computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...

, time is a command in

Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...

and

Unix-like A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...

operating systems. It is used to determine the duration of execution of a particular

command Command may refer to: Computing * Command (computing), a statement in a computer language * command (Unix), a Unix command * COMMAND.COM, the default operating system shell and command-line interpreter for DOS * Command key, a modifier key on A ...

Overview

time(1) can exist as a standalone program (such as GNU time) or as a

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

builtin in most cases (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 call In computing, a system call (syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, accessing a hard disk drive ...

s 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

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.

Usage

To use the command, one simply precedes 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 103.102.166.224 wikipedia.org mail is handled by 50 mx2001.wikimedia.org. wikipedia.org mail is handled by 10 mx1001.wikimedia.org. host wikipedia.org 0.04s user 0.02s system 7% cpu 0.780 total $ (either a standalone program, or when Bash shell is running in POSIX mode AND is invoked as time -p) reports to standard error output.

time -p

Portable scripts should use time -p mode, which uses a different output format, but which is consistent with various implementations: $ time -p sha256sum /bin/ls 12477deb0e25209768cbd79328f943a7ea8533ece70256cdea96fae0ae34d1cc /bin/ls real 0.00 user 0.00 sys 0.00 $

Implementations

GNU time

Current versions of GNU time, report more than just a time by default: $ /usr/bin/time sha256sum /bin/ls 12477deb0e25209768cbd79328f943a7ea8533ece70256cdea96fae0ae34d1cc /bin/ls 0.00user 0.00system 0:00.00elapsed 100%CPU (0avgtext+0avgdata 2156maxresident)k 0inputs+0outputs (0major+96minor)pagefaults 0swaps $ Format of the output for GNU time, can be adjusted using TIME environment variable, and it can include information other than the execution time (i.e. memory usage). This behavior is not available in general

POSIX The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comm ...

-compliant time, or when executing as time -p. Documentation of this can be usually accessed using man 1 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.

Bash

In a popular Unix shell Bash, time is a special keyword, that can be put before a

pipeline A pipeline is a system of Pipe (fluid conveyance), pipes for long-distance transportation of a liquid or gas, typically to a market area for consumption. The latest data from 2014 gives a total of slightly less than of pipeline in 120 countries ...

(or single command), that measures time of entire pipeline, not just a singular (first) command, and uses a different default format, and puts empty line before reporting times: $ time seq 10000000 , wc -l 10000000 real 0m0.078s user 0m0.116s sys 0m0.029s $ The reported time is a time used by both seq and wc -l added up. Format of the output can be adjusted using TIMEFORMAT variable. The is not a builtin, but a special keyword, and can't be treated as a function or command. It also ignores pipeline redirections (even when executed as time -p, unless entire Bash is run in "POSIX mode"). Documentation of this can be accessed using man 1 bash, or within bash itself using help time.

References

* * * {{Unix commands Unix SUS2008 utilities Unix process- and task-management-related software Inferno (operating system) commands