Question

I’d like to use the time command in a bash script to calculate the elapsed time of the script and write that to a log file. I only need the real time, not the user and sys. Also need it in a decent format. e.g 00:00:00:00 (not like the standard output). I appreciate any advice.

The expected format supposed to be 00:00:00.0000 (milliseconds) [hours]:[minutes]:[seconds].[milliseconds]

I’ve already 3 scripts. I saw an example like this:

{ time { # section code goes here } } 2> timing.log

But I only need the real time, not the user and sys. Also need it in a decent format. e.g 00:00:00:00 (not like the standard output).

In other words, I’d like to know how to turn the time output into something easier to process.

Answer 1

You could use the date command to get the current time before and after performing the work to be timed and calculate the difference like this:

#!/bin/bash

# Get time as a UNIX timestamp (seconds elapsed since Jan 1, 1970 0:00 UTC)
T="$(date +%s)"

# Do some work here
sleep 2

T="$(($(date +%s)-T))"
echo "Time in seconds: ${T}"

printf "Pretty format: %02d:%02d:%02d:%02d\n" "$((T/86400))" "$((T/3600%24))" "$((T/60%60))" "$((T%60))""

Notes:
$((…)) can be used for basic arithmetic in bash – caution: do not put spaces before a minus – as this might be interpreted as a command-line option.

See also: http://tldp.org/LDP/abs/html/arithexp.html

EDIT:
Additionally, you may want to take a look at sed to search and extract substrings from the output generated by time.

EDIT:

Example for timing with milliseconds (actually nanoseconds but truncated to milliseconds here). Your version of date has to support the %N format and bash should support large numbers.

# UNIX timestamp concatenated with nanoseconds
T="$(date +%s%N)"

# Do some work here
sleep 2

# Time interval in nanoseconds
T="$(($(date +%s%N)-T))"
# Seconds
S="$((T/1000000000))"
# Milliseconds
M="$((T/1000000))"

echo "Time in nanoseconds: ${T}"
printf "Pretty format: %02d:%02d:%02d:%02d.%03d\n" "$((S/86400))" "$((S/3600%24))" "$((S/60%60))" "$((S%60))" "${M}"

DISCLAIMER:
My original version said

M="$((T%1000000000/1000000))"

but this was edited out because it apparently did not work for some people whereas the new version reportedly did. I did not approve of this because I think that you have to use the remainder only but was outvoted.
Choose whatever fits you.