topic Re: trap in Operating System - HP-UX

trap

Khashru — Mon, 13 Feb 2006 23:44:11 GMT

what does the folloing line mean in /home/user/.profile

trap : 1 2 3

Re: trap

Senthil Kumar .A_1 — Mon, 13 Feb 2006 23:54:20 GMT

Hi Khashru,

Trap command is usually used in shell scripts to trap signals. To know different signal's and its significance try man pag of kill command.

This command command syntax is

trap {commands} {signal number list}

For example let us assume, you shell script creates lot of temp files in the course of execution.When I kill the shell script while it is running using

kill -15 PID, This actually kills the process and exits without any additional execution. Instead you shell scripts sets trap like the folowing..

trap "rm /tmp/tempfile" 15

then the sript will execute the command "rm /tmp/tempfile" when its killed with the signal 15.

I hope this explanation helped,

Regards,
Senthil Kumar .A

Re: trap

Arunvijai_4 — Mon, 13 Feb 2006 23:54:45 GMT

Hi Khashru,

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=30026

-Arun

Re: trap

Arunvijai_4 — Tue, 14 Feb 2006 00:00:50 GMT

Hi Khashru,

Some important signals are,

Signals 1(hangup)
Singnal 2(interrupt)
Singnal 3(quit)
Singnal 0 (shell exit)
Signal 15 (terninate)

-Arun

Re: trap

Khashru — Tue, 14 Feb 2006 00:02:46 GMT

what is : after trap command. it is used to stop ctrl+c.

Re: trap

Sivakumar TS — Tue, 14 Feb 2006 00:04:41 GMT

Hi,

By giving the follwoing enty in profile.

trap " " 1 2 3

you can "trap"/"supress" those signals.

it means... if the user executes Ctrl+C in a menu, the program won ger terminated and he wont get the shell prompt. Its done for security reasons.

With Regards,

Siva.

Re: trap

Khashru — Tue, 14 Feb 2006 00:06:58 GMT

yes. but I typed trap : 1 2 3 not trap "". I want to know why : not ""

Re: trap

Sivakumar TS — Tue, 14 Feb 2006 00:28:56 GMT

Hi Khashru,

: is equivallent to "" itself....

go thru the follwing,

server[/]#trap
trap -- 'echo '\''logout root'\' EXIT
server[/]#trap : 1 2 3
server[/]#trap
trap -- : QUIT
trap -- : INT
trap -- : HUP
trap -- 'echo '\''logout root'\' EXIT
server[/]#trap "" 4
server[/]#trap
trap -- '' ILL
trap -- : QUIT
trap -- : INT
trap -- : HUP
trap -- 'echo '\''logout root'\' EXIT

With Regards,

Siva.

Re: trap

Muthukumar_5 — Tue, 14 Feb 2006 00:29:43 GMT

I typed trap : 1 2 3 not trap "". I want to know why : not ""

--

trap is using syntax as,

trap "function execution" signal number

trap "" 1 2 3

"" -> Null string .. If it gets the singal then it will do anything.

signal()
{
echo "Got signal"
}

trap "signal" 1 2 3

when ever it gets signal it will call signal shell function.

--
Muthu

Re: trap

Muthukumar_5 — Tue, 14 Feb 2006 00:30:47 GMT

It is always better to use man pages to get more information.

See ksh man page for trap command.

% trap [arg] [sig ...]
arg is a command read and executed when the shell
receives signal(s) sig. (Note that arg is scanned once
when the trap is set and once when the trap is taken.)
Each sig can be given as a number or name of the
signal. Trap commands are executed in signal number
order. Any attempt to set a trap on a signal that was
ignored upon entering the current shell has no effect.
If arg is omitted or is -, all traps for sig are reset
to their original values. If arg is the null string,
this signal is ignored by the shell and by the commands
it invokes. If sig is DEBUG, arg is executed after
each command. If sig is ERR, arg is executed whenever
a command has a non-zero exit code. If sig is 0 or
EXIT and the trap statement is executed inside the body
of a function, the command arg is executed after the
function completes. If sig is 0 or EXIT for a trap set
outside any function, the command arg is executed on
exit from the shell. The trap command with no
arguments prints a list of commands associated with
each signal number.

--
Muthu

Re: trap

Sivakumar TS — Tue, 14 Feb 2006 00:32:59 GMT

sorry there was a typo error..

it NOT equivallent.

and setting : dosent trap the signals.

Re: trap

Khashru — Tue, 14 Feb 2006 00:34:51 GMT

but it works. i have used : and it works.

Re: trap

Arunvijai_4 — Tue, 14 Feb 2006 00:35:29 GMT

Check this thread for Control -C

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=943834

-Arun

Re: trap

Arunvijai_4 — Tue, 14 Feb 2006 00:40:48 GMT

Hi Khashru,

Also, worth reading these pages,

http://unix.ittoolbox.com/groups/technical-functional/hp-ux-l/893492
http://unix.ittoolbox.com/groups/technical-functional/hp-ux-l/893922

-Arun