8.6.1. An Array of Command-Line Parameters

# argv.awk - print command-line parameters
BEGIN { for (x = 0; x < ARGC; ++x)
	    print ARGV[x]
	print ARGC
}

This example also prints out the value of ARGC, the number of command-line arguments. Here's an example of how it works on a sample command line:

$ awk -f argv.awk 1234 "John Wayne" Westerns n=44 -
awk
1234
John Wayne
Westerns
n=44
- 
6

As you can see, there are six elements in the array. The first element is the name of the command that invoked the script. The last argument, in this case, is the filename, "-", for standard input. Note the "-f argv.awk" does not appear in the parameter list.

Generally, the value of ARGC will be at least 2. If you don't want to refer to the program name or the filename, you can initialize the counter to 1 and then test against ARGC - 1 to avoid referencing the last parameter (assuming that there is only one filename).

Remember that if you invoke awk from a shell script, the command-line parameters are passed to the shell script and not to awk. You have to pass the shell script's command-line parameters to the awk program inside the shell script. For instance, you can pass all command-line parameters from the shell script to awk, using "$*". Look at the following shell script:

awk '
# argv.sh - print command-line parameters
BEGIN {
	for (x = 0; x < ARGC; ++x)
		print ARGV[x]
	print ARGC
}' $*

This shell script works the same as the first example of invoking awk.

One practical use is to test the command-line parameters in the BEGIN rule using a regular expression. The following example tests that all the parameters, except the first, are integers.

# number.awk - test command-line parameters
BEGIN {
	for (x = 1; x < ARGC; ++x)
		if ( ARGV[x] !~ /^[0-9]+$/ ) {
			print ARGV[x], "is not an integer."
			exit 1
		}
}

If the parameters contain any character that is not a digit, the program will print the message and quit.

After testing the value, you can, of course, assign it to a variable. For instance, we could write a BEGIN procedure of a script that checks the command-line parameters before prompting the user. Let's look at the following shell script that uses the phone and address database from the previous chapter:

awk '# phone - find phone number for person 
# supply name of person on command line or at prompt.
BEGIN { FS = "," 
	# look for parameter
	if ( ARGC > 2 ){ 
		name = ARGV[1]
		delete ARGV[1]
	} else { 

		# loop until we get a name
		while (! name) { 
			printf("Enter a name? ")
			getline name < "-"
		}
	}
}
$1 ~ name {
	print $1, $NF 
}' $* phones.data

8.6.2. An Array of Environment Variables

# environ.awk - print environment variable
BEGIN {
	for (env in ENVIRON)
		print env "=" ENVIRON[env]
}

The index of the array is the variable name. The script generates the same output produced by the env command (printenv on some systems).

$ awk -f environ.awk
DISPLAY=scribe:0.0
FRAME=Shell 3
LOGNAME=dale
MAIL=/usr/mail/dale
PATH=:/bin:/usr/bin:/usr/ucb:/work/bin:/mac/bin:.
TERM=mac2cs
HOME=/work/dale
SHELL=/bin/csh
TZ=PST8PDT
EDITOR=/usr/bin/vi

You can reference any element, using the variable name as the index of the array:

ENVIRON["LOGNAME"]

You can also change any element of the ENVIRON array.

ENVIRON["LOGNAME"] = "Tom"