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Dennis Williamson
Asked: 2009-05-06 21:49:07 +0800 CST

How can I use symbolic signals in an at job?

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In this answer on SO, I gave an example that should have used a -SIGCONT signal in a job submitted to at. However, I found that the process that I had stopped with -SIGSTOP did not continue. When I changed the signal to the numeric value, it worked.

echo "kill -18 $rspid"|at now + 2 minutes

It's a bad idea to use the numeric value because it can be different on different systems.

How can I submit this job with a symbolic signal (-SIGCONT) instead of the numeric one (-18)?

linux unix shell signals at

2 Answers

  1. My guess would be that whatever shell your at job is using is different to the one that you use at the command line, and the at job shell doesn't have a kill built-in that knows about symbolic signal symbols. This is common on Ubuntu, which uses dash for /bin/sh (which is used by at by default) but /bin/bash for interactive shells.

    You can use the 'kill' binary by specifying the path to it:

    echo "/bin/kill -CONT $rspid" | at now + 2 minutes

    Or else specify the shell to execute explicitly:

    echo "/bin/bash -c \"kill -CONT $rspid\"" |at now + 2 minutes

    I thought there was an option to tell at what shell to use, but I can't seem to find any mention of it now. Perhaps a shebang would work...

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  2. Best Answer

    It turns out that dash likes signals in the form -CONT and doesn't understand the -SIGCONT style. Since bash understands both, it may be more portable to use the former.

    In my testing, it didn't make any difference whether I included an explicit path to 'kill' but the way the signal is specified did.

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