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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exit

exit(1)                                                            User Commands                                                           exit(1)

NAME

       exit, return, goto - shell built-in functions to enable the execution of the shell to advance beyond its sequence of steps

SYNOPSIS

   sh
       exit [n]

       return [n]

   csh
       exit [ ( expr )]

       goto label

   ksh
       *exit [n]

       *return [n]

DESCRIPTION

   sh
       exit  will  cause the calling shell or shell script to exit with the exit status specified by n. If n is omitted the exit status is that of
       the last command executed (an EOF will also cause the shell to exit.)

       return causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command  exe-
       cuted.

   csh
       exit  will cause the calling shell or shell script to exit, either with the value of the status variable or with the value specified by the
       expression expr.

       The goto built-in uses a specified label as a search string amongst commands. The shell rewinds its input as much as possible and  searches
       for a line of the form label: possibly preceded by space or tab characters. Execution continues after the indicated line. It is an error to
       jump to a label that occurs between a while or for built-in command and its corresponding end.

   ksh
       exit will cause the calling shell or shell script to exit with the exit status specified by n. The value will be the  least  significant  8
       bits of the specified status. If n is omitted then the exit status is that of the last command executed. When    exit occurs when executing
       a trap, the last command refers to the command that executed before the trap was invoked. An end-of-file will also cause the shell to  exit
       except for a shell which has the ignoreeof option (See set below) turned on.

       return  causes a shell function or '.' script to return to the invoking script with the return status specified by n. The value will be the
       least significant 8 bits of the specified status.  If n is omitted then the return status is that of the last command executed.  If  return
       is invoked while not in a function or a '.' script, then it is the same as an exit.

       On this man page, ksh(1) commands that are preceded by one or two * (asterisks) are treated specially in the following ways:

       1.  Variable assignment lists preceding the command remain in effect when the command completes.

       2.  I/O redirections are processed after variable assignments.

       3.  Errors cause a script that contains them to abort.

       4.  Words,  following a command preceded by ** that are in the format of a variable assignment, are expanded with the same rules as a vari-
           able assignment. This means that tilde substitution is performed after the = sign and word splitting and file name generation  are  not
           performed.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
       |Availability                 |SUNWcsu                      |
       +-----------------------------+-----------------------------+

SEE ALSO

       break(1), csh(1), ksh(1), sh(1), attributes(5)

SunOS 5.10                                                          15 Apr 1994                                                            exit(1)

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