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Can I write the output of the time difference command to a file?

For example, I tried:

$ time > bar.txt

But it only gives the output of to bar.txt.

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4 Answers 4

up vote 7 down vote accepted

In many shells including ksh, zsh and bash, time is a keyword and is used to time pipelines.

time foo | bar

Will time both the foo and bar commands (zsh will show you the breakdown). It reports it on the shell's stderr.

time > bar.txt

Will tell you the time needed to open bar.txt and run

If you want to redirect time's output, you need to redirect stderr in the context where time is started like:

{ time; } 2> bar.txt


2> bar.txt time

works as well but with ksh93 and bash, because it's not in first position, time is not recognised as a keyword, so the time command is used instead (you'll probably notice the different output format, and it doesn't time pipelines).

Note that both would redirect both the output of time and the errors of to bar.txt. If you only want the time output, you'd need:

{ time 2>&3 3>&-; } 3>&2 2> bar.txt

Note that POSIX doesn't specify whether time behaves as a keyword or builtin (whether it times pipelines or single commands). So to be portable (in case you want to use it in a sh script which you want portable to different Unix-like systems), you should probably write it:

command time -p 2> bar.txt

Note that you can't time functions or builtins or pipelines or redirect errors separately there unless you start a separate shell as in:

command time -p sh -c 'f() { blah; }; f | cat'

But that means the timing will also include the startup time of that extra sh.

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Not all versions of time support the -o and --output arguments.

You will want to run the command like this:

(time 1> /dev/null 2> /tmp/logFile

This will put the output of into /dev/null and the results to /tmp/logFile.

If you want both STDERR and STDOUT to go to the log file you can run it like this:

(time &> /tmp/logFile
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To get the output of ls and the output of time in the file:

(time ls) &> log

To get just the output of time in the file:

(time ls) 2> log
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/usr/bin/time outputs to stderr, so you will need to redirect that.

/usr/bin/time 2>bar.txt

If you use bash' time builtin, you need to write it differently:

(time ls) 2>bar.txt

Alternatively, GNU time supports an --output argument:

/usr/bin/time --output bar.txt 
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Hmm, my time doesn't have the output argument and the 2>bar.txt didn't work. –  sheldonk May 9 '13 at 20:06
What version do you have? GNU Time is /usr/bin/time -V. There is also the time command built into bash. –  slm May 9 '13 at 20:26
It's old enough that it doesn't support that argument... I'll look for an update –  sheldonk May 9 '13 at 20:38
That first example didn't work for me either on a Fedora 14 laptop. I don't think it has to do with time, it's how we're piping the output. If you change it to (time ls) 2>bar.txt it works fine. –  slm May 9 '13 at 20:39

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