nice and renice Basics - Controlling Process Priority

nice and renice Basics - Controlling Process Priority
Reading time: About 12 min Difficulty: Intermediate

What Are nice and renice?

nice and renice are Linux commands for controlling a process's CPU scheduling priority. Processes with higher priority receive more CPU time from the scheduler.

Key Takeaways

  • Want to run heavy processing without impacting other services? → Use nice to lower priority at launch
  • Running process consuming too much CPU? → Use renice to lower its priority dynamically
  • Regular users can only lower priority — raising it requires root privileges

What Is a Nice Value?

A nice value is an integer from -20 to 19. Lower values mean higher priority.

Nice Value Priority Typical Use
-20 Highest Real-time processing (root only)
0 Normal (default) Typical processes
10 Lower Background batch jobs
19 Lowest Run only when system is idle

Despite the name, a process with a higher nice value (like 19) is "nicer" to other processes — it voluntarily takes less CPU. Counter-intuitive, but that's the convention.

nice - Set Priority at Launch

Syntax

nice -n <nice_value> <command>

The -n option specifies how much to adjust the nice value relative to the current process's nice value. Omitting -n applies the default +10 increment (lower priority).

Examples

# Run backup with lowered priority (nice=10)
nice -n 10 tar czf /backup/home.tar.gz /home/

# Run at minimum priority (nice=19)
nice -n 19 ./long-batch-job.sh

# Omit -n flag (defaults to +10)
nice ./heavy-script.sh

Check the current nice value

nice
0

Running nice with no arguments prints the current shell's nice value.

renice - Change Priority of a Running Process

Syntax

renice -n <nice_value> -p <PID>
renice -n <nice_value> -u <username>
renice -n <nice_value> -g <groupname>

Change by PID

# Set PID 1234 to nice value 15
renice -n 15 -p 1234
1234 (process ID) old priority 0, new priority 15

Change all processes for a user

# Set all processes owned by "worker" to nice value 10
renice -n 10 -u worker

Combine with pgrep to target a process by name:

renice -n 19 -p $(pgrep heavy-job)

How to Check Nice Values

Using top

The NI column in top shows the nice value. The PR column shows the kernel's computed scheduling priority (PR = 20 + NI).

top
  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
 1234 user      30  10  102400  12000   8000 R  15.0   0.2   0:05.23 tar

Using ps

ps -eo pid,ni,comm
  PID  NI COMMAND
    1   0 systemd
 1234  10 tar
 5678   0 bash

Permission Rules

Priority adjustments are subject to the following restrictions:

Action Regular User Root
Lower priority (increase nice value) Yes Yes
Raise priority (decrease nice value) No Yes
Modify another user's processes No Yes

Attempting to raise priority as a regular user fails:

renice -n -5 -p 1234
renice: failed to set priority for 1234 (process ID): Permission denied

Practical Patterns

# Launch nightly batch at low priority to avoid impacting services
nice -n 15 /opt/scripts/nightly-backup.sh &

# A running job is too heavy — deprioritize it without stopping
renice -n 19 -p $(pgrep heavy-job)

# Compress large log file in the background without slowing other work
nice -n 10 gzip /var/log/large.log &

Avoid These Mistakes

  • Applying nice -n -20 to a production service as root — it can starve other services of CPU
  • Relying on nice alone to cap CPU usage — use cgroups or cpulimit for hard limits

Next Reading