Bash loop with limits extracted from variable and specific number format

Question

I want to loop using limits extracted from two variables, and get the loop variable have a specific number format, e.g.=001 instead of just 1.

How can I do that?

Answer 1

The following code does it:

date_start=20160701
date_end=20160731

for ((year=${date_start:0:4}; year<=${date_end:0:4}; year++))
do
  yyyy=$(printf "%04d" $year)
  for ((month=${date_start:4:2}; month<=${date_end:4:2}; month++))
  do
    mm=$(printf "%02d" $month)
    for ((day=${date_start:6:2}; day<=${date_end:6:2}; day++))
    do
      dd=$(printf "%02d" $day)

      echo ${yyyy}${mm}${dd}

    done
  done
done

My question: is there a better way to do this inside a Bash script? If yes, why that way is better?

(Linux Ubuntu 20.04.03, Bash 5.1.16)

Answer 2

If you want to iterate through a range of dates (represented as YYYYMMDD) and you have GNU date, then this may be what you want:

#!/bin/bash

date_start=20160701
date_end=20160731

for ((cur_date = date_start; cur_date <= date_end;)); do
    echo "$cur_date"
    cur_date=$(date -d "$cur_date next day" +%Y%m%d)
done