How do I measure duration in seconds in a shell script?

Question

I wish to find out how long an operation takes in a Linux shell script. How can I do this?

Answer 1

GNU time

I'm also a big fun of the GNU time command: https://www.gnu.org/software/time/ which offers some important options compared to the time Bash built-in.

Sample usage:

env time --format '%e' --output time.log sleep 1

Output:

1.00

Explanation:

• env: to find /usr/bin/time instead of the Bash built-in

• --format '%e': print time in seconds, see man time.

  This is often what I want when benchmarking: a single number rather than minutes + seconds.

And an important pattern I often use is:

bench-cmd() (
  logfile=time.log
  echo "cmd $@" >> "$logfile"
  printf 'time ' >> "$logfile"
  bench_cmd="env time --append --format '%e' --output '$logfile' $@"
  eval "$bench_cmd"
  echo >> "$logfile"
)

rm -f time.log
bench-cmd sleep 1
bench-cmd sleep 2
bench-cmd sleep 3
cat time.log

GitHub upstream.

Output:

cmd sleep 1
time 1.00

cmd sleep 2
time 2.00

cmd sleep 3
time 3.00

Explanation:

• --output: output the time to a file.

  By default, the output goes to stderr, so this option is important to separate the timing from the stderr of the command.

• --append: append to the file instead of overwriting.

  This allows me to concentrate the entire benchmark output in a single file.