Exit codes on the Linux command line are numeric values that provide feedback on whether the command just run was successful or ran into some variety of problem. If youâve never noticed them, donât be surprised. The more obvious error messages like “command not found” will generally tell you all you need to know when something goes wrong, but the stealthy exit codes have a distinct advantage. For one thing, they can be checked in scripts to note errors that might be encountered (more on this below).
To get started, sit down at your Linux system and open a terminal window. Then type “pwd”. You should be rewarded by a quick display of your current location in the file system â undoubtedly your home directory. Before you do anything else, however, type “echo $?”. The system should result in the display of a zero (0). Yes, thatâs the exit code and itâs meant to indicate that the pwd command you just used ran without any errors. An exit code of 0 always indicates that no problems were encountered.
Any exit code other than 0, on the other hand, indicates that some problem occurred. What problem depends on the command that was run. Executables donât always use the same code for the same problem. The numeric range for exit codes is 0-255, so any code from 1-255 means something went wrong. Hereâs a list of some of the exit codes that you might run into from time to time.
Code Description 0 success 1 generic error 2 syntax error 126 permissions issue -- the requested command (file) can't be executed (but was found) 127 command (file) not found 128 fatal error, amount above 128 is signal number 128 + N the shell was terminated by the signal N (also used like this by various other programs) 130 process terminated by SIGINT (^c on keyboard) 137 process terminated by SIGKILL 143 process terminated by SIGTERM 255 wrong argument to the exit builtin (see code 128)
Now letâs run some tests and see how this works.
First, weâll try to display a file that doesnât exist â leading to an exit code of 1.
$ cat noSuchFile cat: noSuchFile: No such file or directory $ echo $? 1
Next, weâll try to list the same nonexistent file.
$ ls noSuchFile ls: cannot access 'noSuchFile': No such file or directory $ echo $? 2
The command below tries to run a file which isnât executable.
$ friends bash: ./friends: Permission denied $ echo $? 126
The command below tries to execute a nonexistent file.
$ ./noSuchFile -bash: ./noSuchFile: No such file or directory $ echo $? 127
In the example below, I start a second shell inside my current shell by typing â/bin/bashâ and then exit it with a ^c (control-c). In this case, the exit code returned is 130.
$ ^C $ echo $? 130
In the next example, I start a second bash shell and ask it to exit with a particular code. We then display the exit code and see that it returned the code requested.
$ bash -c "exit 42" $ echo $? 42
Next, I try doing the same thing with a series of exit codes.
$ bash -c "exit 255" $ echo $? 255 $ bash -c "exit 256" $ echo $? 0 $ bash -c "exit 266" $ echo $? 10 $ bash -c "exit 267" $ echo $? 11
Notice how the exit codes above 255 are all reduced by 256.
Using exit codes in scripts
The script below attempts to run a second script (called âmyscriptâ), but sends all of that scriptâs output to /dev/null since that scriptâs output is not of interest in this example. It does, however, capture the return code so that it can itself exit with that same code to note that âmyscriptâ failed.
#!/bin/bash # send output to "bit bucket" myscript &>/dev/null exitcode=$? if [ $exitcode -eq 0 ]; then echo "myscript ran successfully" else echo "myscript exited with exit code $exitcode" exit $exitcode fi
Wrap-up
Knowing how to work with and display exit codes on the Linux command line or in scripts can help to make the various kinds of errors more obvious.