Lower a Command's Priority with nice

A batch job hogging the CPU while someone else works is the exact problem nice was built to solve. Start the program at a less favourable scheduling priority and let interactive work jump the queue. It does not cap CPU usage, pause a process, or make a command safe to run as root.

You need a POSIX-like shell, GNU coreutils nice, and the ps command for the verification examples. Allow about five minutes. The examples only start short-lived shell processes and write no files, so no elevated privileges are needed.

1. Check the current niceness

Run nice without a command. It prints the niceness of the current shell process:

$ /usr/bin/nice
0

GNU nice accepts values from -20 to 19. A lower number is more favourable to the process; a higher number makes it more willing to yield when competing for CPU. The normal starting value is usually 0, but the inherited value is what actually matters for a particular shell.

2. Start one command with a lower priority

Pass an adjustment with -n or --adjustment. The adjustment is added to the inherited niceness. With no option, GNU nice adds 10:

$ /usr/bin/nice -n 7 sh -c 'ps -o ni= -p $$'
  7
$ /usr/bin/nice sh -c 'ps -o ni= -p $$'
 10

• First command starts a shell at niceness 7.
• Second command uses the default adjustment and starts its child at niceness 10.
• The leading spaces in ps's output are display padding, not part of the value.

Put the command and its arguments after the adjustment, and quote anything that belongs to the child shell, as in this harmless status example:

$ /usr/bin/nice -n 12 sh -c 'printf "child niceness: "; ps -o ni= -p $$'
child niceness:   12

3. Choose the adjustment carefully

For ordinary background work, a positive adjustment such as 5, 10 or 12 is the usual range. It changes the scheduling preference for this process and any descendants that inherit it. It does not guarantee less CPU: if the machine is otherwise idle, the program can still use an available core.

Do not treat negative adjustments as a routine performance switch. A negative value asks for more favourable scheduling, and an unprivileged user normally cannot grant itself that. Test the request without hiding the diagnostic:

$ /usr/bin/nice -n -5 true
/usr/bin/nice: cannot set niceness: Permission denied
$ printf 'exit status: %s\n' "$?"
exit status: 125

The exact diagnostic can vary by system, but GNU nice documents status 125 for a failure in nice itself. An administrator may be able to use a negative adjustment under the host's policy; do not add sudo merely to make a batch job faster. Elevated privileges change the security boundary and can give the child access it did not otherwise have.

4. Keep the shell and exit status straight

Some shells provide their own nice builtin, which can take different options. Compare or bypass it with command -V nice and the explicit /usr/bin/nice path:

$ command -V nice
$ /usr/bin/nice --version
nice (GNU coreutils) 9.4

The installed package here is coreutils 9.4. The command's documented exit status is normally the exit status of the child, so a successful launch does not turn a failed workload into success. These two checks show the distinction:

$ /usr/bin/nice sh -c 'exit 23'
$ printf 'child status: %s\n' "$?"
child status: 23
$ /usr/bin/nice definitely-not-a-command >/dev/null 2>&1
$ printf 'missing command status: %s\n' "$?"
missing command status: 127

GNU nice uses 127 when the command cannot be found and 126 when it is found but cannot be invoked. It uses 125 for a failure in nice itself. Preserve and check these statuses in scripts instead of checking only whether the child printed anything.

5. Undo the example

There is no persistent configuration to undo: niceness belongs to a process, and it disappears the moment the process exits. To stop a long-running test, use its normal application shutdown or send a signal to the specific process after checking its PID. Do not kill a production process just because it happens to have a different niceness.

For a command you have not yet run, just drop the /usr/bin/nice ... prefix. For a command already running at a lower priority, renice is the separate tool for changing an existing process, with its own permissions and targeting risks.