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.
;-)
|
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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.
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