Mac OS X Shell Script Measure Time Elapsed

Question

How can I measure the time elapsed in milliseconds in a shell script in Mac OS X?

Answer 1

Use the time command (manpage). This will be much cheaper than invoking ruby just to tell you elapsed time:

$ time a_command

To "extract" the real time from the command do (untested):

real_time=$(time a_command | grep ^real | awk 'print $2')

(where a_command can be a shell function if necessary)

This will return the value in minutes and seconds, so if you want the result in milliseconds then use python (or your favourite scripting language) to run the process with timing functions around the outside of the sub-process invocation and you will not incur the cost invoking the scripting language just to get the current time. See this answer and this answer for details.

Answer 2

You may use:

start_ms=$(ruby -e 'puts (Time.now.to_f * 1000).to_i')
# do some work
end_ms=$(ruby -e 'puts (Time.now.to_f * 1000).to_i')
elapsed_ms=$((end_ms - start_ms))
echo "$elapsed_ms ms passed"

OR only shell builtins (works in bash and zsh):

start_ns=$(date +%s%N)
# do some work
end_ns=$(date +%s%N)
elapsed_ms=$(((end_ns - start_ns) / 1000000))