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Joshua Enfield
Asked: 2010-08-28 09:42:11 +0800 CST

Redirect output of time command in unix into a variable in bash?

  • 772

I just want to capture the output of a time command i.e:

X=$(time ls)

or

$(time ls) | grep real

The time function spits it to the console though. How do I do this?

linux bash

5 Answers

  1. Best Answer

    X=`(time ls) 2>&1 | grep real`

    • 13

  2. See BashFAQ/032.

    $ # captures output of command and time
$ time=$( TIMEFORMAT="%R"; { time ls; } 2>&1 )    # note the curly braces

$ # captures the time only, passes stdout through
$ exec 3>&1 4>&2
$ time=$(TIMEFORMAT="%R"; { time ls 1>&3 2>&4; } 2>&1)
bar baz
$ exec 3>&- 4>&-

    The time will look like "0.000" using TIMEFORMAT="%R" which will be the "real" time.

    • 11

  3. Time writes its output to STDERR rather than STDOUT. Making matters worse, by default 'time' is a shell builtin command, so if you attempt 'time ls 2>&1' the '2>&1' only applies to 'ls'.

    The solution would probably be something like:

    /usr/bin/time -f 'real %e' -o OUTPUT_FILE ls > /dev/null 2>&1<br>
REALTIME=$(cat OUTPUT_FILE | cut -f 2 -d ' ')

    There are more fancy ways to do it, but that is the clear/simple way.

    • 4

  4. @Dennis Williamson's answer is great, but it doesn't help you store the command's output in one variable, and the output of time in another variable. This is actually not possible using file descriptors.

    If you want to record how long a program takes to run, you could do this by just subtracting the start time from the end time. Here is a simple example that shows how many milliseconds a program took to run:

    START_TIME=$(date +%s%3N)
OUTPUT=$(ls -l)
ELAPSED_TIME=$(expr $(date +%s%3N) - $START_TIME)
echo "Command finished in $ELAPSED_TIME milliseconds"

    This is not quite as accurate as the time command, but it should work perfectly fine for most bash scripts.

    Unfortunately Mac's date command doesn't support the %N format, but you can install coreutils (brew install coreutils) and use gdate:

    START_TIME=$(gdate +%s%3N)
OUTPUT=$(ls -l)
ELAPSED_TIME=$(expr $(gdate +%s%3N) - $START_TIME)
echo "Command finished in $ELAPSED_TIME milliseconds"
    • 2

  5. The cleanest approach is to combine a temp file and use GNU's time.

    command="sleep 1"
saved_timing=`mktemp`
/usr/bin/time --format="%E real" -o "$saved_timing" $command
elapsed=$(cat "$saved_timing")
echo $elapsed

    This will not bypass normal command output and you don't have to care about the file either since it will be deleted on it own.

    • 2