shell - How can I increment a number in a while-loop while preserving leading zeroes (BASH < V4) -

i trying write bash script downloads transcripts of podcast curl. transcript files have name differs 3 digits: filename[three-digits].txt

• from filename001.txt
• to.... filename440.txt.

i store 3 digits number in variable , increment variable in while loop. how can increment number without losing leading zeroes?

#!/bin/bash clear  # [...] code handling storage  episode=001 last=440  secnow_transcript_url="https://www.grc.com/sn/sn-" last_token=".txt"  while [ $episode -le $last ];     curl -x $secnow_transcript_url$episode$last_token > # storage location     episode=$[$episode+001];     sleep 60 # don't stress server much! done 

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i searched lot , discovered nice approaches of others, solve problem, out of curiosity love know if there solution problem keeps while-loop, despite for-loop more appropriate in first place, know range, day come, when need while loop! :-)

#!/bin/bash episode in $(seq -w 01 05);     curl -x $secnow_transcript_url$episode$last_token > # ... done 

or few digits (becomes unpractical more digits)

#!/bin/bash episode in 00{1..9} 0{10..99} {100..440};     curl -x $secnow_transcript_url$episode$last_token > # ... done 

you can use $((10#$n)) remove 0 padding (and calculations), , printf add 0 padding back. here both put increment 0 padded number in while loop:

n="0000123" digits=${#n} # number of digits, here calculated original number while sleep 1     n=$(printf "%0${digits}d\n" "$((10#$n + 1))")     echo "$n" done