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It's my understanding that a unix shell shell script can, and should, issue an exit code. An exit code of zero means the program completed successfully, while a positive integer exit code (1-255) means the program exited with an error.

Do any -- unixes? shells? (unsure on the terminology to use) --, reserve any of these 255 error exits code for specific meanings? i.e. I'm creating a command line program that I'd like to be a well behaved unix citizen -- can I safely have my program return any non-zero exit code and have that code mean something to my program? Or are there certain exit codes my program shouldn't use, because the system itself will interpret them in a particular way?

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(EDIT, stolen from the answer, referred to by the question comment, which ultimately pointed here).

In shell scripting, fake exit statuses over 128 are provided for certain conditions, so it's probably best to avoid these. (ssh uses 255 though, which is not used this way).

The two exit statuses below 128 are also used to represent certain shell errors, so you generally don't want to use those codes either.

BSD went on to try and standardize some exit codes, starting at 64. I don't see any problem using these if you fell in love with them (search sysexit.h, it's not a formal standard so you'll have to copy it). And if you're not using this, then "less than 64" is a nicer round number than 126 :-P. That said, neither the shell nor the kernel will care about these at all. They're used between co-operating programs - specifically this was intended for mailer components. If you want more than 63 "error" statuses, I wouldn't worry about these ones being "reserved" in Unix.

The system call interface itself aka kernel, doesn't really care. In the system calls, special exit statuses are encoded out of band of the 0-255 range. This is how the shell detects and prints the status "Killed", for a command terminated by SIGKILL. This out-of-band information cannot be faked by any simple exit call.

If status is not NULL, wait() and waitpid() store status information in the int to which it points. This integer can be inspected with the following macros (which take the integer itself as an argument, not a pointer to it, as is done in wait() and waitpid()!):

WIFEXITED(status) returns true if the child terminated normally, that is, by calling exit(3) or _exit(2), or by returning from main().

WEXITSTATUS(status) returns the exit status of the child. This consists of the least significant 8 bits of the status argument that the child specified in a call to exit(3) or _exit(2) or as the argument for a return statement in main(). This macro should only be employed if WIFEXITED returned true.

WIFSIGNALED(status) returns true if the child process was terminated by a signal.

WTERMSIG(status) returns the number of the signal that caused the child process to terminate. This macro should only be employed if WIFSIGNALED returned true.

WCOREDUMP(status) returns true if the child produced a core dump. This macro should only be employed if WIFSIGNALED returned true. This macro is not specified in POSIX.1-2001 and is not available on some UNIX implementations (e.g., AIX, SunOS). Only use this enclosed in #ifdef WCOREDUMP ... #endif.

WIFSTOPPED(status) returns true if the child process was stopped by delivery of a signal; this is only possible if the call was done using WUNTRACED or when the child is being traced (see ptrace(2)).

WSTOPSIG(status) returns the number of the signal which caused the child to stop. This macro should only be employed if WIFSTOPPED returned true.

WIFCONTINUED(status) (since Linux 2.6.10) returns true if the child process was resumed by delivery of SIGCONT.

and

ssh exits with the exit status of the remote command or with 255 if an error occurred.

and

$ a
bash: a: command not found...
$ echo $?
127
$ /dev/null
bash: /dev/null: Permission denied
$ echo $?
126
  • 1
    Another loose convention is that 2 means an inability to parse arguments – Daniel H Feb 17 '17 at 19:53

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