Shell Programming and Scripting

What does $! return?

Killing background processes

Code:

root@:~# sleep 100 &
[1] 13829
root@:~# echo $!
13829
root@:~# kill $!
root@:~#

Thanks...
Does it kill all the background process or the latest?

only the latest. but why didnt you try yourself ?. If you want to come along with unix you need to be keen to experiment. relying on the forum may be comfortable, but dosnt bring you any further.

Thanks,

But i have a query here
Please view this code:-

#!/usr/bin/ksh
read a &
sleep 5
kill $!


Now in these 5 secs if there is another background process invoked in the system will that process be killed?

no $! is only set within the current shell processes running in different shells are not affected. there might be a problem with your script when you sleep to long and murphy is on the way. for ex. when you sleep 3600 and read finishes after a few seconds another process may have been executed on the system using the same pid. if you want to be precize you have to test if $! points to your read process.
* is the process $PID a child of this shell ?
you can determine the PID of your shell with echo $$

to make it easy

Code:

read ....
for i in 1 2 3 4 ; do 
 sleep $i
 pgrep -P $$ read || break  # stop waiting if read has finished
done
 pgrep -P $$ read && pkill -P $$ read