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Shell Programming for the Uninitiated
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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.

true
false
Two simple UNIX utilities do nothing but return an exit status. true returns a status of 0 (zero); false returns 1 (one). There are GNU versions on the CD-ROM-and no, they don't have any amazing extra features. ;-)

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.

- JP


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