time (Unix)

time is a command in the Unix operating systems. It is used to determine the duration of execution of a particular command.

Usage

To use the command, simply precede any command by the word time, such as:

$ time ls

When the command completes, time will report how long it took to execute the ls command in terms of user CPU time, system CPU time, and real time. The output format varies between different versions of the command, and some give additional statistics, as in this example:

$ time host wikipedia.org
wikipedia.org has address 207.142.131.235
0.000u 0.000s 0:00.17 0.0%      0+0k 0+0io 0pf+0w
$

time(1) can exist as a standalone program (such as GNU time) or as a shell builtin in most case (e.g. in sh, bash, tcsh or in zsh).

User time vs system time

The total CPU time is the combination of the amount of time the CPU or CPUs spent performing some action for a program and the amount of time they spent performing system calls for the kernel on the program's behalf. When a program loops through an array, it is accumulating user CPU time. Conversely, when a program executes a system call such as exec or fork, it is accumulating system CPU time.

Real time vs CPU time

The term "real time" in this context refers to elapsed wall-clock time, like using a stop watch. The total CPU time (user time + sys time) may be more or less than that value. Because a program may spend some time waiting and not executing at all (whether in user mode or system mode) the real time may be greater than the total CPU time. Because a program may fork children whose CPU times (both user and sys) are added to the values reported by the time command, but on a multicore system these tasks are run in parallel, the total CPU time may be greater than the real time.

Method of operation

According to the source code of the GNU implementation of time, most information shown by time is derived from the wait3 system call. On systems that do not have a wait3 call that returns status information, the times system call is used instead.

References

time: time a simple command – Commands & Utilities Reference, The Single UNIX Specification, Issue 7 from The Open Group
time(1): time a simple command or give resource usage – Linux Programmer's Manual – User Commands

Unix command-line interface programs and shell builtins
File system	cat chmod chown chgrp cksum cmp cp dd du df file fuser ln ls mkdir mv pax pwd rm rmdir split tee touch type umask
Processes	at bg crontab fg kill nice ps time
User environment	env exit logname mesg talk tput uname who write
Text processing	awk basename comm csplit cut diff dirname ed ex fold head iconv join m4 more nl paste printf sed sort strings tail tr uniq vi wc xargs
Shell builtins	alias cd echo test unset wait
Searching	find grep
Documentation	man
Software development	ar ctags lex make nm strip yacc
Miscellaneous	bc cal expr lp od sleep true and false
Categories Standard Unix programs Unix SUS2008 utilities List

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