44.7 Exit Status of UNIX Processes

When a UNIX process (command) runs, it can return a numeric status value to the process that called (started) it. The status can tell the calling process whether the command succeeded or failed. Many (but not all) UNIX commands return a status of zero if everything was okay or non-zero (1, 2, etc.) if something went wrong. A few commands, like grep and diff, return a different non-zero status for different kinds of problems; see your online manual pages to find out.

The Bourne shell puts the exit status of the previous command in the question mark (?) variable. You can get its value by preceding it with a dollar sign ($), just like any other shell variable. For example, when cp copies a file, it sets the status to 0. If something goes wrong, cp sets the status to 1:

$ cp afile /tmp
$ echo $?

$ cp afiel /tmp
cp: afiel: No such file or directory
$ echo $?
1

In the C shell, use the status variable instead:

% cp afiel /tmp
cp: afiel: No such file or directory
% echo $status
1

Of course, you usually don't have to display the exit status in this way, because there are several ways (44.8, 44.9, 44.10) to use the exit status of one command as a condition of further execution.

The exit status of pipelines (1.4) is the status of the last command in the pipeline. [2] You can't test the exit status of a background job in the Bourne shell unless you use the wait command to wait for it (in effect, to bring the job out of the background).

[2] I've seen a few places where that wasn't true - in an early Korn Shell, I think, and a couple of other places too - but that was a long time ago.