TRAP(1SH)               COMMAND REFERENCE               TRAP(1SH)



NAME
     trap - execute command on a given signal (sh built-in)

SYNOPSIS
     trap [ arg ] [ n ]

DESCRIPTION
     Arg is a command to be read and executed when the shell
     receives the signal(s) n.  (Note that arg ss scanned once
     when the trap is set and once when the trap is taken.) Trap
     commands are executed in order of signal number. If arg is
     absent, all trap(s) n are reset to their original values. If
     arg is the null string or nonexistent, this signal is
     ignored by the shell and by invoked commands. If n is 0, the
     command arg is executed on exit from the shell; otherwise
     upon receipt of signal n as numbered in signal(3c).  Trap
     with no arguments prints a list of commands associated with
     each signal number.

EXAMPLES
     The following command causes interrupts (signal number 2) to
     be ignored:



          trap '' 2


     The following shell script shows a major use for the trap
     command. The script puts a copy of the standard input into a
     temporary file and then copies the data to the standard
     output preceded by the current date. If the interrupt or
     hangup signal (signal number 1) is sent to the process, the
     temporary file is removed and the exit code is 1. When the
     process is finished, the temporary file is removed and the
     exit code is 1.



          #!/bin/sh
          Excode=1
          trap 'rm -f /tmp/slowpr.$$;exit $Excode' 0 1 2
          while read f
          do
               echo "$f" >> /tmp/slowpr.$$
          done
          date
          cat /tmp/slowpr.$$
          Excode=0






Printed 10/17/86                                                1

TRAP(1SH)               COMMAND REFERENCE               TRAP(1SH)



RETURN VALUE
     [NO_ERRS]      Command completed without error.

     [1]            One or more of the signal numbers is invalid.


CAVEATS
     It is very important to note that command and variable
     substitutions are made when the trap command is interpreted
     and also when the corresponding commands are executed.
     Take, for example, the following commands:


          trap "rm -f `ls -d /tmp/foo.*`" 0
          trap 'rm -f `ls -d /tmp/foo.*`" 0

     The first command tells the shell to remove all files that
     matched the pattern /tmp/foo.* at the time the trap command
     was interpreted. The second trap command tells the shell to
     remove all files that matched pattern at the time the shell
     script exits.  This means that in the first case, new files
     created by the shell script after the trap command was
     executed will not be removed.

     Signal number 11 (segmentation violation) can not be trapped
     at this time.  Attempts to do so result in an error.

SEE ALSO
     break(1sh), cd(1sh), chdir(1sh), continue(1sh), csh(1csh),
     echo(1sh), eval(1sh), exec(1sh), exit(1sh), export(1sh),
     hash(1sh), login(1), onintr(1csh), pwd(1sh), read(1sh),
     readonly(1sh), return(1sh), set(1sh), sh(1sh), shift(1sh),
     test(1sh), times(1sh), type(1sh), ulimit(1sh), umask(1sh),
     unset(1sh), wait(1sh), which(1sh), execve(2), signal(3c).





















Printed 10/17/86                                                2

%%index%%
na:72,102;
sy:174,114;
de:288,841;
ex:1129,914;
rv:2187,201;
ca:2388,1043;
se:3431,903;
%%index%%000000000121