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Editorial Team
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Asked: 2026-05-21T06:38:21+00:00

Use trap to capture signals like this: i=-1;while((++i<33)); do trap echo $i >> log.txt

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Use trap to capture signals like this:

i=-1;while((++i<33));
do
    trap "echo $i >> log.txt" $i;
done

And close the terminal by force.

The content in log.txt is then (under redhat linux):

1

18

1

17

0

Where these signals from?

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1 Answer

  1. Editorial Team

    The first signal is SIGHUP; that gets sent to all processes in the process group when the terminal disconnects (hangs up – hence HUP).

    The second signal is SIGCONT (thanks, SiegeX, for the numbers). This is slightly surprising; it suggests you had a job stopped in the background which had to be allowed to run again.

    The third signal is another SIGHUP. This was likely sent to ensure that the continued process got its turn to exit, but was sent to the whole process group. (See the POSIX standard for information on process groups, etc.).

    The fourth signals is a SIGCHLD, indicating that a child process died and the corpse is available (well, the status is available).

    The final signal, 0, is the shells internal pseudo-signal indicating that it is exiting.

    You can do:

    trap 'echo Bye' 0

    to echo ‘Bye’ when the shell exits under control for any reason. You chose to echo the signal number to the file instead. Since the shell exits at this point, that is the last signal message that is seen. Its parent process should get a SIGCHLD signal because the shell died.

    FWIW, on MacOS X 10.6.7, I ran your test. There isn’t a signal 32 on MacOS X, and some of the mappings are different, and the sequence of signals sent is also different:

    $ i=-1;while((++i<33));
> do
>     trap "echo $i >> log.txt" $i;
> done
-sh: trap: 32: invalid signal specification
$ trap
trap -- 'echo 0 >> log.txt' EXIT
trap -- 'echo 1 >> log.txt' HUP
trap -- 'echo 2 >> log.txt' INT
trap -- 'echo 3 >> log.txt' QUIT
trap -- 'echo 4 >> log.txt' ILL
trap -- 'echo 5 >> log.txt' TRAP
trap -- 'echo 6 >> log.txt' ABRT
trap -- 'echo 7 >> log.txt' EMT
trap -- 'echo 8 >> log.txt' FPE
trap -- 'echo 9 >> log.txt' KILL
trap -- 'echo 10 >> log.txt' BUS
trap -- 'echo 11 >> log.txt' SEGV
trap -- 'echo 12 >> log.txt' SYS
trap -- 'echo 13 >> log.txt' PIPE
trap -- 'echo 14 >> log.txt' ALRM
trap -- 'echo 15 >> log.txt' TERM
trap -- 'echo 16 >> log.txt' URG
trap -- 'echo 17 >> log.txt' STOP
trap -- 'echo 19 >> log.txt' CONT
trap -- 'echo 20 >> log.txt' CHLD
trap -- 'echo 23 >> log.txt' IO
trap -- 'echo 24 >> log.txt' XCPU
trap -- 'echo 25 >> log.txt' XFSZ
trap -- 'echo 26 >> log.txt' VTALRM
trap -- 'echo 27 >> log.txt' PROF
trap -- 'echo 28 >> log.txt' WINCH
trap -- 'echo 29 >> log.txt' INFO
trap -- 'echo 30 >> log.txt' USR1
trap -- 'echo 31 >> log.txt' USR2
$

    The signals captured in one run were:

    2
1
20
0

    In a second run, I got:

    20
1
20
0

    The SIGINT first is surprising — I don’t think I can explain that unless it simply means an incomplete write of some sort (it should have read 20 but the SIGHUP caused a problem). I’m not sure that I can explain the SIGCHLD signals either; the SIGHUP and ‘exit’ trap are as before.

    To some extent, though, the signals are system specific – or so it seems. The SIGHUP is common and constant, though.

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