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 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 `ls -Fs' and show just the user, system, and total
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
shell need to use an explicit path in order to run the external
command and not the shell builtin variant. On system where
is installed in
the first example would become
/usr/bin/time wc /etc/hosts
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.