Signal

Bash Shell#

By default, the Bash Shell ignores the SIGQUIT(3) and SIGTERM(15) signals, so executing the following commands will not have any effect ($$ is the process ID of the current Shell).

kill -3 $$

kill -15 $$

If the SIGHUP(1) signal is received, the Bash Shell will exit, but before exiting, it will pass the SIGHUP signal to all processes and scripts started by that Shell.

kill -1 $$

TRAP#

The trap command can be used to catch signals.

trap commands signals

Catch Signals#

The SIGINT signal is the signal for Ctrl+C.

#!/usr/bin/env bash

trap "echo ' Trapped Ctrl-C'" SIGINT

count=1
while [ $count -le 5 ]
do
    echo "loop #$count"
    sleep 1
    count=$[ $count + 1 ]
done

loop #1
loop #2
^C Trapped Ctrl-C
loop #3
loop #4
^C Trapped Ctrl-C
loop #5

Exit Signals#

You can catch the signals when the script exits.

trap "echo ' Trapped Ctrl-C'" EXIT

Remove Signals#

Use -- to remove signals.

trap -- SIGINT

NOHUP#

No Hang Up, processes or scripts started by this command will ignore the SIGHUP signal. In other words, even if the terminal exits, the processes or scripts started by that terminal will not exit.

run a command immune to hangups, with output to a non-tty

nohup ./foo

By default, STDOUT and STDERR are redirected to nohup.out.

-rw------- 1 kuga kuga   40 Sep  2 18:46 nohup.out

It’s better to redirect it yourself.

nohup ./foo > out.log 2>&1

-rw-rw-r-- 1 kuga kuga   62 Sep  2 18:50 out.log

Nice Value#

A parameter that controls the priority of a process. The higher the nice value, the lower the priority of the process, and the less CPU resources it is allocated.

Range#

The nice value ranges from -20 to 19:

• -20: Highest priority.
• 0: Default priority.
• 19: Lowest priority.

Start Process Priority#

nice -n VALUE command

nice -n 10 ./foo

Modify Process Priority#

renice VALUE -p PID

renice 5 -p 404

Permissions#

• Regular users can only lower the priority of a process.
• Only root users can raise the priority of a process.