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PROLOG
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NAME
time — time a simple command
SYNOPSIS
time [−p] utility [argument...]
DESCRIPTION
The time utility shall invoke the utility named by the utility operand with arguments supplied as the
argument operands and write a message to standard error that lists timing statistics for the utility. The
message shall include the following information:
* The elapsed (real) time between invocation of utility and its termination.
* The User CPU time, equivalent to the sum of the tms_utime and tms_cutime fields returned by the
times() function defined in the System Interfaces volume of POSIX.1‐2008 for the process in which
utility is executed.
* The System CPU time, equivalent to the sum of the tms_stime and tms_cstime fields returned by the
times() function for the process in which utility is executed.
The precision of the timing shall be no less than the granularity defined for the size of the clock tick
unit on the system, but the results shall be reported in terms of standard time units (for example, 0.02
seconds, 00:00:00.02, 1m33.75s, 365.21 seconds), not numbers of clock ticks.
When time is used as part of a pipeline, the times reported are unspecified, except when it is the sole
command within a grouping command (see Section 2.9.4.1, Grouping Commands) in that pipeline. For example,
the commands on the left are unspecified; those on the right report on utilities a and c, respectively:
time a | b | c { time a; } | b | c
a | b | time c a | b | (time c)
OPTIONS
The time utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section 12.2, Utility
Syntax Guidelines.
The following option shall be supported:
−p Write the timing output to standard error in the format shown in the STDERR section.
OPERANDS
The following operands shall be supported:
utility The name of a utility that is to be invoked. If the utility operand names any of the special
built-in utilities in Section 2.14, Special Built-In Utilities, the results are undefined.
argument Any string to be supplied as an argument when invoking the utility named by the utility
operand.
STDIN
Not used.
INPUT FILES
None.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of time:
LANG Provide a default value for the internationalization variables that are unset or null. (See the
Base Definitions volume of POSIX.1‐2008, Section 8.2, Internationalization 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 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 and
informative messages written to standard error.
LC_NUMERIC
Determine the locale for numeric formatting.
NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.
PATH Determine the search path that shall be used to locate the utility to be invoked; see the Base
Definitions volume of POSIX.1‐2008, Chapter 8, Environment Variables.
ASYNCHRONOUS EVENTS
Default.
STDOUT
Not used.
STDERR
The standard error shall be used to write the timing statistics. If −p is specified, the following format
shall be used in the POSIX locale:
"real %f\nuser %f\nsys %f\n", <real seconds>, <user seconds>,
<system seconds>
where each floating-point number shall be expressed in seconds. The precision used may be less than the
default six digits of %f, but shall be sufficiently precise to accommodate the size of the clock tick on
the system (for example, if there were 60 clock ticks per second, at least two digits shall follow the
radix character). The number of digits following the radix character shall be no less than one, even if
this always results in a trailing zero. The implementation may append white space and additional
information following the format shown here. The implementation may also prepend a single empty line
before the format shown here.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
If the utility utility is invoked, the exit status of time shall be the exit status of utility;
otherwise, the time utility shall exit with one of the following values:
1‐125 An error occurred in the time 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 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 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.
EXAMPLES
It is frequently desirable to apply time to pipelines or lists of commands. This can be done by placing
pipelines and command lists in a single file; this file can then be invoked as a utility, and the time
applies to everything in the file.
Alternatively, the following command can be used to apply time to a complex command:
time sh −c 'complex-command-line'
RATIONALE
When the time utility was originally proposed to be included in the ISO POSIX‐2:1993 standard, questions
were raised about its suitability for inclusion on the grounds that it was not useful for conforming
applications, specifically:
* The underlying CPU definitions from the System Interfaces volume of POSIX.1‐2008 are vague, so the
numeric output could not be compared accurately between systems or even between invocations.
* The creation of portable benchmark programs was outside the scope this volume of POSIX.1‐2008.
However, time does fit in the scope of user portability. Human judgement can be applied to the analysis
of the output, and it could be very useful in hands-on debugging of applications or in providing
subjective measures of system performance. Hence it has been included in this volume of POSIX.1‐2008.
The default output format has been left unspecified because historical implementations differ greatly in
their style of depicting this numeric output. The −p option was invented to provide scripts with a common
means of obtaining this information.
In the KornShell, time is a shell reserved word that can be used to time an entire pipeline, rather than
just a simple command. The POSIX definition has been worded to allow this implementation. Consideration
was given to invalidating this approach because of the historical model from the C shell and System V
shell. However, since the System V time utility historically has not produced accurate results in
pipeline timing (because the constituent processes are not all owned by the same parent process, as
allowed by POSIX), it did not seem worthwhile to break historical KornShell usage.
The term utility is used, rather than command, to highlight the fact that shell compound commands,
pipelines, special built-ins, and so on, cannot be used directly. However, utility includes user
application programs and shell scripts, not just the standard utilities.
FUTURE DIRECTIONS
None.
SEE ALSO
Chapter 2, Shell Command Language, sh
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment Variables, Section 12.2, Utility
Syntax Guidelines
The System Interfaces volume of POSIX.1‐2008, times()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
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