Reference:
http://manpages.ubuntu.com/manpages/maverick/man1/time.1.html
NAME
time - run programs and summarize system resource usage
SYNOPSIS
time [ -apqvV ] [ -f FORMAT ] [ -o FILE ]
[ --append ] [ --verbose ] [ --quiet ] [ --portability ]
[ --format=FORMAT ] [ --output=FILE ] [ --version ]
[ --help ] COMMAND [ ARGS ]
DESCRIPTION
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.
OPTIONS
-o FILE, --output=FILE
Write the resource use statistics to FILE instead of to the
standard error stream. By default, this overwrites the file,
destroying the file's previous contents. This option is useful
for collecting information on interactive programs and programs
that produce output on the standard error stream.
-a, --append
Append the resource use information to the output file instead
of overwriting
it. This option is only useful with the `-o' or `--output'
option.
-f FORMAT, --format FORMAT
Use FORMAT as the format string that controls the output of
time. See the below more information.
--help Print a summary of the command line options and exit.
-p, --portability
Use the following format string, for conformance with POSIX
standard 1003.2:
real %e
user %U
sys %S
-v, --verbose
Use the built-in verbose format, which displays each available
piece of information on the program's resource use on its own
line, with an English description of its meaning.
--quiet
Do not report the status of the program even if it is different
from zero.
-V, --version
Print the version number of time and exit.
FORMATTING THE OUTPUT
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 `0).
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:
% A literal `%'.
C Name and command line arguments of the command being
timed.
D Average size of the process's unshared data area, in
Kilobytes.
E Elapsed real (wall clock) time used by the process, in
[hours:]minutes:seconds.
F Number of major, or I/O-requiring, page faults that
occurred while the process was running. These are faults
where the page has actually migrated out of primary
memory.
I Number of file system inputs by the process.
K Average total (data+stack+text) memory use of the
process, in Kilobytes.
M Maximum resident set size of the process during its
lifetime, in Kilobytes.
O Number of file system outputs by the process.
P Percentage of the CPU that this job got. This is just
user + system times divided by the total running time. It
also prints a percentage sign.
R Number of minor, or recoverable, page faults. These are
pages that are not valid (so they fault) but which have
not yet been claimed by other virtual pages. Thus the
data in the page is still valid but the system tables
must be updated.
S Total number of CPU-seconds used by the system on behalf
of the process (in kernel mode), in seconds.
U Total number of CPU-seconds that the process used
directly (in user mode), in seconds.
W Number of times the process was swapped out of main
memory.
X Average amount of shared text in the process, in
Kilobytes.
Z System's page size, in bytes. This is a per-system
constant, but varies between systems.
c Number of times the process was context-switched
involuntarily (because the time slice expired).
e Elapsed real (wall clock) time used by the process, in
seconds.
k Number of signals delivered to the process.
p Average unshared stack size of the process, in Kilobytes.
r Number of socket messages received by the process.
s Number of socket messages sent by the process.
t Average resident set size of the process, in Kilobytes.
w Number of times that the program was context-switched
voluntarily, for instance while waiting for an I/O
operation to complete.
x Exit status of the command.
EXAMPLES
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 "%E real,%U user,%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="%E,%k" # If using bash or ksh
setenv TIME "%E,%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
ACCURACY
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.
DIAGNOSTICS
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.
AUTHOR
time was written by David MacKenzie. This man page was added by Dirk
Eddelbuettel <edd@debian.org>, the Debian GNU/Linux maintainer, for use
by the Debian GNU/Linux distribution but may of course be used by
others.
SEE ALSO
tcsh(1), printf(3)
