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Linux Time Command

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The time command runs the specified program command with the given arguments. If the command exits with non-zero status, this utility displays a warning message and exit status. When command finishes, time writes a message to standard error giving timing statistics about this program run. These statistics consist of

Note: bash have a built-in time command that provides less functionality than the command described here. To access the utility, you may need to specify its pathname (something like /usr/bin/time).

It can measure:

The syntax of ‘time’ is :

/usr/bin/time [options] program [arguments]

Command options:

When the ‘time’ command is run, following is the kind of output it gives :

# /usr/bin/time ls
anaconda-ks.cfg  bin  install.log  install.log.syslog  mbox
0.00user 0.00system 0:00.00elapsed 0%CPU (0avgtext+0avgdata 3888maxresident)k
0inputs+0outputs (0major+304minor)pagefaults 0swaps

As we can see above, apart from executing the command, the last two lines of the output are the resource information that ‘time’ command outputs.

Note: In the above example, the command ‘time’ was run without any options. So this is a default output generated by the ‘time’ command, which is not formatted properly.

As we can see from the output, the default format of the output generated is :

%Uuser %Ssystem %Eelapsed %PCPU (%Xtext+%Ddata %Mmax)k
%Iinputs+%Ooutputs (%Fmajor+%Rminor)pagefaults %Wswaps

The Format Option

This option lets the user to decide the output generated by ‘time’ command. In the last section we discussed the default format that is used in output. Here in this section, we will learn how to specify customized formats.

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.

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).

For example :

$ /usr/bin/time -f "\t%U user,\t%S system,\t%x status" date
Sun Jan 22 17:46:58 IST 2012
	0.00 user,	0.00 system,	0 status

So we see that in the above example, we tried to change the output format by using a different output format.


Since we discussed above that ‘time’ utility displays information about the resource usage by a program, In this section lets list the resources that can be tracked by this utility and the corresponding specifiers.

From the man page :

So we can see that there is a long list of resources whose usage can be tracked by the ‘time’ utility.

Why /usr/bin/time? (Instead of just time)

Lets not use /usr/bin/time and use ‘time’ instead.

$ time -f "\t%U user,\t%S system,\t%x status" date
-f: command not found 

real	0m0.255s
user	0m0.230s
sys	0m0.030s

As seen from the output above, the ‘time’ command when used without the complete path (/usr/bin/time) spits out an error regarding the ‘-f’ flag. Also the format of output is neither the one specified by us in the command nor the default format we discussed earlier. This lead to a confusion over how this output got generated.

When ‘time’ command is executed without the complete path (/usr/bin/time), then its the built-in ‘time’ command of the bash shell that is executed.

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time(1) - Linux man page



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