How to get date n months before in Unix?

Shell Programming and Scripting
1

Hi,

I need the date n months before currrent date
I need to store date in a varible say x
and then get the date 6 month before and get the result in the format 25-Aug-2009

Please Advice

2

If you have GNU date you can do something like:

date -d "2 month ago" +%F

otherwise you can check one of the links of:

http://www.unix.com/answers-frequently-asked-questions/13785-yesterdays-date-date-arithmetic.html

3

Hi Franklin,

I have tried with the above command and the article too doesn't help in terms of finding the date n month before the current date. i am using AIX unix and the commands as mentioned for Free bSD and gnu dates aren't working.

4

Maybe there are shorter solutions with perl but with awk you can do something like this in your script:

x=11

newdate=$(echo $x | awk -v date="$(date "+%d %m %Y")" -v OFS="-" '
{ 
  num=$0
  split(date, a); d=a[1]; m=a[2]; y=a[3]
  m-=num
  while(m < 1) {m+=12; y--}
  printf("%02d%s%02d%s%s\n", d, OFS, m, OFS, y)
}')

echo $newdate

You can change the format in the printf statement, the variable OFS is the separator.
Play around with it to get the desired format.

5

There is a problem with the end of month. You would need to add some code so that the function doesn't return invalid dates like "31-Nov-2009".

6

Thanks a lot Franklin this is working fine!!

---------- Post updated at 05:49 AM ---------- Previous update was at 05:48 AM ----------

Ygor thanks for the input i am working on adding that check

7

Oops, right, it's much more complicated than that :smiley: but here we go:

x=11
newdate=$(echo $x | 
awk -v date="$(date "+%d %m %Y")" -v OFS="-"  '
{ 
  num=$0
  split(date, a); d=a[1]; m=a[2]; y=a[3]
  m-=num
  while(m < 1) {m+=12; y--}
  if(m == 2 && d >= 29){
    if((y % 4 == 0 && y % 100 != 0) || (y % 400 == 0)){
      d-=29
      m++
    }
    else {
      d-=28
      m++
    }
  }
  if(d == 31 && (m < 8 && m % 2 == 0) || m > 7 && m % 2 ) {
    d=1
    m++
  }  
  if(m > 12)y++
  while(m < 1) {m+=12; y--}
  printf("%02d%s%02d%s%s\n", d, OFS, m, OFS, y)
}')

echo $newdate
8

Thanks a lot Franklin!!!