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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
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.
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
$ time host wikipedia.org wikipedia.org has address 18.104.22.168 0.000u 0.000s 0:00.17 0.0% 0+0k 0+0io 0pf+0w $
time (either a standalone program, or when Bash shell is running in POSIX mode AND time is invoked as
time -p) reports to standard error output.
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 $
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-complaint time, or when executing as
Documentation of this time 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.
In a popular Unix shell Bash,
time is a special keyword, that can be put before a pipeline (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
wc -l added up. Format of the output can be adjusted using
The time 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 time can be accessed using
man 1 bash, or within bash itself using
- The Single UNIX Specification, Issue 7 from The Open Group : time a simple command – Commands & Utilities Reference,
- Linux User's Manual – User Commands : time a simple command or give resource usage –
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