Date calculations in BASH script

Crashman · 07-02-2004, 07:53 PM

Hey all,

I've been fiddling with this way to long now with no luck. It seems like it should be simple, but my brain must think otherwise.....

I'm trying to perform a simple date calculation in a BASH script that tells how many days from now until my anniversary... before I forget that too!

I thought a simple script like this would work:

let y=date --date'03 Oct' +%j
let x=date +%j
let z=`expr $y - $x`
echo "You have $z days untill your anniversary"

But, BAH... doesn't work at all.

Any help would be greatly appreciated.
Thanks

Dark_Helmet · 07-02-2004, 09:01 PM

Easily corrected...

Code:

#!/bin/bash

y=$(date --date '03 Oct' +%j)
x=$(date +%j)

((z=${y} - ${x}))

echo "You have ${z} days until your anniversary"

exit 0

Crashman · 07-02-2004, 09:12 PM

Thanks alot for that... you can't believe how nuts that one snippet of code has been driving me. LOL

Sometimes syntax is taxing....

Crashman

Dark_Helmet · 07-02-2004, 09:41 PM

Yup, I know the feeling. I've come close to ripping out my hair at times trying to get the right number of backslashes for certain kinds of strings of text. Many here have shared your grief at one time or another.

As a side note, I thought I should also mention you can use backticks instead of the $( ) format. For example:

Code:

y=`date +%j`

It's a little less intrusive (visually speaking), but can cause some syntax grief because it's commonly mistaken as a single quote character.

Anyway, glad it's working for you. Just make sure the wife doesn't know you're "cheating"

jlliagre · 07-03-2004, 10:15 AM

I don't fully agree on the advice to give up the $() syntax.

Actually, it is an improvement of the original single backquote one, with which it was very hard to use imbricated scripts:

ex:
$(command $(cmd2 $(cmd3)))
`command \`cmd2 \\\`cmd3\\\`\``

It is also easier to find the start and ending part of the subcommand with editors like vi that jumps to the other end (command %).

Finally, it also has a useful shortcut $(<file) equivalent to `cat file`.

my 2 cents