[string] Convert string to date in bash

I have a string in the format "yyyymmdd". It is a string in bash and I want to get it converted into a date so that all other date functions can be used on it.

"20121212" string into "20121212" date with format "%Y%m%d".

We can use date -d option

1) Change format to "%Y-%m-%d" format i.e 20121212 to 2012-12-12

date -d '20121212' +'%Y-%m-%d'

2)Get next or last day from a given date=20121212. Like get a date 7 days in past with specific format

date -d '20121212 -7 days' +'%Y-%m-%d'

3) If we are getting date in some variable say dat

dat2=$(date -d "$dat -1 days" +'%Y%m%d')

date only work with GNU date (usually comes with Linux)

for OS X, two choices:

1. change command (verified)

   #!/bin/sh
   #DATE=20090801204150
   #date -jf "%Y%m%d%H%M%S" $DATE "+date \"%A,%_d %B %Y %H:%M:%S\""
   date "Saturday, 1 August 2009 20:41:50"
   

   http://www.unix.com/shell-programming-and-scripting/116310-date-conversion.html

2. Download the GNU Utilities from Coreutils - GNU core utilities (not verified yet) http://www.unix.com/emergency-unix-and-linux-support/199565-convert-string-date-add-1-a.html

just use the -d option of the date command, e.g.

date -d '20121212' +'%Y %m'