Why does kill not work from script, but does work from terminal?

Question

I have the following contrived script to illustrate my issue:

#!/bin/bash
set -eux
sudo sleep 120 &
spid=$!
sleep 1
sudo kill $spid
wait $!

This will print

$ ./test.sh 
+ spid=21931
+ sleep 1
+ sudo sleep 120
+ sudo kill 21931
+ wait 21931

and then hang on wait until the sleep 120 times out. However, when I run sudo kill 21931 from another terminal the sleep process is killed immediately. I expected the sudo kill $spid line in the script to also kill the sleep process immediately. Why doesn't this work and how do I make this work?

(Might be relevant: I see this behaviour bash 4.3.42 and dash 0.5.7 on Ubuntu 15.10.)

Answer 1

the reason could be found in source code of sudo:

/*
 * Do not forward signals sent by a process in the command's process
 * group, as we don't want the command to indirectly kill itself.
 * For example, this can happen with some versions of reboot that
 * call kill(-1, SIGTERM) to kill all other processes.
 */

Answer 2

The difference could be in the /etc/sudoers file. Maybe your user is allowed to run kill with nopasswd on one machine, but not on the other.

Answer 3

I confirm this strange behaviour occurs as well on Ubuntu 17.10 bash 4.4.12(1)-release (x86_64-pc-linux-gnu).

I wasn't able to track down the real reason but it seems it has something to do with process signals. By default, kill sends out signal 15 – TERM which stands for a soft kill. However, if I use a hard 9 – KILL, the sleep process gets immediately killed.

#!/bin/bash
set -eux
sudo sleep 120 &
spid=$!
sleep 1
sudo kill -9 $spid
wait $!

prints out

$ ./test.sh 
+ spid=4342
+ sleep 1
+ sudo sleep 120
+ sudo kill -9 4342
./test.sh: line 6:  4342 Killed                  sudo sleep 120
+ wait 4342

Answer 4

I had a similar problem and landed here. The other answers didn't help. My script was piping IDs into kill along the lines of:

echo $some_output | awk '{print $1}' | xargs kill -9

This worked from the terminal but not from a script. After finding this answer that said kill doesn't read PIDs from stdin, I changed the script to this and it fixed the problem:

pids=`echo $some_output | awk '{print $1}'`

for p in $pids; do
    echo "Killing $p"
    kill -9 $p
done

Hopefully, that helps someone who landed here like I did!