Subtracting two timestamps in bash script

问题

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 like Subtract (say Timestamp2 - Timestamp1) it doesn't give me the answer, I do not have experience work with bash script programming.

Any suggestions, recommendations will be much appreciated.

Here is the screen shot of my problem.

Here is an example :

Start Time = 17:53:01.166721
End Time = 17:53:01.369787

The expected result for End Time - Start Time is either 0:00:0.203066 or 0.203066.

回答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 "12:20:45.12345"  ==>  1485105645123450000

Then you can subtract them directly:

echo $(( $(timestamp "${Stime[$i]}") - $(timestamp "${Rtime[$i]}") ))

Note that since you are not keeping track of the day but only of the time, you will have errors when, for example, the start time is 23:59:59 and the end time is 00:00:01.

回答2:

If you need to use bash, you need to convert the timestamp into an integer value:

$ time="12:34:56.789"
$ IFS=":." read -r h m s ms <<<"$time"
$ echo $h $m $s $ms
12 34 56 789
$ milliseconds=$(( (h*3600 + m*60 + s)*1000 + ms ))
$ echo $milliseconds
45296789

Then you can subtract to get a number of milliseconds representing the diff.

Convert to seconds with some arithmetic and string construction:

$ seconds="$((milliseconds / 1000)).$((milliseconds % 1000))"
$ echo $seconds
45296.789

To address gniourf_gniourf's valid point (in a clunky way):

$ time="12:34:56.7"
$ IFS=":." read -r h m s ms <<<"$time"
$ ms="${ms}000"; ms=${ms:0:3}
$ echo $h $m $s $ms
12 34 56 700
# .......^^^

Also, strings that are invalid octal numbers like "08" and "09" will throw errors in bash arithmetic, so explicitly declare they are base 10

milliseconds=$(( (10#$h*3600 + 10#$m*60 + 10#$s)*1000 + 10#$ms ))

回答3:

Assuming your variables are integers, you need to enclose your calculation in an arithmetic evaluation for that to work.

echo $(( VAR1 - VAR2 ))

Or, if you want to assign the value :

VAR3=$(( VAR1 - VAR2 ))

来源：https://stackoverflow.com/questions/41793634/subtracting-two-timestamps-in-bash-script

标签

bash

shell

date

math

timestamp