Time (Unix)

In computing, codice_1 is a command in Unix and Unix-like operating systems. It is used to determine the duration of execution of a particular command.
Overview.
codice_2 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 codice_3 or codice_4, 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 codice_1 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, simply precede any command by the word codice_1, such as:
$ time ls
When the command completes, codice_1 will report how long it took to execute the codice_8 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
time (either a standalone program, or when Bash shell is running in POSIX mode AND time is invoked as codice_9) reports to standard error output.
time -p.
Portable scripts should use codice_9 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 codice_11 environment variable, and it can include information other than the execution time (i.e. memory usage). This behavior is not available in general POSIX-compliant time, or when executing as codice_9.
Documentation of this time can be usually accessed using codice_13.
Method of operation.
According to the source code of the GNU implementation of codice_1, most information shown by codice_1 is derived from the codice_16 system call. On systems that do not have a codice_16 call that returns status information, the codice_18 system call is used instead.
Bash.
In a popular Unix shell Bash, codice_1 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 codice_20 and codice_21 added up. Format of the output can be adjusted using codice_22 variable.
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 codice_9, unless entire Bash is run in "POSIX mode").
Documentation of this time can be accessed using codice_24, or within bash itself using codice_25.