How Linux Works

Exit Codes

When a Unix program finishes, it leaves an exit code for the parent process that started the program. The exit code is a number and is sometimes called an error code. When the exit code is zero (0), this means that the program ran without problems. However, if the program has an error, it usually exits with some number other than 0.

The exit code of the last command is held in the $? special variable, so you can check it out for yourself at your shell prompt:

$ ls / > /dev/null
$ echo $?
0
$ ls /asdfasdf > /dev/null
ls: /asdfasdf: No such file or directory
$ echo $?
1

You can see that the successful command returned 0 and the unsuccessful command returned 1.

If you intend to use the exit code of a command, you must use or store the code ...