and uses fields.
awk's built-in variables are:
ARGC
The number of command line arguments (not including options or the
awk program itself).
ARGIND
The index in ARGV of the current file being processed.
It is always true that FILENAME == ARGV[ARGIND].
ARGV
The 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 to use when converting numbers to strings.
FIELDWIDTHS
A space separated list of numbers describing the fixed-width input data.
ENVIRON
An array containing the values of the environment variables. The array
is indexed by variable name, each element being the value of that
variable. Thus, the environment variable HOME would be in
ENVIRON["HOME"]. Its value might be /u/close.
Changing this array does not affect the environment seen by programs
which gawk spawns via redirection or the system function.
(This may change in a future version of gawk.)
Some operating systems do not have environment variables.
The array ENVIRON is empty when running on these systems.
ERRNO
The system error message when an error occurs using getline
or close.
FILENAME
The name of the current input file. If no files are specified on the command
line, the value of FILENAME is -.
FNR
The input record number in the current input file.
FS
The input field separator, a blank by default.
IGNORECASE
The case-sensitivity flag for regular expression operations. If
IGNORECASE has a nonzero value, then pattern matching in rules,
field splitting with FS, regular expression matching with
~ and !~, and the gsub, index, match,
split and sub predefined functions 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 for the print statement,
"%.6g" by default.
OFS
The output field separator, a blank by default.
ORS
The output record separator, by default a newline.
RS
The input record separator, by default a newline. RS is exceptional
in that 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, then the newline
character always acts as a field separator, in addition to whatever value
FS may have.
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 string used to separate multiple subscripts in array elements, by
default "\034".
See section Built-in Variables, for more information.
Arrays are subscripted with an expression between square brackets
([ and ]). Array subscripts are always strings;
numbers are converted to strings as necessary, following the standard
conversion rules
(see section Conversion of Strings and Numbers).
If you use multiple expressions separated by commas inside the square
brackets, then the array subscript is a string consisting of the
concatenation of the individual subscript values, converted to strings,
separated by the subscript separator (the value of SUBSEP).
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
to test for existence of an element.
The in construct may also be used in a for loop to iterate
over all the elements of an array.
See section Scanning all Elements of an Array.
An element may be deleted from an array using the delete statement.
The value of an awk expression is always either a number
or a string.
Certain contexts (such as arithmetic operators) require numeric
values. They convert strings to numbers by interpreting the text
of the string as a numeral. If the string does not look like a
numeral, it converts to 0.
Certain contexts (such as concatenation) require string values.
They convert numbers to strings by effectively printing them
with sprintf.
See section Conversion of Strings and Numbers, for the details.
To force conversion of a string value to a number, simply add 0
to it. If the value you start with is already a number, this
does not change it.
To force conversion of a numeric value to a string, concatenate it with
the null string.
The awk language defines comparisons as being done numerically if
both operands are numeric, or if one is numeric and the other is a numeric
string. Otherwise one or both operands are converted to strings and a
string comparison is performed.
Uninitialized variables have the string value "" (the null, or
empty, string). In contexts where a number is required, this is
equivalent to 0.
See section Variables, for more information on variable naming and initialization;
see section Conversion of Strings and Numbers, for more information
on how variable values are interpreted.
An awk program is mostly composed of rules, each consisting of a
pattern followed by an action. The action is 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 is executed for every single line of input. A missing action is
equivalent to this action,
{ print }
which prints the entire line.
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 is 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 rule (the
usual case), and to the rule statements.
See section Comments in awk Programs's commenting convention;
see section awk Statements versus Lines.
awk patterns may be one of the following:
/regular expression/
relational expression
pattern && pattern
pattern || pattern
pattern ? pattern : pattern
(pattern)
! pattern
pattern1, pattern2
BEGIN
END
BEGIN and END are two special kinds of patterns that are not
tested against the input. The action parts of all BEGIN rules are
merged as if all the statements had been written in a single BEGIN
rule. They are executed before any of the input is read. Similarly, all the
END rules 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 rules cannot have missing action parts.
For /regular-expression/ patterns, the associated statement is
executed for each input line that matches the regular expression. Regular
expressions are extensions of those in egrep, 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 matches, then the second pattern is matched against the input
record; otherwise, the third is matched. Only one of the second and
third patterns is matched.
The pattern1, pattern2 form of a pattern is called a
range pattern. It matches all input lines starting with a line that
matches pattern1, and continuing until a line that matches
pattern2, inclusive. A range pattern cannot be used as an operand
to any of the pattern operators.
See section Patterns, for a full description of the pattern part of awk
rules.
Regular expressions are the extended kind found in egrep.
They are composed of characters as follows:
c
matches the character c (assuming c is a character with no
special meaning in regexps).
\c
matches the literal character c.
.
matches any character except newline.
^
matches the beginning of a line or a string.
$
matches the end of a line or a string.
[abc...]
matches any of the characters abc... (character class).
[^abc...]
matches any character except abc... and newline (negated
character class).
r1|r2
matches either r1 or r2 (alternation).
r1r2
matches r1, and then r2 (concatenation).
r+
matches one or more r's.
r*
matches zero or more r's.
r?
matches zero or one r's.
(r)
matches r (grouping).
See section Regular Expressions as Patterns, for a more detailed
explanation of regular expressions.
The escape sequences allowed in string constants are also valid in
regular expressions (see section Constant Expressions).
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.
The operators in awk, in order of increasing precedence, are:
= += -= *= /= %= ^=
Assignment. Both absolute assignment (var=value)
and operator assignment (the other forms) are supported.
?:
A conditional expression, as in C. 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.
||
Logical ``or''.
&&
Logical ``and''.
~ !~
Regular expression match, negated match.
< <= > >= != ==
The usual relational operators.
blank
String concatenation.
+ -
Addition and subtraction.
* / %
Multiplication, division, and modulus.
+ - !
Unary plus, unary minus, and logical negation.
^
Exponentiation (** may also be used, and **= for the assignment
operator, but they are not specified in the POSIX standard).
++ --
Increment and decrement, both prefix and postfix.
$
Field reference.
See section Expressions as Action Statements, for a full
description of all the operators listed above.
See section Examining Fields, for a description of the field
reference operator.
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]
exit [ expression ]
{ statements }
See section Control Statements in Actions, for a full description
of all the control statements listed above.
The input/output statements are as follows:
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 NF, 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 rule(s), if any,
are executed.
next file
Stop processing the current input file. The next input record read comes
from the next input file. FILENAME is updated, FNR is set 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 rule(s), if any,
are executed.
print
Prints the current record.
print expr-list
Prints expressions.
print expr-list > file
Prints expressions on file.
printf fmt, expr-list
Format and print.
printf fmt, expr-list > file
Format and print on file.
Other input/output redirections are also allowed. For print and
printf, >> file appends output to the file,
and | command writes on a pipe. In a similar fashion,
command | getline pipes input into getline.
getline returns 0 on end of file, and -1 on an error.
See section Explicit Input with getline statement.
See section Printing Output, for a full description of print and
printf. Finally, see section The next Statement statement works.
The awk printf statement and sprintf function
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
A floating point number of the form
[-]d.ddddddE[+-]dd.
%f
A floating point number of the form
[-]ddd.dddddd.
%g
Use %e or %f conversion, whichever produces a shorter string,
with nonsignificant zeros suppressed.
%o
An unsigned octal number (again, an integer).
%s
A character string.
%x
An unsigned hexadecimal number (an integer).
%X
Like %x, except use A through F instead of a
through f for decimal 10 through 15.
%%
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.
width
The field should be padded to this width. If width has a leading zero,
then the field is padded with zeros. Otherwise it is padded with blanks.
.prec
A number indicating the maximum width of strings or digits to the right
of the decimal point.
Either or both of the width and prec values may be specified
as *. In that case, the particular value is taken from the argument
list.
Note Using printf Statements for Fancier Printing, for examples and for a more detailed description.
When doing I/O redirection from either print or printf into a
file, or via getline from a file, gawk recognizes certain special
file names internally. These file names allow access to open file descriptors
inherited from gawk's parent process (usually the shell). The
file names are:
/dev/stdin
The standard input.
/dev/stdout
The standard output.
/dev/stderr
The standard error output.
/dev/fd/n
The file denoted by the open file descriptor n.
In addition the following files provide process related information
about the running gawk program.
/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 blanks. The fields represent the
following information:
$1
The value of the getuid system call.
$2
The value of the geteuid system call.
$3
The value of the getgid system call.
$4
The value of the getegid system call.
If there are any additional fields, they are the group IDs returned by
getgroups system call.
(Multiple groups may not be supported on all systems.)
These file names may also be used on the command line to name data files.
These file names are only recognized internally if you do not
actually have files by these names on your system.
See section Standard I/O Streams, for a longer description that
provides the motivation for this feature.
awk has the following predefined arithmetic functions:
atan2(y, x)
returns the arctangent of y/x in radians.
cos(expr)
returns the cosine 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 in radians.
sqrt(expr)
the square root function.
srand(expr)
use expr as a new seed for the random number generator. If no expr
is provided, the time of day is used. The return value is the previous
seed for the random number generator.
awk has the following predefined string functions:
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.
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. The length of $0
is returned if no argument is 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.
sprintf(fmt, expr-list)
prints expr-list according to fmt, and returns the resulting string.
sub(r, s, t)
this is just like gsub, but only the first matching substring is
replaced.
substr(s, i, n)
returns the 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.
Nonalphabetic 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.
Nonalphabetic characters are left unchanged.
system(cmd-line)
Execute the command cmd-line, and return the exit status.
The following two functions are available for getting the current
time of day, and for formatting time stamps.
systime()
returns the current time of day as the number of seconds since a particular
epoch (Midnight, January 1, 1970 UTC, on POSIX systems).
strftime(format, timestamp)
formats timestamp according to the specification in format.
The current time of day is used if no timestamp is supplied.
See section Functions for Dealing with Time Stamps, for the
details on the conversion specifiers that strftime accepts.
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
Formfeed.
\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 a
string containing the ASCII ESC (escape) character. (The \x
escape sequence is not in POSIX awk.)
\ddd
The character represented by the 1-, 2-, or 3-digit sequence of octal
digits. Thus, "\033" is also a string containing the ASCII ESC
(escape) character.
\c
The literal character c.
The escape sequences may also be used inside constant regular expressions
(e.g., the regexp /[@ \t\f\n\r\v]/ matches whitespace
characters).
See section Constant Expressions.
Functions in awk are defined as follows:
function name(parameter list) { statements }
Actual parameters supplied in the function call are used to instantiate
the formal parameters declared in the function. Arrays are passed by
reference, other variables are passed by value.
If there are fewer arguments passed than there are names in parameter-list,
the extra names are given the null string as value. Extra names have the
effect of local variables.
The open-parenthesis in a function call of a user-defined function must
immediately follow the function name, without any intervening white space.
This is to avoid a syntactic ambiguity with the concatenation operator.
The word func may be used in place of function (but not in
POSIX awk).
Use the return statement to return a value from a function.
See section User-defined Functions, for a more complete description.
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 arguments, but even without parentheses!
a = length
is the same as either of
a = length()
a = length($0)
This feature is marked as ``deprecated'' in the POSIX standard, and
gawk will issue a warning about its use if -W lint is
specified on the command line.
The other feature is the use of the continue statement 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 support this usage if -W posix
has not been specified.
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