nice (Unix)

nice
Developer(s)	AT&T Bell Laboratories
Initial release	November 1973; 51 years ago
Operating system	Unix and Unix-like
Platform	Cross-platform
Type	Command
License	coreutils: GNU GPL v3 4.4BSD: BSD License

nice is a program found on Unix and Unix-like operating systems such as Linux. It directly maps to a kernel call of the same name. nice is used to invoke a utility or shell script with a particular CPU priority, thus giving the process more or less CPU time than other processes. A niceness of -20 is the lowest niceness, or highest priority. The default niceness for processes is inherited from its parent process and is usually 0.

Systems have diverged on what priority is the lowest. Linux systems document a niceness of 19 as the lowest priority,^[1] BSD systems document 20 as the lowest priority.^[2] In both cases, the "lowest" priority is documented as running only when nothing else wants to.

Etymology

Niceness value is a number attached to processes in *nix systems, that is used along with other data (such as the amount of I/O done by each process) by the kernel process scheduler to calculate a process' 'true priority'—which is used to decide how much CPU time is allocated to it.

The program's name, nice, is an allusion to its task of modifying a process' niceness value.

The term niceness itself originates from the idea that a process with a higher niceness value is nicer to other processes in the system and to users by virtue of demanding less CPU power—freeing up processing time and power for the more demanding programs, who would in this case be less nice to the system from a CPU usage perspective.^[3]

Use and effect

nice becomes useful when several processes are demanding more resources than the CPU can provide. In this state, a higher-priority process will get a larger chunk of the CPU time than a lower-priority process. Only the superuser (root) may set the niceness to a lower value (i.e. a higher priority). On Linux it is possible to change /etc/security/limits.conf to allow other users or groups to set low nice values.^[4]

If a user wanted to compress a large file without slowing down other processes, they might run the following:

$ nice -n 19 tar cvzf archive.tgz largefile

The exact mathematical effect of setting a particular niceness value for a process depends on the details of how the scheduler is designed on that implementation of Unix. A particular operating system's scheduler will also have various heuristics built into it (e.g. to favor processes that are mostly I/O-bound over processes that are CPU-bound). As a simple example, when two otherwise identical CPU-bound processes are running simultaneously on a single-CPU Linux system, each one's share of the CPU time will be proportional to 20 − p, where p is the process' priority. Thus a process, run with nice +15, will receive 25% of the CPU time allocated to a normal-priority process: (20 − 15)/(20 − 0) = 0.25.^[5] On the BSD 4.x scheduler, on the other hand, the ratio in the same example is about ten to one.^{[citation needed]}

Similar commands

The related renice program can be used to change the priority of a process that is already running.^[1]

Linux also has an ionice program, which affects scheduling of I/O rather than CPU time.^[6]

References

^ ^a ^b renice(1) – Linux General Commands Manual
^ "renice(8) - NetBSD Man Pages". NetBSD. October 22, 2020. Retrieved April 18, 2023.
^ Jerry Peek, Shelley Powers, Tim O'Reilly and Mike Loukides (2002). Unix Power Tools. O'Reilly, p. 507.
^ limits.conf(5) – Linux File Formats Manual
^ Silberschatz, Abraham; Galvin, Peter B.; Gagne, Greg (2013). Operating system concepts (ninth ed.). Hoboken, N.J: Wiley. p. 294. ISBN 978-1-118-06333-0.
^ ionice(1) – Linux General Commands Manual

External links

nice: invoke a utility with an altered nice value – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group

≥

[linuxrenice-1] renice(1) – Linux General Commands Manual

[2] "renice(8) - NetBSD Man Pages". NetBSD. October 22, 2020. Retrieved April 18, 2023.

[3] Jerry Peek, Shelley Powers, Tim O'Reilly and Mike Loukides (2002). Unix Power Tools. O'Reilly, p. 507.

[4] limits.conf(5) – Linux File Formats Manual

[5] Silberschatz, Abraham; Galvin, Peter B.; Gagne, Greg (2013). Operating system concepts (ninth ed.). Hoboken, N.J: Wiley. p. 294. ISBN 978-1-118-06333-0.

[6] ionice(1) – Linux General Commands Manual

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v t e Unix command-line interface programs and shell builtins
File system	cat chattr chmod chown chgrp cksum cmp cp dd du df file fuser ln ls mkdir mv pax pwd rm rmdir split tee touch type umask
Processes	at bg crontab fg kill nice ps time
User environment	env exit logname mesg talk tput uname who write
Text processing	awk basename comm csplit cut diff dirname ed ex fold head iconv join m4 more nl paste patch printf read sed sort strings tail tr troff uniq vi wc xargs
Shell builtins	alias cd echo test unset wait
Searching	find grep
Documentation	man
Software development	ar ctags lex make nm strip yacc
Miscellaneous	bc cal expr lp od sleep true and false
Categories Standard Unix programs Unix SUS2008 utilities List

v t e GNU Core Utilities command-line interface programs
File system	chcon chmod chown chgrp cksum cp dd df dir dircolors install ln ls mkdir mkfifo mknod mktemp mv realpath rm rmdir shred sync touch truncate vdir
Text utilities	b2sum base32 base64 cat cksum comm csplit cut expand fmt fold head join md5sum nl numfmt od paste ptx pr sha1sum shuf sort split sum tac tail tr tsort unexpand uniq wc
Shell utilities	arch basename chroot date dirname du echo env expr factor false groups hostid id link logname nice nohup nproc pathchk pinky printenv printf pwd readlink runcon seq sleep stat stdbuf stty tee test timeout true tty uname unlink uptime users who whoami yes

Etymology

Use and effect

Similar commands

See also

References

External links