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Measuring program resource use

The time command runs another program, then displays information about the resources used by that program, collected by the system while the program was running. You can select which information is reported and the format in which it is shown (see section Formatting the output), or have time save the information in a file instead of display it on the screen (see section Using the time command).

The resources that time can report on fall into the general categories of time, memory, I/O, and IPC calls. Some systems do not provide much information about program resource use; time reports unavailable information as zero values (see section Accuracy).

Here is an example of using time to measure the time and other resources used by running the program doit, which is given the arguments `foo' and `3':

time doit foo 3

Mail suggestions and bug reports for GNU time to bug-gnu-utils@prep.ai.mit.edu.

Using the time command

The format of the time command is:

time [-apvV] [-f format] [-o file] [--append] [--verbose]
[--portability] [--format=format] [--output=file]
[--version] [--help] command [arg...]

time runs 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 a format string (see section Formatting the output). 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 (see section Formatting the output).

Options to time must appear on the command line before command. Anything on the command line after command is passed as arguments to command.

-o 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.
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
Use format as the format string that controls the output of time. See section Formatting the output, for more information.
Print a summary of the command line options to time and exit.
Use the following format string, for conformance with POSIX standard 1003.2:
real %e
user %U
sys %S
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.
Print the version number of time and exit.

Formatting the output

The format string 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 C printf 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 builtin time command, are:

A literal `%'.
Name and command line arguments of the command being timed.
Average size of the process's unshared data area, in Kilobytes.
Elapsed real (wall clock) time used by the process, in [hours:]minutes:seconds.
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.
Number of file system inputs by the process.
Average total (data+stack+text) memory use of the process, in Kilobytes.
Maximum resident set size of the process during its lifetime, in Kilobytes.
Number of file system outputs by the process.
Percentage of the CPU that this job got. This is just user + system times divied by the total running time.
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.
Total number of CPU-seconds used by the system on behalf of the process (in kernel mode), in seconds.
Total number of CPU-seconds that the process used directly (in user mode), in seconds.
Number of times the process was swapped out of main memory.
Average amount of shared text in the process, in Kilobytes.
System's page size, in bytes. This is a per-system constant, but varies between systems.
Number of times the process was context-switched involuntarily (because the time slice expired).
Elapsed real (wall clock) time used by the process, in seconds.
Number of signals delivered to the process.
Average unshared stack size of the process, in Kilobytes.
Number of socket messages received by the process.
Number of socket messages sent by the process.
Average resident set size of the process, in Kilobytes.
Number of times that the program was context-switched voluntarily, for instance while waiting for an I/O operation to complete.
Exit status of the command.


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


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 system call. The numbers are only as good as those returned by wait3. On systems that do not have a wait3 call that returns status information, the times system call is used instead. However, it provides much less information than wait3, 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.

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