Running Awk

Running awk

There are two ways to run awk: with an explicit program, or with one or more program files. Here are templates for both of them; items enclosed in `[...]' in these templates are optional.

Besides traditional one-letter POSIX-style options, gawk also supports GNU long options.

awk [options] -f progfile [--] file ...
awk [options] [--] 'program' file ...

It is possible to invoke awk with an empty program:

$ awk " datafile1 datafile2

Doing so makes little sense though; awk will simply exit silently when given an empty program (d.c.). If `--lint' has been specified on the command line, gawk will issue a warning that the program is empty.

Command Line Options

Options begin with a dash, and consist of a single character. GNU style long options consist of two dashes and a keyword. The keyword can be abbreviated, as long the abbreviation allows the option to be uniquely identified. If the option takes an argument, then the keyword is either immediately followed by an equals sign (`=') and the argument's value, or the keyword and the argument's value are separated by whitespace. For brevity, the discussion below only refers to the traditional short options; however the long and short options are interchangeable in all contexts.

Each long option for gawk has a corresponding POSIX-style option. The options and their meanings are as follows:

-F fs

--field-separator fs

Sets the FS variable to fs (see section Specifying How Fields are Separated).

-f source-file

--file source-file

Indicates that the awk program is to be found in source-file instead of in the first non-option argument.

-v var=val

--assign var=val

Sets the variable var to the value val before execution of the program begins. Such variable values are available inside the BEGIN rule (see section Other Command Line Arguments). The `-v' option can only set one variable, but you can use it more than once, setting another variable each time, like this: `awk -v foo=1 -v bar=2 ...'.

-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 provided for compatibility, but otherwise ignored by gawk, since gawk has no predefined limits.

-W gawk-opt

Following the POSIX standard, options that are implementation specific are supplied as arguments to the `-W' option. With gawk, these arguments may be separated by commas, or quoted and separated by whitespace. Case is ignored when processing these options. These options also have corresponding GNU style long options. See below.

--

Signals the end of the command line options. The following arguments are not treated as options even if they begin with `-'. This interpretation of `--' follows the POSIX argument parsing conventions. This is useful if you have file names that start with `-', or in shell scripts, if you have file names that will be specified by the user which could start with `-'.

The following gawk-specific options are available:

-W traditional

-W compat

--traditional

--compat

Specifies compatibility mode, in which the GNU extensions to the awk language are disabled, so that gawk behaves just like the Bell Labs research version of Unix awk. `--traditional' is the preferred form of this option. See section Extensions in gawk Not in POSIX awk, which summarizes the extensions. Also see section Downward Compatibility and Debugging.

-W copyleft

-W copyright

--copyleft

--copyright

Print the short version of the General Public License. This option may disappear in a future version of gawk.

-W help

-W usage

--help

--usage

Print a "usage" message summarizing the short and long style options that gawk accepts, and then exit.

-W lint

--lint

Warn about constructs that are dubious or non-portable to other awk implementations. Some warnings are issued when gawk first reads your program. Others are issued at run-time, as your program executes.

-W lint-old

--lint-old

Warn about constructs that are not available in the original Version 7 Unix version of awk (see section Major Changes between V7 and SVR3.1).

-W posix

--posix

Operate in strict POSIX mode. This disables all gawk extensions (just like `--traditional'), and adds the following additional restrictions:

• \x escape sequences are not recognized (see section Escape Sequences).
• The synonym func for the keyword function is not recognized (see section Function Definition Syntax).
• The operators `**' and `**=' cannot be used in place of `^' and `^=' (see section Arithmetic Operators, and also see section Assignment Expressions).
• Specifying `-Ft' on the command line does not set the value of FS to be a single tab character (see section Specifying How Fields are Separated).
• The fflush built-in function is not supported (see section Built-in Functions for Input/Output).

If you supply both `--traditional' and `--posix' on the command line, `--posix' will take precedence. gawk will also issue a warning if both options are supplied.

-W re-interval

--re-interval

Allow interval expressions (see section Regular Expression Operators), in regexps. Because interval expressions were traditionally not available in awk, gawk does not provide them by default. This prevents old awk programs from breaking.

-W source program-text

--source program-text

Program source code is taken from the program-text. This option allows you to mix source code in files with source code that you enter on the command line. This is particularly useful when you have library functions that you wish to use from your command line programs (see section The AWKPATH Environment Variable).

-W version

--version

Prints version information for this particular copy of gawk. This allows you to determine if your copy of gawk is up to date with respect to whatever the Free Software Foundation is currently distributing. It is also useful for bug reports (see section Reporting Problems and Bugs).

Any other options are flagged as invalid with a warning message, but are otherwise ignored.

In compatibility mode, as a special case, if the value of fs supplied to the `-F' option is `t', then FS is set to the tab character ("\t"). This is only true for `--traditional', and not for `--posix' (see section Specifying How Fields are Separated).

The `-f' option may be used more than once on the command line. If it is, awk reads its program source from all of the named files, as if they had been concatenated together into one big file. This is useful for creating libraries of awk functions. Useful functions can be written once, and then retrieved from a standard place, instead of having to be included into each individual program.

You can type in a program at the terminal and still use library functions, by specifying `-f /dev/tty'. awk will read a file from the terminal to use as part of the awk program. After typing your program, type Control-d (the end-of-file character) to terminate it. (You may also use `-f -' to read program source from the standard input, but then you will not be able to also use the standard input as a source of data.)

Because it is clumsy using the standard awk mechanisms to mix source file and command line awk programs, gawk provides the `--source' option. This does not require you to pre-empt the standard input for your source code, and allows you to easily mix command line and library source code (see section The AWKPATH Environment Variable).

If no `-f' or `--source' option is specified, then gawk will use the first non-option command line argument as the text of the program source code.

If the environment variable POSIXLY_CORRECT exists, then gawk will behave in strict POSIX mode, exactly as if you had supplied the `--posix' command line option. Many GNU programs look for this environment variable to turn on strict POSIX mode. If you supply `--lint' on the command line, and gawk turns on POSIX mode because of POSIXLY_CORRECT, then it will print a warning message indicating that POSIX mode is in effect.

You would typically set this variable in your shell's startup file. For a Bourne compatible shell (such as Bash), you would add these lines to the `.profile' file in your home directory.

POSIXLY_CORRECT=true
export POSIXLY_CORRECT

For a csh compatible shell,(14) you would add this line to the `.login' file in your home directory.

setenv POSIXLY_CORRECT true

Other Command Line Arguments

Any additional arguments on the command line are normally treated as input files to be processed in the order specified. However, an argument that has the form var=value, assigns the value value to the variable var---it does not specify a file at all.

All these arguments are made available to your awk program in the ARGV array (see section Built-in Variables). Command line options and the program text (if present) are omitted from ARGV. All other arguments, including variable assignments, are included. As each element of ARGV is processed, gawk sets the variable ARGIND to the index in ARGV of the current element.

The distinction between file name arguments and variable-assignment arguments is made when awk is about to open the next input file. At that point in execution, it checks the "file name" to see whether it is really a variable assignment; if so, awk sets the variable instead of reading a file.

Therefore, the variables actually receive the given values after all previously specified files have been read. In particular, the values of variables assigned in this fashion are not available inside a BEGIN rule (see section The BEGIN and END Special Patterns), since such rules are run before awk begins scanning the argument list.

The variable values given on the command line are processed for escape sequences (d.c.) (see section Escape Sequences).

In some earlier implementations of awk, when a variable assignment occurred before any file names, the assignment would happen before the BEGIN rule was executed. awk's behavior was thus inconsistent; some command line assignments were available inside the BEGIN rule, while others were not. However, some applications came to depend upon this "feature." When awk was changed to be more consistent, the `-v' option was added to accommodate applications that depended upon the old behavior.

The variable assignment feature is most useful for assigning to variables such as RS, OFS, and ORS, which control input and output formats, before scanning the data files. It is also useful for controlling state if multiple passes are needed over a data file. For example:

awk 'pass == 1  { pass 1 stuff }
     pass == 2  { pass 2 stuff }' pass=1 mydata pass=2 mydata

Given the variable assignment feature, the `-F' option for setting the value of FS is not strictly necessary. It remains for historical compatibility.

The AWKPATH Environment Variable

The previous section described how awk program files can be named on the command line with the `-f' option. In most awk implementations, you must supply a precise path name for each program file, unless the file is in the current directory.

But in gawk, if the file name supplied to the `-f' option does not contain a `/', then gawk searches a list of directories (called the search path), one by one, looking for a file with the specified name.

The search path is a string consisting of directory names separated by colons. gawk gets its search path from the AWKPATH environment variable. If that variable does not exist, gawk uses a default path, which is `.:/usr/local/share/awk'.(15) (Programs written for use by system administrators should use an AWKPATH variable that does not include the current directory, `.'.)

The search path feature is particularly useful for building up libraries of useful awk functions. The library files can be placed in a standard directory that is in the default path, and then specified on the command line with a short file name. Otherwise, the full file name would have to be typed for each file.

By using both the `--source' and `-f' options, your command line awk programs can use facilities in awk library files. See section A Library of awk Functions.

Path searching is not done if gawk is in compatibility mode. This is true for both `--traditional' and `--posix'. See section Command Line Options.

Note: if you want files in the current directory to be found, you must include the current directory in the path, either by including `.' explicitly in the path, or by writing a null entry in the path. (A null entry is indicated by starting or ending the path with a colon, or by placing two colons next to each other (`::').) If the current directory is not included in the path, then files cannot be found in the current directory. This path search mechanism is identical to the shell's.

Starting with version 3.0, if AWKPATH is not defined in the environment, gawk will place its default search path into ENVIRON["AWKPATH"]. This makes it easy to determine the actual search path gawk will use.

Obsolete Options and/or Features

This section describes features and/or command line options from previous releases of gawk that are either not available in the current version, or that are still supported but deprecated (meaning that they will not be in the next release).

For version 3.0 of gawk, there are no command line options or other deprecated features from the previous version of gawk. This section is thus essentially a place holder, in case some option becomes obsolete in a future version of gawk.

Undocumented Options and Features

This section intentionally left blank.

Known Bugs in gawk

• The `-F' option for changing the value of FS (see section Command Line Options) 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 different open files (see section Special File Names in gawk).
• 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.
• The word "GNU" is incorrectly capitalized in at least one file in the source code.