[bash] How to add leading zeros for for-loop in shell?

I have a basic number for loop which increments the variable num by 1 over each iteration...

for (( num=1; num<=5; num++ ))
do
 echo $num
done

Which outputs:

1
2
3
4
5

I'm trying to make it produce the output (add leading zero before $num):

01
02
03
04
05

Without doing:

echo 0$num

From bash 4.0 onward, you can use Brace Expansion with fixed length strings. See below for the original announcement.

It will do just what you need, and does not require anything external to the shell.

$ echo {01..05}
01 02 03 04 05

for num in {01..05}
do
  echo $num
done
01
02
03
04
05

CHANGES, release bash-4.0, section 3

This is a terse description of the new features added to bash-4.0 since the release of bash-3.2.

. . .

z. Brace expansion now allows zero-padding of expanded numeric values and will add the proper number of zeroes to make sure all values contain the same number of digits.

Just a note: I have experienced different behaviours on different versions of bash:

• version 3.1.17(1)-release-(x86_64-suse-linux) and
• Version 4.1.17(9)-release (x86_64-unknown-cygwin))

for the former (3.1) for nn in (00..99) ; do ... works but for nn in (000..999) ; do ... does not work both will work on version 4.1 ; haven't tested printf behaviour (bash --version gave the version info)

Cheers, Jan

seq -w will detect the max input width and normalize the width of the output.

for num in $(seq -w 01 05); do
    ...
done

At time of writing this didn't work on the newest versions of OSX, so you can either install macports and use its version of seq, or you can set the format explicitly:

seq -f '%02g' 1 3
    01
    02
    03

But given the ugliness of format specifications for such a simple problem, I prefer the solution Henk and Adrian gave, which just uses Bash. Apple can't screw this up since there's no generic "unix" version of Bash:

echo {01..05}

Or:

for number in {01..05}; do ...; done

why not printf '%02d' $num? See help printf for this internal bash command.

Use printf command to have 0 padding:

printf "%02d\n" $num

Your for loop will be like this:

for (( num=1; num<=5; num++ )); do printf "%02d\n" $num; done
01
02
03
04
05

I'm not interested in outputting it to the screen (that's what printf is mainly used for, right?) The variable $num is going to be used as a parameter for another program but let me see what I can do with this.

You can still use printf:

for num in {1..5}
do
   value=$(printf "%02d" $num)
   ... Use $value for your purposes
done