- Time "ls":time ls
time run the program COMMAND with any given arguments ARG.... When COMMAND finishes, time displays information about resources used by COMMAND (on the standard error output, by default). If COMMAND exits with non-zero status, time displays a warning message and the exit status.
time determines which information to display about the resources used by the COMMAND from the string FORMAT. If no format is specified on the command line, but the TIME environment variable is set, its value is used as the format. Otherwise, a default format built into time is used.
Options to time must appear on the command line before COMMAND. Anything on the command line after COMMAND is passed as arguments to COMMAND.
The format string FORMAT controls the contents of the time output. The format string can be set using the `-f' or `--format', `-v' or `--verbose', or `-p' or `--portability' options. If they are not given, but the TIME environment variable is set, its value is used as the format string. Otherwise, a built-in default format is used. The default format is: %Uuser %Ssystem %Eelapsed %PCPU (%Xtext+%Ddata %Mmax)k %Iinputs+%Ooutputs (%Fmajor+%Rminor)pagefaults %Wswaps
The format string usually consists of `resource specifiers' interspersed with plain text. A percent sign (`%') in the format string causes the following character to be interpreted as a resource specifier, which is similar to the formatting characters in the printf(3) function.
A backslash (`\') introduces a `backslash escape', which is translated into a single printing character upon output. `\t' outputs a tab character, `\n' outputs a newline, and `\\' outputs a backslash. A backslash followed by any other character outputs a question mark (`?') followed by a backslash, to indicate that an invalid backslash escape was given.
Other text in the format string is copied verbatim to the output. time always prints a newline after printing the resource use information, so normally format strings do not end with a newline character (or `\n').
There are many resource specifications. Not all resources are measured by all versions of Unix, so some of the values might be reported as zero. Any character following a percent sign that is not listed in the table below causes a question mark (`?') to be output, followed by that character, to indicate that an invalid resource specifier was given.
The resource specifiers, which are a superset of those recognized by the tcsh(1) builtin `time' command, are:
To run the command `wc /etc/hosts' and show the default information: time wc /etc/hosts
To run the command `ls -Fs' and show just the user, system, and total time: time -f "\t%E real,\t%U user,\t%S sys" ls -Fs
To edit the file BORK and have `time' append the elapsed time and number of signals to the file `log', reading the format string from the environment variable `TIME': export TIME="\t%E,\t%k" # If using bash or ksh setenv TIME "\t%E,\t%k" # If using csh or tcsh time -a -o log emacs bork
Users of the bash shell need to use an explicit path in order to run the external time command and not the shell builtin variant. On system where time is installed in /usr/bin, the first example would become /usr/bin/time wc /etc/hosts
The elapsed time is not collected atomically with the execution of the program; as a result, in bizarre circumstances (if the time command gets stopped or swapped out in between when the program being timed exits and when time calculates how long it took to run), it could be much larger than the actual execution time.
When the running time of a command is very nearly zero, some values (e.g., the percentage of CPU used) may be reported as either zero (which is wrong) or a question mark.
Most information shown by time is derived from the wait3(2) system call. The numbers are only as good as those returned by wait3(2). On systems that do not have a wait3(2) call that returns status information, the times(2) system call is used instead. However, it provides much less information than wait3(2), so on those systems time reports the majority of the resources as zero.
The `%I' and `%O' values are allegedly only `real' input and output and do not include those supplied by caching devices. The meaning of `real' I/O reported by `%I' and `%O' may be muddled for workstations, especially diskless ones.
The time command returns when the program exits, stops, or is terminated by a signal. If the program exited normally, the return value of time is the return value of the program it executed and measured. Otherwise, the return value is 128 plus the number of the signal which caused the program to stop or terminate.
time was written by David MacKenzie. This man page was added by Dirk Eddelbuettel <firstname.lastname@example.org>, the Debian GNU/Linux maintainer, for use by the Debian GNU/Linux distribution but may of course be used by others.