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1.1.4 Executable 'awk' Programs
-------------------------------

Once you have learned 'awk', you may want to write self-contained 'awk'
scripts, using the '#!' script mechanism.  You can do this on many
systems.(1)  For example, you could update the file 'advice' to look
like this:

     #! /bin/awk -f

     BEGIN { print "Don't Panic!" }

After making this file executable (with the 'chmod' utility), simply
type 'advice' at the shell and the system arranges to run 'awk' as if
you had typed 'awk -f advice':

     $ chmod +x advice
     $ ./advice
     -| Don't Panic!

Self-contained 'awk' scripts are useful when you want to write a program
that users can invoke without their having to know that the program is
written in 'awk'.

                          Understanding '#!'

   'awk' is an "interpreted" language.  This means that the 'awk'
utility reads your program and then processes your data according to the
instructions in your program.  (This is different from a "compiled"
language such as C, where your program is first compiled into machine
code that is executed directly by your system's processor.)  The 'awk'
utility is thus termed an "interpreter".  Many modern languages are
interpreted.

   The line beginning with '#!' lists the full file name of an
interpreter to run and a single optional initial command-line argument
to pass to that interpreter.  The operating system then runs the
interpreter with the given argument and the full argument list of the
executed program.  The first argument in the list is the full file name
of the 'awk' program.  The rest of the argument list contains either
options to 'awk', or data files, or both.  (Note that on many systems
'awk' is found in '/usr/bin' instead of in '/bin'.)

   Some systems limit the length of the interpreter name to 32
characters.  Often, this can be dealt with by using a symbolic link.

   You should not put more than one argument on the '#!' line after the
path to 'awk'.  It does not work.  The operating system treats the rest
of the line as a single argument and passes it to 'awk'.  Doing this
leads to confusing behavior--most likely a usage diagnostic of some sort
from 'awk'.

   Finally, the value of 'ARGV[0]' (*note Built-in Variables::) varies
depending upon your operating system.  Some systems put 'awk' there,
some put the full pathname of 'awk' (such as '/bin/awk'), and some put
the name of your script ('advice').  (d.c.)  Don't rely on the value of
'ARGV[0]' to provide your script name.

   ---------- Footnotes ----------

   (1) The '#!' mechanism works on GNU/Linux systems, BSD-based systems,
and commercial Unix systems.