NICE(P) POSIX Programmer's Manual NICE(P)NAMEnice - invoke a utility with an altered nice value
SYNOPSISnice [-n increment] utility [argument...]
DESCRIPTION
The nice utility shall invoke a utility, requesting that it be run with
a different nice value (see the Base Definitions volume of
IEEE Std 1003.1-2001, Section 3.239, Nice Value). With no options and
only if the user has appropriate privileges, the executed utility shall
be run with a nice value that is some implementation-defined quantity
less than or equal to the nice value of the current process. If the
user lacks appropriate privileges to affect the nice value in the
requested manner, the nice utility shall not affect the nice value; in
this case, a warning message may be written to standard error, but this
shall not prevent the invocation of utility or affect the exit status.
OPTIONS
The nice utility shall conform to the Base Definitions volume of
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following option is supported:
-n increment
A positive or negative decimal integer which shall have the same
effect on the execution of the utility as if the utility had
called the nice() function with the numeric value of the incre‐
ment option-argument.
OPERANDS
The following operands shall be supported:
utility
The name of a utility that is to be invoked. If the utility op‐
erand names any of the special built-in utilities in Special
Built-In Utilities , the results are undefined.
argument
Any string to be supplied as an argument when invoking the util‐
ity named by the utility operand.
STDIN
Not used.
INPUT FILES
None.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of nice:
LANG Provide a default value for the internationalization variables
that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
ables for the precedence of internationalization variables used
to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all
the other internationalization variables.
LC_CTYPE
Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as
opposed to multi-byte characters in arguments).
LC_MESSAGES
Determine the locale that should be used to affect the format
and contents of diagnostic messages written to standard error.
NLSPATH
Determine the location of message catalogs for the processing of
LC_MESSAGES .
PATH Determine the search path used to locate the utility to be
invoked. See the Base Definitions volume of
IEEE Std 1003.1-2001, Chapter 8, Environment Variables.
ASYNCHRONOUS EVENTS
Default.
STDOUT
Not used.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
If utility is invoked, the exit status of nice shall be the exit status
of utility; otherwise, the nice utility shall exit with one of the fol‐
lowing values:
1-125 An error occurred in the nice utility.
126 The utility specified by utility was found but could not be
invoked.
127 The utility specified by utility could not be found.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
The only guaranteed portable uses of this utility are:
nice utility
Run utility with the default lower nice value.
nice-n <positive integer> utility
Run utility with a lower nice value.
On some implementations they have no discernible effect on the invoked
utility and on some others they are exactly equivalent.
Historical systems have frequently supported the <positive integer> up
to 20. Since there is no error penalty associated with guessing a num‐
ber that is too high, users without access to the system conformance
document (to see what limits are actually in place) could use the his‐
torical 1 to 20 range or attempt to use very large numbers if the job
should be truly low priority.
The nice value of a process can be displayed using the command:
ps -onice
The command, env, nice, nohup, time, and xargs utilities have been
specified to use exit code 127 if an error occurs so that applications
can distinguish "failure to find a utility" from "invoked utility
exited with an error indication". The value 127 was chosen because it
is not commonly used for other meanings; most utilities use small val‐
ues for "normal error conditions" and the values above 128 can be con‐
fused with termination due to receipt of a signal. The value 126 was
chosen in a similar manner to indicate that the utility could be found,
but not invoked. Some scripts produce meaningful error messages differ‐
entiating the 126 and 127 cases. The distinction between exit codes 126
and 127 is based on KornShell practice that uses 127 when all attempts
to exec the utility fail with [ENOENT], and uses 126 when any attempt
to exec the utility fails for any other reason.
EXAMPLES
None.
RATIONALE
Due to the text about the limits of the nice value being implementa‐
tion-defined, nice is not actually required to change the nice value of
the executed command; the limits could be zero differences from the
system default, although the implementor is required to document this
fact in the conformance document.
The 4.3 BSD version of nice does not check whether increment is a valid
decimal integer. The command nice-x utility, for example, would be
treated the same as the command nice--1 utility. If the user does not
have appropriate privileges, this results in a "permission denied"
error. This is considered a bug.
When a user without appropriate privileges gives a negative increment,
System V treats it like the command nice-0 utility, while 4.3 BSD
writes a "permission denied" message and does not run the utility. Nei‐
ther was considered clearly superior, so the behavior was left unspeci‐
fied.
The C shell has a built-in version of nice that has a different inter‐
face from the one described in this volume of IEEE Std 1003.1-2001.
The term "utility" is used, rather than "command", to highlight the
fact that shell compound commands, pipelines, and so on, cannot be
used. Special built-ins also cannot be used. However, "utility"
includes user application programs and shell scripts, not just utili‐
ties defined in this volume of IEEE Std 1003.1-2001.
Historical implementations of nice provide a nice value range of 40 or
41 discrete steps, with the default nice value being the midpoint of
that range. By default, they lower the nice value of the executed util‐
ity by 10.
Some historical documentation states that the increment value must be
within a fixed range. This is misleading; the valid increment values on
any invocation are determined by the current process nice value, which
is not always the default.
The definition of nice value is not intended to suggest that all pro‐
cesses in a system have priorities that are comparable. Scheduling
policy extensions such as the realtime priorities in the System Inter‐
faces volume of IEEE Std 1003.1-2001 make the notion of a single under‐
lying priority for all scheduling policies problematic. Some implemen‐
tations may implement the nice-related features to affect all processes
on the system, others to affect just the general time-sharing activi‐
ties implied by this volume of IEEE Std 1003.1-2001, and others may
have no effect at all. Because of the use of "implementation-defined"
in nice and renice, a wide range of implementation strategies are pos‐
sible.
FUTURE DIRECTIONS
None.
SEE ALSO
Shell Command Language , renice , the System Interfaces volume of
IEEE Std 1003.1-2001, nice()COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 NICE(P)
