Bash - Prepend zero to single digit in while loop

I made this script exif_script And want to change ep variable to double digit Eg 01 instread of 1.

#!/bin/bash x=0 ep=1 while [ $x -le 11 ] do echo "Welcome $x times" date --date="$x week" +"%Y:%m:%d" exiftool -exififd:dateTimeOriginal="$(date --date="$x week" +"%Y:%m:%d") 00" $1$ep* x=$(( $x + 1 )) ep=$(( $ep + 1 )) done 

3 Answers

You can use shell parameter expansion to add zero-padding to numbers.

For example, to print number in a 5-digits format:

$ zeroos="00000" $ number="123" $ temp_num="$zeroos$number" $ echo "${temp_num:(-5)}" 00123 # Or instead of the "5" constant, use $zeroos length $ echo "${temp_num:(-${#zeroos})}" 00123 

Update:

Above example cuts off the front-most digits of $number, when the number becomes longer (with more digits) than $zeroos:

$ number="1234567" # Changed to 7 digits echo "${temp_num:(-5)}" 34567 # Resulted in a wrong number! 

A safer way:

$ number="123" $ echo "${zeroos:${#number}:${#zeroos}}${number}" 00123 $ number="1234567" $ echo "${zeroos:${#number}:${#zeroos}}${number}" 1234567 

Note 1:

A slightly different variant, to use the first digits of $zeroos resp. $century (instead of the last ones) would be:

$ century="2000" $ year="1" $ echo "${century:0:-${#year}}${year}" 2001 

This comes in handy for example, if you have mixed dates with 1-, 2- or even 4-digit years, but want them to be 4 digits in any case, without the need of conditions (for checking their actual digits) and any arithmetic.
The downside is, if ${#year} gets greater than ${#century}, this results in an error.

Note 2:

When using printf (as suggested on other answers), be aware that it could be mush slower on a large scale (see an example).

Using your script, the following will work.

#!/bin/bash x=0 ep=1 while [ $x -le 11 ] do ep_padded=$(printf '%02d' $ep) echo "Welcome $x times" date --date="$x week" +"%Y:%m:%d" exiftool -exififd:dateTimeOriginal="$(date --date="$x week" +"%Y:%m:%d") 00" $1$ep_padded* x=$(( $x + 1 )) ep=$(( $ep + 1 )) done 

You can use the following form to increment your variables too

x=$((++x)) ep=$((++ep)) 

set your variable as below, this will add a padding zero, you can add more zeros in case of your expectations.

ep="$(printf '%02d' $((++ep)) )"