Subtracting two timestamps in bash script

Question

I have a file that contains a set of Timestamps in format of H:M:S.MS, I can read and print all of the saved Timestamps but when I do some arithmetic operation

Answer 1

If you want to process the date using simple command line tools, you need to convert the timestamps into some easy-to-deal-with format, like epoch-based.

You can do this with the date command, which you can wrap in a function like so:

timestamp() { 
    date '+%s%N' --date="$1"
}
# timestamp '2020-01-01 12:20:45.12345' => 1577899245123450000

Then you can subtract them directly:

echo $(( $(timestamp "$etime") - $(timestamp "$stime") ))

---

Note that the %N format specifier (nanoseconds) is a GNU extension and is not handled in the default macOS date command. Either (a) brew install coreutils and use gdate in the function above or (b) use this alternative function on macOS (but note it lacks support for sub-second measurements):

timestamp() {
    local format='%Y-%m-%d %H:%M:%S'     # set to whatever format is used
    date -j -f "$format" "$1" '+%s'
}
# timestamp '2020-01-01 12:20:45' => 1577899245