time (Unix)
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time is a command in the Unix operating systems. It is used to determine the duration of execution of a particular command.
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[edit] 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 (e.g. in tcsh or in zsh).
[edit] User Time vs System Time
The term 'user CPU time' can be a bit misleading at first. To be clear the total time (real CPU time), is the combination of the amount of time the CPU spends performing some action for a program and the amount of time the CPU spends 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.
[edit] 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.
[edit] See also
[edit] References
- : time a simple command – Commands & Utilities Reference, The Single UNIX® Specification, Issue 6 from The Open Group
- : time a simple command or give resource usage – Linux User's Manual
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