CODEWITHSUNDEEP
linuxbashshell
Buddy
Buddy

Reputation: 43

Shell - Subtracting an integer from a date

I am trying to subtract an integer from a date. Basically I am trying to say that if it is the before the 15th of the month, so subtract 1 from the month. So if the day is 05-05-2016 I want to use 04 as the month.

Month=`date +%m`
Day=`date +%d`

If [ $Day -lt 15 ]
    then
        Output_Month=$Month - 1
fi

This does not seem to be working because I assume they are in two different formats (date and integer). How can I either subtract a month or convert the month into an integer?

Upvotes: 0

Views: 238

Answers (4)

sjsam
sjsam

Reputation: 21955

if [ "$Day" -lt "15" ] # No harm double quoting $Day, note this is integer comparison
    then
        (( Output_Month = Month - 1 )) #You may omit $ inside ((..)) construct
fi

Upvotes: 0

Michael Zarubin
Michael Zarubin

Reputation: 386

The date command is quite smart, you can just write:

if [ $Day -lt 15 ]; then
    Output_Month=$(date -d "-1 month" +%m)
fi

Upvotes: 2

P.P
P.P

Reputation: 121357

First of all, you have typo: It's if (lowercase), not If. To do arithmetic, you can use the $((..)) construct. So, it could be written as:

Month=`date +%-m`
Day=`date +%d`

if [ $Day -lt 15 ]
    then
        Output_Month=$((Month - 1))
fi

Also, note that I used the - in calculating the Month. It's because date +%d prints with a leading 0 and any number with a leading is an octal number. So, when you have Month as 08 or 09 then it'll be an error. Using the - suppresses the leading 0.

Upvotes: 2

ojblass
ojblass

Reputation: 21620

Let is a bit picky about spaces before and after arithmetic operators. This should help you get your answer:

#!/bin/ksh

Month=`date +%m`
Day=`date +%d`

if [ $Day -lt 15 ]
then
   let Output_Month=$Month-1
   echo $Output_Month
else
   let Output_Month=$Month+1
   echo $Output_Month
fi

I added the control block for testing because today is obviously above the target date of 15. It is the 27th so to get any output I had to populate the else clause.

Upvotes: 0

Related Questions