How to add leading zeros for for-loop in shell

Question

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

User · Answer

Use printf command to have 0 padding   printf   02d n   num   Your for loop will be like this   for    num 1  num lt  5  num       do printf   02d n   num  done 01 02 03 04 05

User · Answer

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

User · Answer

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

User · Answer

why not printf   02d   num  See help printf for this internal bash command

User · Answer

Use the following syntax    for i in  01  05   do echo  quot  i quot   done 01 02 03 04 05  Disclaimer  Leading zeros only work in  gt  bash-4  If you want to use printf  nothing prevents you from putting its result in a variable for further use    foo   printf  quot  02d quot  5    echo  quot   foo  quot  05

User · Answer

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  quot unix quot  version of Bash  echo  01  05   Or  for number in  01  05   do      done

User · Answer

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

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

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