Make directory named 0001 instead of 1 in a bash script loop

Your current script uses only a decimal value - this obviously translates to the value without leading zeroes.

You would have to pad the string to the length of 4 characters, with leading zeroes. You do this by using backticks `` to get the result of the printf call printf %04d $j.

As a result, you should have the full command:

for i in *.jpg; do let j+=1; mkdir `printf %04d $j`; done

Source: StackOverflow: bash - Padding zeros in a string

(Obviously also see the further answers to that question, if you want a more in-depth solution, but this definitely works perfectly for this use case.)

Using bash, your best option is:

mkdir {0001..0666}

to create dirs with name 0001 to 0666 (with zero padding).

If you want directories named prefix0001suffix, prefix0002suffix, ... then:

mkdir prefix{0001..0666}suffix

will do.

If you only want odd number directory names

mkdir {0001..0666..2}

and so on... See Brace Expansion in the bash reference manual.

As FEichinger points out, you need to know in advance the number of directories. If you want a solution that is close to yours, but really safe and using more modern bash idiom:

for i in *.jpg; do ((++j)); mkdir $(printf "%04d" $j) ; done