NAME [Toc] [Back]
nice - run a command at nondefault priority
SYNOPSIS [Toc] [Back]
nice [-priority_change] command [command_args]
nice [-n priority_change] command [command_args]
DESCRIPTION [Toc] [Back]
The nice command executes command at a nondefault CPU scheduling
priority. (The name is derived from being "nice" to other system
users by running large programs at lower priority.)
Arguments [Toc] [Back]
The command-line arguments are as follows:
priority_change The difference between the system nice value
(relative priority) of the current (or parent)
process and the actual system nice value at which
command is to run.
An unsigned value increases the system nice value
for command, causing it to run at lower priority.
A negative value requires superuser privileges,
and assigns a lower system nice value (higher
priority) to command. If the current process is
not privileged, the value is silently treated as
if it were 0.
If the value of priority_change would result in a
system nice value outside the range 0 through 39,
the corresponding limit value of 0 or 39 is used
Note that a positive priority_change (lower
priority) has a single - option character before
the numeric value; a negative (higher priority)
priority_change has two: the option character
followed by the minus sign (--). If -
priority_change is not specified, it defaults to
command A program, HP-UX command, user shell script, etc.
to be executed at the nondefault priority.
command can be run as a foreground or background
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If command is run as a background process, any
nice priority_change made by the shell (ksh
executes all background processes via nice -4) is
in addition to that specified in the nice command
command_args Any arguments recognized by command.
Process Priorities [Toc] [Back]
All processes have an associated system nice value which is used to
compute the instantaneous-priority of the process when it is scheduled
to run. Normally, all processes inherit the system nice value of
their parent process when they are spawned. The shell (sh, csh, ksh,
etc.) can create a child process with a different priority from the
current shell process by spawning the child process via the nice
command. If the priority_change value is unsigned (positive), the
child process is nicer (lower in priority) relative to the parent. If
the priority_change value is negative, the child process runs at a
higher priority with a greater share of available system resources.
To spawn a higher priority child process, the parent process must be
owned by a user who has the appropriate privileges.
At boot-up, the system starts the init process at a system nice value
of 20 (system default). On most systems, all processes (down to the
login shells) inherit this priority. Starting from their individual
login shell processes, users can alter the system nice value of
descendent processes to as much as 39, or, with appropriate
privileges, as little as 0. A system nice value of 0 establishes an
extremely high priority, whereas a value of 39 indicates a very low
Ordinary users can only increase the system nice value of any child
process relative to the current process; i.e., priority_change must be
a positive (unsigned) value, resulting in a lower priority. To start
a child process at a lower system nice value (higher priority) than
the current process, the user must have the appropriate privileges,
regardless of the relative nice-priority value desired.
For example, using the command
from a login shell whose current nice value is 20 spawns a subshell
with a system nice value of 30. Attempting to use
nice --2 ksh
from the new shell to spawn another subshell whose system nice value
would be 28, is rejected (unless the user has appropriate privileges),
even though the resulting system nice value would be less than the
priority of the original login shell process.
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The system nice value for current processes is listed under the NI
column produced by the ps -l command (see ps(1)).
Background Processes [Toc] [Back]
Foreground processes are run at same system nice value as the parent
shell. Background processes spawned by ksh run at the equivalent of a
nice -4 by default. If a background process is started via nice from
ksh, any priority_change specified in the nice command is added to
default nice -4. Thus the command
nice 12 command &
runs at a system nice value of 36 if executed from ksh.
EXTERNAL INFLUENCES [Toc] [Back]
LC_MESSAGES determines the language in which messages are displayed.
If LC_MESSAGES is not specified in the environment or is set to the
empty string, the value of LANG is used as a default for each
unspecified or empty variable. If LANG is not specified or is set to
the empty string, a default of "C" (see lang(5)) is used instead of
If any internationalization variable contains an invalid setting, nice
behaves as if all internationalization variables are set to "C". See
International Code Set Support [Toc] [Back]
Single- and multi-byte character code sets are supported.
RETURN VALUE [Toc] [Back]
nice returns the value returned by command.
EXAMPLES [Toc] [Back]
The following examples assume the current process is running with a
system nice value of 20 and nice is executed from the Korn shell (see
Run a program named prog in the current directory at the default
priority_change of 10 (system nice value of 30):
nice ./prog prog_args
Run the same program in the background using a system nice value of 36
(priority_change=12 plus 4 for the Korn shell):
nice -12 ./prog prog_args &
As a user with appropriate privileges, run prog as a foreground
process with a system nice value of 6:
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nice --14 ./prog prog_args
WARNINGS [Toc] [Back]
The C shell, csh, has a built-in nice command with different syntax.
See csh(1) for details.
SEE ALSO [Toc] [Back]
csh(1), ksh(1), nohup(1), sh-posix(1), sh(1), renice(1M), nice(2).
STANDARDS CONFORMANCE [Toc] [Back]
nice: SVID2, SVID3, XPG4
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