ARGC and ARGV ¶Built-in Variables That Convey Information
presented the following program describing the information contained in ARGC
and ARGV:
$ awk 'BEGIN {
> for (i = 0; i < ARGC; i++)
> print ARGV[i]
> }' inventory-shipped mail-list
-| awk
-| inventory-shipped
-| mail-list
In this example, ARGV[0] contains ‘awk’, ARGV[1]
contains ‘inventory-shipped’, and ARGV[2] contains
‘mail-list’.
Notice that the awk program is not entered in ARGV. The
other command-line options, with their arguments, are also not
entered. This includes variable assignments done with the -v
option (see Command-Line Options).
Normal variable assignments on the command line are
treated as arguments and do show up in the ARGV array.
Given the following program in a file named showargs.awk:
BEGIN {
printf "A=%d, B=%d\n", A, B
for (i = 0; i < ARGC; i++)
printf "\tARGV[%d] = %s\n", i, ARGV[i]
}
END { printf "A=%d, B=%d\n", A, B }
Running it produces the following:
$ awk -v A=1 -f showargs.awk B=2 /dev/null -| A=1, B=0 -| ARGV[0] = awk -| ARGV[1] = B=2 -| ARGV[2] = /dev/null -| A=1, B=2
A program can alter ARGC and the elements of ARGV.
Each time awk reaches the end of an input file, it uses the next
element of ARGV as the name of the next input file. By storing a
different string there, a program can change which files are read.
Use "-" to represent the standard input. Storing
additional elements and incrementing ARGC causes
additional files to be read.
If the value of ARGC is decreased, that eliminates input files
from the end of the list. By recording the old value of ARGC
elsewhere, a program can treat the eliminated arguments as
something other than file names.
To eliminate a file from the middle of the list, store the null string
("") into ARGV in place of the file’s name. As a
special feature, awk ignores file names that have been
replaced with the null string.
Another option is to
use the delete statement to remove elements from
ARGV (see The delete Statement).
All of these actions are typically done in the BEGIN rule,
before actual processing of the input begins.
See Splitting a Large File into Pieces and
see Duplicating Output into Multiple Files
for examples
of each way of removing elements from ARGV.
To actually get options into an awk program,
end the awk options with -- and then supply
the awk program’s options, in the following manner:
awk -f myprog.awk -- -v -q file1 file2 ...
The following fragment processes ARGV in order to examine, and
then remove, the previously mentioned command-line options:
BEGIN {
for (i = 1; i < ARGC; i++) {
if (ARGV[i] == "-v")
verbose = 1
else if (ARGV[i] == "-q")
debug = 1
else if (ARGV[i] ~ /^-./) {
e = sprintf("%s: unrecognized option -- %c",
ARGV[0], substr(ARGV[i], 2, 1))
print e > "/dev/stderr"
} else
break
delete ARGV[i]
}
}
Ending the awk options with -- isn’t
necessary in gawk. Unless --posix has
been specified, gawk silently puts any unrecognized options
into ARGV for the awk program to deal with. As soon
as it sees an unknown option, gawk stops looking for other
options that it might otherwise recognize. The previous command line with
gawk would be:
gawk -f myprog.awk -q -v file1 file2 ...
Because -q is not a valid gawk option, it and the
following -v are passed on to the awk program.
(See Processing Command-Line Options for an awk library function that
parses command-line options.)
When designing your program, you should choose options that don’t
conflict with gawk’s, because it will process any options
that it accepts before passing the rest of the command line on to
your program. Using ‘#!’ with the -E option may help
(see Executable awk Programs
and
see Command-Line Options).