[linux] How to redirect the output of the time command to a file in Linux?

Just a little question about timing programs on Linux: the time command allows to measure the execution time of a program:

[ed@lbox200 ~]$ time sleep 1

real    0m1.004s
user    0m0.000s
sys     0m0.004s

Which works fine. But if I try to redirect the output to a file, it fails.

[ed@lbox200 ~]$ time sleep 1 > time.txt

real    0m1.004s
user    0m0.001s
sys     0m0.004s

[ed@lbox200 ~]$ cat time.txt 
[ed@lbox200 ~]$ 

I know there are other implementations of time with the option -o to write a file but my question is about the command without those options.

Any suggestions ?

Simple. The GNU time utility has an option for that.

But you have to ensure that you are not using your shell's builtin time command, at least the bash builtin does not provide that option! That's why you need to give the full path of the time utility:

/usr/bin/time -o time.txt sleep 1

&>out time command >/dev/null

in your case

&>out time sleep 1 >/dev/null

then

cat out

If you are using GNU time instead of the bash built-in, try

time -o outfile command

(Note: GNU time formats a little differently than the bash built-in).

If you are using csh you can use:

/usr/bin/time --output=outfile -p $SHELL  -c 'your command'

For example:

/usr/bin/time --output=outtime.txt -p csh -c 'cat file'

I ended up using:

/usr/bin/time -ao output_file.txt -f "Operation took: %E" echo lol

• Where "a" is append
• Where "o" is proceeded by the file name to append to
• Where "f" is format with a printf-like syntax
• Where "%E" produces 0:00:00; hours:minutes:seconds
• I had to invoke /usr/bin/time because the bash "time" was trampling it and doesn't have the same options
• I was just trying to get output to file, not the same thing as OP

If you don't want to touch the original process' stdout and stderr, you can redirect stderr to file descriptor 3 and back:

$ { time { perl -le "print 'foo'; warn 'bar';" 2>&3; }; } 3>&2 2> time.out
foo
bar at -e line 1.
$ cat time.out

real    0m0.009s
user    0m0.004s
sys     0m0.000s

You could use that for a wrapper (e.g. for cronjobs) to monitor runtimes:

#!/bin/bash

echo "[$(date)]" "$@" >> /my/runtime.log

{ time { "$@" 2>&3; }; } 3>&2 2>> /my/runtime.log

#!/bin/bash

set -e

_onexit() {
    [[ $TMPD ]] && rm -rf "$TMPD"
}

TMPD="$(mktemp -d)"
trap _onexit EXIT

_time_2() {
    "$@" 2>&3
}

_time_1() {
    time _time_2 "$@"
}

_time() {
    declare time_label="$1"
    shift
    exec 3>&2
    _time_1 "$@" 2>"$TMPD/timing.$time_label"
    echo "time[$time_label]"
    cat "$TMPD/timing.$time_label"
}

_time a _do_something
_time b _do_another_thing
_time c _finish_up

This has the benefit of not spawning sub shells, and the final pipeline has it's stderr restored to the real stderr.

If you care about the command's error output you can separate them like this while still using the built-in time command.

{ time your_command 2> command.err ; } 2> time.log

or

{ time your_command 2>1 ; } 2> time.log

As you see the command's errors go to a file (since stderr is used for time).

Unfortunately you can't send it to another handle (like 3>&2) since that will not exist anymore outside the {...}

That said, if you can use GNU time, just do what @Tim Ludwinski said.

\time -o time.log command

If you want just the time in a shell variable then this works:

var=`{ time <command> ; } 2>&1 1>/dev/null`

Wrap time and the command you are timing in a set of brackets.

For example, the following times ls and writes the result of ls and the results of the timing into outfile:

$ (time ls) > outfile 2>&1

Or, if you'd like to separate the output of the command from the captured output from time:

$ (time ls) > ls_results 2> time_results

Since the output of 'time' command is error output, redirect it as standard output would be more intuitive to do further processing.

{ time sleep 1; } 2>&1 |  cat > time.txt