How to shield the kill output [duplicate]

Question

I run a script like:

sleep 20 &
PID=$!
kill -9 $PID >/dev/null 2>&1

I dont want the script show the output like:

line 51: 22943 Killed  sleep

I have no idea why this happen, I have redirect the output to /dev/null

Answer 1

The message isn't coming from either kill or the background command, it's coming from bash when it discovers that one of its background jobs has been killed. To avoid the message, use disown to remove it from bash's job control:

sleep 20 &
PID=$!
disown $PID
kill -9 $PID

Answer 2

This can be done using 'wait' + redirection of wait to /dev/null :

sleep 2 &
PID=$!
kill -9 $PID
wait $PID 2>/dev/null
sleep 2
sleep 2
sleep 2

This script will not give the "killed" message:

-bash-4.1$ ./test
-bash-4.1$ 

While, if you try to use something like:

sleep 2 &
PID=$!
kill -9 $PID 2>/dev/null
sleep 2
sleep 2
sleep 2

It will output the message:

-bash-4.1$ ./test
./test: line 4:  5520 Killed                  sleep 2
-bash-4.1$

I like this solution much more than using 'disown' which may have other implications.

Idea source: https://stackoverflow.com/a/5722850/1208218