ksh trap signal

What's Signal

When a signal is received, a script can do one of three actions:

1. Ignore it and do nothing.
2. Catch the signal using trap and take appropriate action.
3. Take the default action.

All the above is true except for the following signals:

SIGKILL (signal 9)

SIGSTOP (signal 17)

SIGCONT (signal 19)

These cannot be caught and always uses the default action.

Signal Syntax

trap [ command ] signal [ signal ... ]

• Command can be explicit commands or a function
  • Command

    trap 'echo signal received, now exiting..; exit' 2 6
    

• • function

    function mysignal {
        echo "in mysignal function"
        exit
    }
    trap 'mysignal' 2 6
    

• Signal can be a signal number or signal name

  Signals numbers from 0 to 31, "0" being a pseudo-signal meaning "program termination", or using name, HUP for HANGUP signal, TERM for the SIGTERM signal etc.

trap 'echo signal received, now exiting..; exit' 2 6
trap 'echo signal received, now exiting..; exit' SIGINT SIGQUIT

What will script do when a signal(for example 15) is received:

1. The script would trap the signal 15, and execute the command "rm -f $Tmp", thus removing the temporary file.
2. it would continue with the next script command. This could cause strange results, because the (probably needed) temporary file $Tmp is gone. Another point is that somebody explicitly tried to terminate the script, a fact it deliberately ignores.

How to ignore and reset default signal handler

To ignore a signal, use two single quotes in place of the command:

trap ''  signals

To reset a trap use:

trap -  signals

KSH Signal Code Samples

 1  #!/bin/ksh
 2
 3  function mysignal {
 4          echo "in mysignal"
 5          #exit
 6  }
 7
 8  trap "mysignal" SIGINT SIGQUIT
 9
10  function mainfun {
11          sleep 10
12  }
13
14  mainfun
15
16  echo "End of script"

When script is running, and CTRL-2 is pressed, function mysignal will be called, and after function is finished, control-flow come to the statement immediately after 'mainfun', so the final "End of script" is executed.

So if line 5 (#exit) is not commented out, line 16 will not be executed, because script will exit at line 5.