Exit Status Codes

When any process finishes executing, it exits with a numeric value. This can be called the exit code, status code, exit status code, or error code.

Usually, an exit status code of zero (0) means that no errors were encountered, and a non-zero code means that something went wrong. Therefore, it may be easiest to think of this as the error code, with 0 meaning no errors.

Be aware that program authors can use this value as they see fit, so the status code may indicate something else, such as the number of data items processed.

The special variable $? is set to the exit status code of the last command executed by the shell.

For example:

$ ls -d /etc
/etc
$ echo $?
0

$ ls -d /this/does/not/exist
ls: cannot access '/this/does/not/exist': No such file or directory
$ echo $?
2

Why is this important? Because exit status codes are the key to conditional logic (if...) and looping (for/while/until/...) in bash scripting.