This is as much a reminder for myself as anything. I needed to create 31 folders named 00 through 31 for each day of the month in a web-directory used to store audio files from CAPE-1 transmissions. The first thought that came to mind was to write a quick one-liner bash command/script to create those folders. So, without much thought, I quickly typed the following into a shell on my web server:
for ((x=1;x<=31;x+=1)); do mkdir $x; done
OK, well that did create 31 directories, but the first 9 were named 1, 2, 3, etc. instead of 01, 02, 03 like I wanted. After a quick search on zero-padding in bash it seemed no one had posted anything about it except to say it was tricky. Being a C programmer before a bash script writer, I thought to myself, “Wouldn’t it be nice if I could use a printf() statement in bash?” Then I remembered that you can, only usually it is used for formatting user interface statements. I could not think of any reason it would not work to deliver the argument to a mkdir statement, so I tried:
$ for ((x=1;x< =31;x+=1)); do mkdir `printf "%02d" $x`; done
01 03 05 07 09 11 13 15 17 19 21 23 25 27 29 31
02 04 06 08 10 12 14 16 18 20 22 24 26 28 30
And it works! Breaking the command down gives a standard bash for loop with the mkdir command as the internal command inside the loop. As an argument to the mkdir command, however, is a printf statement which given the formatting prints a zero-padded (the 0 after %) two-character long (the 2 after the 0) integer, x. Remember, the `s are back-ticks (below the tilde on most keyboards), not single quotation marks (‘).
It seems like there should be a better way of doing this, but quickest way I found to add zero padding to a number in bash is to use the provided printf which uses the same formatting as the C language printf statement. More usage and examples are available at the ss64.org bash printf reference page.