Ubuntu Manpage: time - time a simple command

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 IEEE Std 1003.1-2001 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 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 IEEE Std 1003.1-2001, 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 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 IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001, 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.

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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001.

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
IEEE Std 1003.1-2001.

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.

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                                               TIME(P)