date +%s.%N will give you, eg., 1364391019.877418748. The %N is thenumber of nanoseconds elapsed in the current second. Notice it is 9 digits,and by default date will pad this with zeros if it is less than 100000000. This is actually a problem if we want to do math with the number, because bash treats numbers with a leading zero as octal. This padding can be disabled by using a hyphen in the field spec, so:
echo $((`date +%s`*1000+`date +%-N`/1000000))would naively give you milliseconds since the epoch.
However, as Stephane Chazelas points out in comment below, that's two different date calls which will yield two slightly different times. Ifthe second has rolled over in between them, the calculation will be anentire second off. So:
echo $(($(date +'%s * 1000 + %-N / 1000000')))Or optimized (thanks to comments below, though this should have been obvious):
echo $(( $(date '+%s%N') / 1000000));