UNIX for Dummies Questions & Answers

Hi friends,

I would like to compare two dates in an IF statement.
This is what I am trying, but it doesn't work.

Code:

date=20120122
minus=6
if [ $date < `date -d "-'$minus' days" +%Y%m%d` ]; then
 ...
fi

what would the IF clause looks like?
Thanks!

In what way does it "not work"?

example:

Code:

if [ '20120122' < '20120120' ]; then
   echo 'yes!'
else 
   echo 'no'
fi

The word 'yes!' is displayed after execution.

Use perl for date arithmetic instead of doing it in the shell...

I'm not sure if perl is supported in our environment. Most of our scripts are .sh or .ksh only and the admin wants to keep it that way. Thanks

An if-clause would look similar to the one given in this post: https://www.unix.com/shell-programmin...pare-date.html

< is considered as a redirection operator when used in single square parenthesis - [ condition ]. Use -gt, -lt, -ge, -le, -eq, -ne for numeric comparisons.

Or alternatively, you can use < to mean less than when used in double square parenthesis - [[ condition ]].

So its either:

Code:

[[ $date < `date -d "-$minus days" +%Y%m%d` ]]

Or

Code:

[ $date -lt `date -d "-$minus days" +%Y%m%d` ]

For interest; double square parenthesis is not a POSIX standard and is not available in older shells. Its a powerful feature of modern BASH and KSH. Read these links:
1. http://mywiki.wooledge.org/BashFAQ/031
2. https://www.unix.com/302076732-post.html#post302076732