How can I use numbers with leading zeros in a loop, e.g. 01, 02?

As always, there are different ways to solve the problem, each with its own advantages and disadvantages.

If there are not many numbers, BraceExpansion can be used:

    for i in 0{1,2,3,4,5,6,7,8,9} 10
        echo $i



This gets tedious for large sequences, but there are other ways, too. If you have the printf command (which is a Bash builtin, and is also POSIX standard), it can be used to format a number:

    for ((i=1; i<=10; i++))     # Bash 2 for-loop syntax
        printf "%02d " "$i"

In Bash 3, you can use ranges inside brace expansion. Also, since printf will implicitly loop if given more arguments than format specifiers, you can simplify this enormously:

   printf "%03d\n" {1..300}     # Bash 3 brace expansion

The KornShell and KornShell93 have the typeset command to specify the number of leading zeros:

    $ typeset -Z3 i=4
    $ echo $i

If the command seq(1) is available (it's part of GNU sh-utils/coreutils), you can use it as follows:

    seq -w 1 10

or, for arbitrary numbers of leading zeros (here: 3):

    seq -f "%03g" 1 10

Combining printf with seq(1), you can do things like this:

   printf "%03d\n" $(seq 300)

(That may be helpful if your version of seq(1) lacks printf-style format specifiers. Since it's a nonstandard external tool, it's good to keep your options open.)

Be warned however that seq might be considered bad style, it's even mentioned in ["Don't Ever Do These"].

Finally, the following example works with any BourneShell derived shell to zero-pad each line to three bytes:

   while test $i -le 10
       echo "00$i"
       i=`expr $i + 1`
   done |
       sed 's/.*\(...\)$/\1/g'

In this example, the number of '.' inside the parentheses in the sed command determines how many total bytes from the echo command (at the end of each line) will be kept and printed.

Now, since the number one reason this question is asked is for downloading images in bulk, you can use the printf command with xargs(1) and wget(1) to fetch files:

   printf "%03d\n" {$START..$END} | xargs -i% wget $LOCATION/%

Or, in a slightly more general case:

   for i in {1..100}; do
      wget "$prefix$(printf %03d $i).jpg"
      # other commands