All about exit code Post: 17031

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Top Forums Shell Programming and Scripting All about exit code Post 17031 by Perderabo on Sunday 10th of March 2002 01:31:26 PM

03-10-2002

Administrator Emeritus

I just read that intro page. It's rather poorly written. But an accurate answer is going to take a while.

From the standpoint of a C programmer who is using the system call interface, when a program invokes exit() or _exit(), the argument to exit is anded with 0377 and the result is the exit code for the process. If a process dies as the result of a default action of a signal it will get an exit code which is non-zero but which has non of the lower 8 bits set. You're supposed to use the macros which are mentioned on wait(2) man page to portably determine the signal number. It is one or the other. Sending a signal to a process which has started the kernel portion of the exit() system call has no effect. All 8 bits are available to the programmer resulting in values 0 through 255 being legal.

And only a process that is running can invoke the exit() call. If a C program tries to run a non-existant program or one without the proper execute bits set, the exec() system call itself will fail and the program will never run.

You are asking this question in the shell programming forum, though. A programmer who writes a shell sees the above interface. But a programmer who uses a shell is at the mercy of the shell's designer.

If you type "./perderabo" you will get an error message telling you that the shell can't find the file perderabo. But most shells will also set the exit code to some non-zero number. Nothing ran, the shell either noticed that the file wasn't there via stat() or it attempted the exec() which failed. But it will set $? (or $status) anyway. This is kinda useful I guess.

The New Kornshell Command and Programming Language by Morris Bolsky and David Korn says:

Quote:

0 Normal exit.
1-125 Failure.
126 A command was found but cannot be executed.
127 A command could not be found.
128-255 Failure.
256 and above A command has exited because of reciept of a signal.
Version: With the 11/16/88 version of ksh, 129-160 indicated that a command had exited because of a signal....A command that could not be found or could not execute had a return value of 1.

So exit codes used by the shell for processes killed by signals are not fixed even within different versions of ksh, let alone across all shells.

What I do is use "exit 0" for success and "exit n" for failure where n is a small integer. I rarely go above 10 and have never reached 20. I rely on shells to be able to determine the difference between a zero exit status and a non-zero exit status. But I almost never test for different non-zero values. I will display a non-zero exit code where it can be seen by a human who can (perhaps) use it to understand what is happening.

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LEARN ABOUT CENTOS

on_exit

ON_EXIT(3)						     Linux Programmer's Manual							ON_EXIT(3)

NAME

       on_exit - register a function to be called at normal process termination

SYNOPSIS

       #include <stdlib.h>

       int on_exit(void (*function)(int , void *), void *arg);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       on_exit(): _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

       The  on_exit() function registers the given function to be called at normal process termination, whether via exit(3) or via return from the
       program's main().  The function is passed the status argument given to the last call to exit(3) and the arg argument from on_exit().

       The same function may be registered multiple times: it is called once for each registration.

       When a child process is created via fork(2), it inherits copies of its parent's registrations.  Upon  a	successful  call  to  one  of  the
       exec(3) functions, all registrations are removed.

RETURN VALUE

       The on_exit() function returns the value 0 if successful; otherwise it returns a nonzero value.

CONFORMING TO

       This  function  comes  from  SunOS 4, but is also present in libc4, libc5 and glibc.  It no longer occurs in Solaris (SunOS 5).	Avoid this
       function, and use the standard atexit(3) instead.

SEE ALSO

       _exit(2), atexit(3), exit(3)

COLOPHON

       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2008-12-05								ON_EXIT(3)

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