BASH ... Date /days subtraction Addition and compare

em31amit · 05-23-2012, 05:07 PM

I am trying to create a bash shell script and as my requirement i need to subtract some days from current date and compare it to another date which is i am getting into last command.

But i am not getting any logic and a way to do it with in bash shell script. How can i implement it.

sycamorex · 05-23-2012, 05:12 PM

What have you done so far?

Here's something on comparing dates in Bash:
http://stackoverflow.com/questions/5...date-variables

jlinkels · 05-23-2012, 06:53 PM

Calculations with dates always ends up with nasty code for the umpteenth exception you haven't thought about.

For internal calculations, convert dates to unix timestamps, using the date command. After you are done with your calculations, for presentation purposes, convert them back to human readable dates.

jlinkels

Nominal Animal · 05-24-2012, 12:06 AM

GNU date is very useful for this:

Code:

date +%s -d "Fri May 11 12:34:56 2012"
date +%s -d "5 days ago"
date +%s

The output when using +%s is the number of seconds since the Epoch, so you can use $(( $(date +%s -d "later") - $(date +%s -d "earlier") )) in Bash to calculate the number of seconds between later and earlier.

Note that this will not work on all date variants; it will only work on GNU date, as far as I know.

To test if you have GNU date in Bash, you can use

Code:

if [ -n "date --version 2>/dev/null | grep -e GNU" ]; then
    # Yes, you have GNU 'date'
else
    # Oops, not a GNU 'date'!
fi

dru8274 · 05-24-2012, 02:02 AM

This tute shows that the "date" command has its own internal kind of arithmetic. http://linuxconfig.org/addition-and-...x-date-command An example:

Code:

$ date; date --date="5 hours ago" +%H%M-%d%m%Y
Mon Mar  7 09:54:42 EST 2011
0454-07032011

em31amit · 05-24-2012, 01:12 PM

Thanks All, I'll try .