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.1-2024 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 in any of the following circumstances, via a simple command for which the word time is the command name (see 2.9.1.1 Order of Processing), and none of the characters in the word time is quoted, the results (including parsing of later words) are unspecified:

• The simple command for which the word time is the command name includes one or more redirections (see 2.7 Redirection) or is (directly) part of a pipeline (see 2.9.2 Pipelines).

• The next word that follows time would, if the word time were not present, be recognized as a reserved word (see 2.4 Reserved Words) or a control operator (see XBD 3.85 Control Operator).

Since these limitations only apply when time is executed via a simple command for which the word time is the command name and none of the characters in the word time is quoted, they can be avoided by quoting all or part of the word time, by arranging for the command name not to be time (for example, by having the command name be a word expansion), or by executing time via another utility such as command or env.

The limitations on redirections and pipelines can also be overcome by embedding the simple command within a compound command—most commonly a grouping command (see 2.9.4.1 Grouping Commands)—and applying the redirections or piping to the compound command instead.

Note that in no circumstances where the results are specified is it possible to apply different redirections to the time utility than are applied to the utility it invokes.

The following examples (where a and b are assumed to be the names of utilities found by searching PATH ) show unspecified usages:

time a arg1 arg2 | b    # part of a pipeline
a | time -p b           # part of a pipeline
time a >/dev/null       # output redirection
</dev/null time a       # input redirection
time while anything...  # reserved word after time
time ( cmd )            # control operator after time
time;                   # control operator after time
time shift              # special built-in utility
time -p cd /            # intrinsic utility

The following examples have specified results and can be used as alternatives for the first four of the above when the time utility as specified here is intended to be invoked:

{ time a arg1 arg2; } | b
t=time; a | $t -p b
command time a >/dev/null
</dev/null \time a

OPTIONS

The time utility shall conform to XBD 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 a special built-in utility (see 2.15 Special Built-In Utilities), an intrinsic utility (see 1.7 Intrinsic Utilities), or a function (see 2.9.5 Function Definition Command), the results are unspecified.

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 XBD 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

^[XSI] Determine the location of messages objects and message catalogs.

PATH

Determine the search path that shall be used to locate the utility to be invoked; see XBD 8. Environment Variables.

ASYNCHRONOUS EVENTS

Default.

STDOUT

Not used.

STDERR

If the utility utility is invoked, the standard error shall be used to write the timing statistics and may be used to write a diagnostic message if the utility terminates abnormally; otherwise, the standard error shall be used to write diagnostic messages and may also be used to write the timing statistics.

If -p is specified, the following format shall be used for the timing statistics 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.

APPLICATION USAGE

The command, env, nice, nohup, time, timeout, and xargs utilities have been specified to use exit code 127 if a utility to be invoked cannot be found, 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.1-2024 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.1-2024.

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.1-2024.

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

2. Shell Command Language, sh

XBD 8. Environment Variables, 12.2 Utility Syntax Guidelines

XSH times

CHANGE HISTORY

First released in Issue 2.

Issue 6

This utility is marked as part of the User Portability Utilities option.

Issue 7

The time utility is moved from the User Portability Utilities option to the Base. User Portability Utilities is now an option for interactive utilities.

SD5-XCU-ERN-115 is applied, updating the example in the DESCRIPTION.

POSIX.1-2008, Technical Corrigendum 1, XCU/TC1-2008/0144 [266] is applied.

POSIX.1-2008, Technical Corrigendum 2, XCU/TC2-2008/0194 [723] is applied.

Issue 8

Austin Group Defect 267 is applied, allowing time to be a reserved word.

Austin Group Defect 1122 is applied, changing the description of NLSPATH .

Austin Group Defect 1530 is applied, changing "sh -c" to "sh -c --".

Austin Group Defect 1586 is applied, adding the timeout utility.

Austin Group Defect 1594 is applied, changing the APPLICATION USAGE section.