Other Arguments (The GNU Awk User’s Guide)
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2.3 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. (See Assigning Variables on the Command Line.) In the following example, count=1
is a variable assignment, not a file name:
awk -f program.awk file1 count=1 file2
As a side point, should you really need to have awk
process a file named count=1
(or any file whose name looks like a variable assignment), precede the file name with ‘./
’, like so:
awk -f program.awk file1 ./count=1 file2
All the command-line arguments are made available to your awk
program in the ARGV
array (see section Predefined 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 ARGIND
to the index in ARGV
of the current element. (gawk
makes the full command line, including program text and options, available in PROCINFO["argv"]
; see section Built-in Variables That Convey Information.)
Changing ARGC
and ARGV
in your awk
program lets you control how awk
processes the input files; this is described in more detail in Using ARGC and ARGV.
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), because such rules are run before awk
begins scanning the argument list.
The variable values given on the command line are processed for escape sequences (see section Escape Sequences). (d.c.)
In some very early 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. Unfortunately, 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.
Next: Naming Standard Input, Previous: Options, Up: Invoking Gawk [Contents][Index]