time command runs another program, then displays information
about the resources used by that program, collected by the system while
the program was running. You can select which information is reported
and the format in which it is shown (see section Formatting the output), or have
time save the information in a file instead of display it on the
screen (see section Using the
The resources that
time can report on fall into the general
categories of time, memory, I/O, and IPC calls. Some systems do not
provide much information about program resource use;
reports unavailable information as zero values (see section Accuracy).
Here is an example of using
time to measure the time and other
resources used by running the program
doit, which is given the
arguments `foo' and `3':
time doit foo 3
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The format of the
time command is:
time [-apvV] [-f format] [-o file] [--append] [--verbose] [--portability] [--format=format] [--output=file] [--version] [--help] command [arg...]
time runs the program command, with any given arguments
arg.... When command finishes,
information about resources used by command (on the standard error
output, by default). If command exits with non-zero status,
time displays a warning message and the exit status.
time determines which information to display about the resources
used by the command from a format string (see section Formatting the output).
If no format is specified on the command line, but the
environment variable is set, its value is used as the format.
Otherwise, a default format built into
time is used
(see section Formatting the output).
time must appear on the command line before
command. Anything on the command line after command is
passed as arguments to command.
time. See section Formatting the output, for more information.
real %e user %U sys %S
The format string controls the contents of the
The format string can be set using the `-f' or `--format',
`-v' or `--verbose', or `-p' or `--portability'
options. If they are not given, but the
variable is set, its value is used as the format string. Otherwise, a
built-in default format is used. The default format is:
%Uuser %Ssystem %Eelapsed %PCPU (%Xtext+%Ddata %Mmax)k %Iinputs+%Ooutputs (%Fmajor+%Rminor)pagefaults %Wswaps
The format string usually consists of resource specifiers
interspersed with plain text. A percent sign (`%') in the format
string causes the following character to be interpreted as a resource
specifier, which is similar to the formatting characters in the C
A backslash (`\') introduces a backslash escape, which is translated into a single printing character upon output. `\t' outputs a tab character, `\n' outputs a newline, and `\\' outputs a backslash. A backslash followed by any other character outputs a question mark (`?') followed by a backslash, to indicate that an invalid backslash escape was given.
Other text in the format string is copied verbatim to the output.
time always prints a newline after printing the resource use
information, so normally format strings do not end with a newline
character (or `\n').
There are many resource specifications. Not all resources are measured by all versions of Unix, so some of the values might be reported as zero. Any character following a percent sign that is not listed in the table below causes a question mark (`?') to be output, followed by that character, to indicate that an invalid resource specifier was given.
The resource specifiers, which are a superset of those recognized by the
time command, are:
To run the command `wc /etc/hosts' and show the default information:
time wc /etc/hosts
To run the command `ls -Fs' and show just the user, system, and total time:
time -f "\t%E real,\t%U user,\t%S sys" ls -Fs
To edit the file bork and have
time append the elapsed time
and number of signals to the file `log', reading the format string
from the environment variable
export TIME="\t%E,\t%k" # If using bash or ksh setenv TIME "\t%E,\t%k" # If using csh or tcsh time -a -o log emacs bork
The elapsed time is not collected atomically with the execution of the
program; as a result, in bizarre circumstances (if the
command gets stopped or swapped out in between when the program being
timed exits and when
time calculates how long it took to run), it
could be much larger than the actual execution time.
When the running time of a command is very nearly zero, some values (e.g., the percentage of CPU used) may be reported as either zero (which is wrong) or a question mark.
Most information shown by
time is derived from the
system call. The numbers are only as good as those returned by
wait3. On systems that do not have a
wait3 call that
returns status information, the
times system call is used
instead. However, it provides much less information than
wait3, so on those systems
time reports the majority
of the resources as zero.
The `%I' and `%O' values are allegedly only "real" input and output and do not include those supplied by caching devices. The meaning of "real" I/O reported by `%I' and `%O' may be muddled for workstations, especially diskless ones.