Unix in a Nutshell, 4th Edition

Command-Line Syntax

The syntax for invoking awk has two forms:

    awk  [options]  'script'  var=value  file(s)
    awk  [options]  -f scriptfile  var=value  file(s)

You can specify a script directly on the command line, or you can store a script in a scriptfile and specify it with -f. POSIX awk allows multiple -f scripts. Variables can be assigned a value on the command line. The value can be a string or numeric constant, a shell variable ($ name), or a command substitution (' cmd '), but the value is available only after the BEGIN statement is executed.

awk operates on one or more files. If none are specified (or if - is specified), awk reads from the standard input.