What are exit codes in Linux?

Overview

In Linux, an exit code indicates the response from the command or a script after execution. It ranges from 0 to 255. The exit codes help us determine whether a process ran:

1. Successfully.
2. Failed due to an error.
3. Or another state that gives us an indication of an unexpected result.

Note: Exit codes are also known as return codes.

We can get the process's exit code once the command or script execution is complete using $?.

Example

Here's an example:

Note: The terminal is attached to the end of the shot. Please copy the commands from above and paste them into the terminal to try them.

Explanation

In the above example:

1. We ran the command echo “Hello There!” and it printed Hello There! on the following line. Then we ran the command echo $? and it provided output as 0, which indicates that the command ran successfully.
2. We ran the command cat demofile.txt, and it gave us an error cat: demofile.txt: No such file or directory, and the return code is 1, indicating a failure.

Reserved exit codes

Linux has some reserved exit codes with a special meaning. Here's the list of exit codes:

Exit codes are usually used in the shell script to check the status and take actions accordingly. We run multiple commands in a shell script to check for an everyday use case and see if the command runs successfully. Here's an example:

Explanation

In the code above, we check if the return code is equal to 0. If it is, then we echo OK. Otherwise, we echo FAIL.