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Linux 2.6 - man page for nice (linux section 1posix)

NICE(P) 			    POSIX Programmer's Manual				  NICE(P)

       nice - invoke a utility with an altered nice value

       nice [-n increment] utility [argument...]

       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.

       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 exe-
	      cution of the utility as if the utility had called the  nice()  function	with  the
	      numeric value of the increment option-argument.

       The following operands shall be supported:

	      The  name  of  a utility that is to be invoked. If the utility operand names any of
	      the special built-in utilities in Special Built-In  Utilities  ,	the  results  are

	      Any  string  to  be  supplied as an argument when invoking the utility named by the
	      utility operand.

       Not used.


       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, Inter-
	      nationalization Variables 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 interna-
	      tionalization variables.

	      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 argu-

	      Determine the locale that should be used to affect the format and contents of diag-
	      nostic messages written to standard error.

	      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.


       Not used.

       The standard error shall be used only for diagnostic messages.



       If utility is invoked, the exit status of nice shall be the exit status of utility; other-
       wise, the nice utility shall exit with one of the following 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.


       The following sections are informative.

       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 number that is too high, users without
       access to the system conformance document (to see what limits are actually in place) could
       use the historical 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 -o nice

       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 util-
       ity" 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 values for
       "normal error conditions" and the values above 128 can be confused 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
       differentiating	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.


       Due  to	the text about the limits of the nice value being implementation-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 docu-
       ment 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  "permis-
       sion 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. Neither was considered clearly superior, so the behavior was
       left unspecified.

       The C shell has a built-in version of nice that has a different	interface  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 com-
       pound 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
       utilities 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 utility 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  processes  in	a  system
       have  priorities  that  are comparable.	Scheduling policy extensions such as the realtime
       priorities in the System Interfaces volume of IEEE Std 1003.1-2001 make the  notion  of	a
       single  underlying  priority for all scheduling policies problematic. Some implementations
       may implement the nice-related features to affect all processes on the system,  others  to
       affect	just   the   general   time-sharing   activities   implied   by  this  volume  of
       IEEE Std 1003.1-2001, and others may have no effect at all. Because of the use of  "imple-
       mentation-defined"  in nice and renice, a wide range of implementation strategies are pos-


       Shell Command Language , renice , the System Interfaces	volume	of  IEEE Std 1003.1-2001,

       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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					  NICE(P)

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