How to increase the priority of a running process in Linux

Kernel decides how much processor time is required for a process based on
the nice value. Possible nice value range is: -20 to 20. A process that has a
nice value of -20 is very high priority. The process that has a nice value of 20
is very low priority.

$ ps axl
Use ps axl to display the nice value of all running process.
F   UID   PID  PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
4     0     1     0  20   0   2100   724 -      Ss   ?          0:06 init [2]  
5     0     2     0  15  -5      0     0 -      S<   ?          0:00 [kthreadd]
1     0    47     2  15  -5      0     0 -      S<   ?          0:00 [kacpid]
1     0    48     2  15  -5      0     0 -      S<   ?          0:00 [kacpi_notify]
1     0   125     2  15  -5      0     0 -      S<   ?          0:00 [kseriod]
1     0   166     2  20   0      0     0 -      S    ?          0:00 [pdflush]
[Note: 6th column 'NI' is the nice.]

To increase the priority, give a lower nice value as shown below. However, only root can increase the priority of a running process, else you’ll get error messages.

$ ps axl | grep test.sh
1     0    12345     2  15  10      0     0 -      S<   ?     0:00 test.sh
[Note: 6th column with value 10 is the nice value for the shell-script.]

Now decrease the nice value to increase the priority of the process. Use the PID of the process.

$ su -
# renice -5 -p 12345

$ ps axl | grep test.sh 

1     0    12345     2  15  -5      0     0 -      S<   ?     0:00 test.sh
[ Note: The 6th column now shows a lower nice value of -5 (increased priority)]