Are there any standard exit status codes in Linux?

Question

A process is considered to have completed correctly in Linux if its exit status was 0.

I\'ve seen that segmentation faults often result in an exit status of 11, thou

Answer 1

8 bits of the return code and 8 bits of the number of the killing signal are mixed into a single value on the return from wait(2) & co..

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <signal.h>

int main() {
    int status;

    pid_t child = fork();
    if (child <= 0)
        exit(42);
    waitpid(child, &status, 0);
    if (WIFEXITED(status))
        printf("first child exited with %u\n", WEXITSTATUS(status));
    /* prints: "first child exited with 42" */

    child = fork();
    if (child <= 0)
        kill(getpid(), SIGSEGV);
    waitpid(child, &status, 0);
    if (WIFSIGNALED(status))
        printf("second child died with %u\n", WTERMSIG(status));
    /* prints: "second child died with 11" */
}

How are you determining the exit status? Traditionally, the shell only stores an 8-bit return code, but sets the high bit if the process was abnormally terminated.

$ sh -c 'exit 42'; echo $?
42
$ sh -c 'kill -SEGV $$'; echo $?
Segmentation fault
139
$ expr 139 - 128
11

If you're seeing anything other than this, then the program probably has a SIGSEGV signal handler which then calls exit normally, so it isn't actually getting killed by the signal. (Programs can chose to handle any signals aside from SIGKILL and SIGSTOP.)

Answer 2

Programs return a 16 bit exit code. If the program was killed with a signal then the high order byte contains the signal used, otherwise the low order byte is the exit status returned by the programmer.

How that exit code is assigned to the status variable $? is then up to the shell. Bash keeps the lower 7 bits of the status and then uses 128 + (signal nr) for indicating a signal.

The only "standard" convention for programs is 0 for success, non-zero for error. Another convention used is to return errno on error.

Answer 3

sysexits.h has a list of standard exit codes. It seems to date back to at least 1993 and some big projects like Postfix use it, so I imagine it's the way to go.

From the OpenBSD man page:

According to style(9), it is not good practice to call exit(3) with arbi- trary values to indicate a failure condition when ending a program. In- stead, the pre-defined exit codes from sysexits should be used, so the caller of the process can get a rough estimation about the failure class without looking up the source code.

Answer 4

When Linux returns 0, it means success. Anything else means failure, each program has its own exit codes, so it would been quite long to list them all... !

About the 11 error code, it's indeed the segmentation fault number, mostly meaning that the program accessed a memory location that was not assigned.