Signals, kill and process priority

What is a signal?

A signal is a tiny notification the kernel delivers to a process. The process can catch most signals and react: save its work, close files, then exit. Two signals cannot be caught: SIGKILL ends the process immediately, and SIGSTOP pauses it.

The numbers above are for Linux on x86 and ARM. Names are safer than numbers in scripts: kill -TERM is clearer than kill -15. kill -l lists all signals.

Sending signals

$ kill 9812                 # SIGTERM: please stop
$ kill -SIGTERM 9812         # the same, written out
$ kill -9 9812               # SIGKILL: only if it ignores TERM
$ pkill -u reports java      # by name, only processes of user reports
$ killall httpd              # every process named httpd
$ pkill -t pts/3             # everything on terminal pts/3 (log a user out)
$ pgrep -a sshd              # check before you pkill
842 sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups

For services managed by systemd, use systemctl stop or systemctl kill instead of kill. systemd knows all the processes of the service and will not restart it unexpectedly.

Priority: nice and renice

When several processes want the CPU, the scheduler gives more time to those with higher priority. The nice value is how you influence it. It goes from -20 (least nice to others, most CPU) to 19 (very nice, gives way). The default is 0. A normal user can only make their processes nicer (raise the number); only root can lower it below the current value.

$ nice -n 10 tar -czf /backup/home.tgz /home     # start with nice 10
$ ps -o pid,ni,comm -p 7001
  PID  NI COMMAND
 7001  10 tar
# renice -n -5 -p 1301                            # root: give PID 1301 more priority
1301 (process ID) old priority 0, new priority -5
$ top     # the NI column shows the nice value; PR is the kernel priority

nice only matters when the CPU is busy. On an idle server a nice 19 job runs at full speed. It does not limit memory or disk use; for those, systemd resource controls (for example CPUQuota= or MemoryMax= in a unit) are the modern tool.

Common mistakes

• Using kill -9 as the first choice. Use it only when TERM does not work.
• Thinking a higher nice number means higher priority. It is the opposite: 19 is the lowest priority.
• Trying renice -n -5 as a normal user and getting "Permission denied". Only root can lower the nice value.