Linux shell script to add leading zeros to file names

Question

I have a folder with about 1,700 files. They are all named like 1.txt or 1497.txt, etc. I would like to rename all the files so that all the filenames are four digits long.

I.e., 23.txt becomes 0023.txt.

What is a shell script that will do this? Or a related question: How do I use grep to only match lines that contain \d.txt (i.e., one digit, then a period, then the letters txt)?

Here's what I have so far:

for a in [command i need help with] do   mv $a 000$a done 

Basically, run that three times, with commands there to find one digit, two digits, and three digit filenames (with the number of initial zeros changed).

Answer 1

Try:

for a in [0-9]*.txt; do     mv $a `printf %04d.%s ${a%.*} ${a##*.}` done 

Change the filename pattern ([0-9]*.txt) as necessary.

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A general-purpose enumerated rename that makes no assumptions about the initial set of filenames:

X=1; for i in *.txt; do   mv $i $(printf %04d.%s ${X%.*} ${i##*.})   let X="$X+1" done 

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