Advanced Programming in the UNIX® Environment

ptg10805159

Section 8.12 Interpreter Files 263

When/bin/awkis executed, its command-line arguments are

/bin/awk -f /usr/local/bin/awkexample file1 FILENAME2 f3

The pathname of the interpreter file (/usr/local/bin/awkexample) is passed to the

interpreter.The filename portion of this pathname (what we typed to the shell) isn’t

adequate, because the interpreter (/bin/awkin this example) can’t be expected to use

thePATHvariable to locate files. When it reads the interpreter file,awkignores the first

line, since the pound sign isawk’s comment character.

We can verify these command-line arguments with the following commands:

$/bin/su become superuser

Password: enter superuser password

#mv /usr/bin/awk /usr/bin/awk.save save the original program

#cp /home/sar/bin/echoarg /usr/bin/awk and replace it temporarily

#suspend suspend the superuser shell

[1] + Stopped /bin/su using job control

$awkexample file1 FILENAME2 f3

argv[0]: /bin/awk

argv[1]: -f

argv[2]: /usr/local/bin/awkexample

argv[3]: file1

argv[4]: FILENAME2

argv[5]: f3

$fg resume superuser shell using job control

/bin/su

#mv /usr/bin/awk.save /usr/bin/awk restorethe original program

#exit and exit the superuser shell

In this example, the-foption for the interpreter is required. As we said, this tellsawk

where to look for theawkprogram. If we remove the-foption from the interpreter

file, an error message usually results when we try to run it. The exact text of the

message varies, depending on wherethe interpreter file is stored and whether the

remaining arguments represent existing files. This is because the command-line

arguments in this case are

/bin/awk /usr/local/bin/awkexample file1 FILENAME2 f3

andawkis trying to interpret the string/usr/local/bin/awkexampleas anawk

program. If we couldn’t pass at least a single optional argument to the interpreter (-f

in this case), these interpreter files would be usable only with the shells.

Areinterpreter files required? Notreally.They provide an efficiency gain for the

user at some expense in the kernel (since it’s the kernel that recognizes these files).

Interpreter files areuseful for the following reasons.

1. They hide that certain programs arescripts in some other language. For
   example, to execute the program in Figure8.21, we just say
   awkexampleoptional-arguments