Bash: calculate the time elapsed between two timestamps

Question

I have written a script that notifies me when a value is not within a given range. All values "out of range" are logged in a set of per day files.

Every line is timestamped in a proprietary reverse way: yyyymmddHHMMSS

Now, I would like to refine the script, and receive notifications just when at least 60 minutes are passed since the last notification for the given out of range value.

I already solved the issue to print the logs in reverse ordered way with:

for i in $(ls -t /var/log/logfolder/*); do zcat $i|tac|grep \!\!\!|grep --color KEYFORVALUE; done

that results in:

...
20170817041001 - WARNING: KEYFORVALUE=252.36 is not between 225 and 245 (!!!)
20170817040001 - WARNING: KEYFORVALUE=254.35 is not between 225 and 245 (!!!)
20170817035001 - WARNING: KEYFORVALUE=254.55 is not between 225 and 245 (!!!)
20170817034001 - WARNING: KEYFORVALUE=254.58 is not between 225 and 245 (!!!)
20170817033001 - WARNING: KEYFORVALUE=255.32 is not between 225 and 245 (!!!)
20170817032001 - WARNING: KEYFORVALUE=254.99 is not between 225 and 245 (!!!)
20170817031001 - WARNING: KEYFORVALUE=255.95 is not between 225 and 245 (!!!)
20170817030001 - WARNING: KEYFORVALUE=255.43 is not between 225 and 245 (!!!)
20170817025001 - WARNING: KEYFORVALUE=255.26 is not between 225 and 245 (!!!)
20170817024001 - WARNING: KEYFORVALUE=255.42 is not between 225 and 245 (!!!)
20170817012001 - WARNING: KEYFORVALUE=252.04 is not between 225 and 245 (!!!)
...

Anyway, I'm stuck at calculating the number of seconds between two of those timestamps, for instance:

20170817040001
20160312000101

What should I do in order to calculate the time elapsed between two timestamps?

Solution 1

This will give you the date in seconds (since the UNIX epoch)

date --date '2017-08-17 04:00:01' +%s    # "1502938801"

And this will give you the date as a readable string from a number of seconds

date --date '@1502938801'    # "17 Aug 2017 04:00:01"

So all that's needed is to convert your date/timestamp into a format that GNU date can understand, use maths to determine the difference, and output the result

datetime1=20170817040001
datetime2=20160312000101

# bash string manipulation
datestamp1="${datetime1:0:4}-${datetime1:4:2}-${datetime1:6:2} ${datetime1:8:2}:${datetime1:10:2}:${datetime1:12:2}"
datestamp2="${datetime2:0:4}-${datetime2:4:2}-${datetime2:6:2} ${datetime2:8:2}:${datetime2:10:2}:${datetime2:12:2}"

# otherwise use sed
# datestamp1=$(echo "$datetime1" | sed -nr 's/(....)(..)(..)(..)(..)(..)/\1-\2-\3 \4:\5:\6/p')
# datestamp2=$(echo "$datetime2" | sed -nr 's/(....)(..)(..)(..)(..)(..)/\1-\2-\3 \4:\5:\6/p')

seconds1=$(date --date "$datestamp1" +%s)
seconds2=$(date --date "$datestamp2" +%s)

delta=$((seconds1 - seconds2))
echo "$delta seconds"    # "45197940 seconds"

We've not provided timezone information here so it assumes local timezone. Your values for the seconds from the datetime will probably be different to mine. (If your values are UTC then you can use date --utc.)

Solution 2

This is easy with datediff command provided in dateutils package.

ddiff -i '%Y%m%d%H%M%S' 20170817040001 20160312000101

Comments

[111---]

Are you looking for time delta expressed in a particular format, I.E. "HH:MM:SS"? Also, why not just log UNIX timestamps?

[jesse_b]

This question should help you solve this issue: unix.stackexchange.com/questions/383795/…

[Marco]

@roaima Ubuntu Linux. I expect the script to work in any GNU Linux.

[Marco]

@datUser for ease of use (I'll have to make calculations on this delta) seconds would be nice.

[111---]

@Marco then if you are writing the script I would consider adding a timestamp column and save some awkward bash scripting to do the math from a datetime value. Not trying to be snarky here, honest suggestion. Then math is a breeze.

[Marco]

@datUser I already have logs that I'd like to keep. I should find a way to automatically calculate the unix timestamp from my custom one, and batchly edit the logs.

[jesse_b]

@Marco see the link in my comment for several ways to calculate the unix timestamp.

[Marco]

@Jesse_b found it, TY!

[jesse_b]

Oh I also just noticed you are parsing ls in your script. You shouldn't do this. You can do for i in "/var/log/logfolder/*"; instead. mywiki.wooledge.org/ParsingLs

[Marco]

@Jesse_b, with your for, how would I ensure I'm reading files in the right reversed time order? I don't know how to, and that's why I use to use ls when I work in folders just containing properly formatted filenames.

[jesse_b]

You can try to put it in a subshell and pipe it into sort. $(find /var/log/logfolder -type f -name "*" | sort -n) maybe?

[roaima]

@Jesse_b that's not really any different to using ls -t; you're still parsing the output of a command that generates a list of filenames. (Oh, -name "*" is almost a no-op; you wouldn't need it here. And how would sort -n be able to sort file names numerically?)

[jesse_b]

I dunno...Hence why I said I didn't know.

[Marco]

When I started writing this post, I was almost sure it would have ended with sed doing the dirty job. TY @roaima

[Marco]

this on ubuntu 16.04 LTS user@host:/~# apt-get install dateutils [...] Setting up dateutils (0.2.5-1) ... user@host:/~# datediff datediff: command not found user@host:/~#

[αғsнιη]

sorry, the command-line command is ddiff

[MatsK]

Or dateutils.ddiff

[Marco]

This solution is very intresting too, but I keep the sed one in my script; as I find it's most compatible with any GNU Linux and saves me one more dependency.

[Marco]

Also, please note that the correct name for this command in Linux Ubuntu 16.04 is dateutils.ddiff

[Scott - Слава Україні]

Come on, think about it. For simplicity, look at only HHMMSS (assume two entries from the same day). What’s the elapsed time between 12:00:00 and 12:01:00? It’s one minute; i.e., 60 seconds. But 120100−120000 is 100.

[Marco]

Obviously he didn't read the question because the main point is exactly getting rid of this customized timestamp. BTW after years I want to inform people that I solved the problem switching to a Unix timestamp, it is not as human readable as this custom one, but you'd better write a log parser script that shows you date in human readable form and let the system work on its own true low level time handling (seconds elapsed since 1/1/70).

[ychaouche]

Anyway I can use dateutils.ddiff to compute the difference between two time durations ?