Bash Shell Current Date Minus Number of Days - bash

Question

I am new to bash and shell but I am running a debian install and I am trying to make a script which can find a date in the past without having to install any additional packages. From tutorials I have got to this stage:
#!/bin/sh
#
# BACKUP DB TO S3
#
# VARIABLES
TYPE="DATABASE"
DAYS="30"
# GET CURRENT DATETIME
CURRENTDATE="$(date +%Y%m%d%H%M%S)"
# GENERATE PAST DATE FROM DAYS CONTSTANT
OLDERDATE=`expr $CURRENTDATE - $DAYS'
# CALL PYTHON SCRIPT WITH OLDERDATE ARGUMENT
python script.py $OLDERDATE
Where I am getting stuck is the fact that my "days" is just the number 30 and isnt datetime formattted, so when I come to minus it from the currentdate variable it obviously isnt compatible.
Would anyone be kind enough to help me find a way to get this working as it should?

Answer 1

Try
date -d '30 days ago'
should do on debian.

Answer 2

Try doing this :
#!/bin/sh
#
# BACKUP DB TO S3
#
# VARIABLES
TYPE="DATABASE"
DAYS="30"
# GET CURRENT DATETIME
CURRENTDATE="$(date +%Y%m%d%H%M%S)"
# GENERATE PAST DATE FROM DAYS CONSTANT
OLDERDATE="$(date "+%Y%m%d%H%M%S" -d "$DAYS days ago")"
# CALL PYTHON SCRIPT WITH OLDERDATE ARGUMENT
python script.py "$OLDERDATE"
See info coreutils 'date invocation' | less +/28.7\ Relative\ items\ in\ date\ strings

Answer 3

You can use the following script:
#!/bin/bash
days=73
while [ ${days} -ge 0 ]; do
date -d "${days} days ago" +'%F'
days=$((days-1))
done

Answer 4

You could modify the python script instead -- that way you would not depend on particular implementation of date