#!/bin/awk -f

"#!" is followed by the pathname that locates your version of awk and then the -f option. After this line, you specify the awk script:

#!/bin/awk -f
{ print $1 }

Note that no quotes are necessary around the script. All lines in the file after the first one will be executed as though they were specified in a separate script file.

A few years ago, there was an interesting discussion on the Net about the use of the "#!" syntax that clarified how it works. The discussion was prompted by a 4.2BSD user's observation that the shell script below fails:

#!/bin/awk
{ print $1 }

while the one below works:

#!/bin/sh
/bin/awk '{ print $1 }'

The two responses that we saw were by Chris Torek and Guy Harris and we will try to summarize their explanation. The first script fails because it passes the filename of the script as the first parameter (argv[1] in C) and awk interprets it as the input file and not the script file. Because no script has been supplied, awk produces a syntax error message. In other words, if the name of the shell script is "myscript," then the first script executes as:

/bin/awk myscript

If the script was changed to add the -f option, it looks like this:

#!/bin/awk -f
{ print $1 }

Then you enter the following command:

$ myscript myfile

It then executes as though you had typed:

/bin/awk -f myscript myfile

NOTE: You can put only one parameter on the "#!" line. This line is processed directly by the UNIX kernel; it is not processed by the shell and thus cannot contain arbitrary shell constructs.

The "#!" syntax allows you to create shell scripts that pass command-line parameters transparently to awk. In other words, you can pass awk parameters from the command line that invokes the shell script.

For instance, we demonstrate passing parameters by changing our sample awk script to expect a parameter n:

{ print $1*n }

Assuming that we have a test file in which the first field contains a number that can be multiplied by n, we can invoke the program, as follows:

$ myscript n=4 myfile

This spares us from having to pass "$1" as a shell variable and assigning it to n as an awk parameter inside the shell script.