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BNFILE=$(echo "${FNFILE%.txt}" | sed 's/[1-9][0-9]\{,10\}$//')
OLDNUM=$(echo "${FNFILE%.txt}" | sed 's/.*[^1-9]\([0-9][0-9]*\)/\1/')

The reason is that "012" is not a number. "12" is a number and you can increment that to "13". But "012" and "013" are strings and you cannot do calculations with strings.

You haven't told us yet...

For your latest request - increment the last number in the file names - , on top of the "parameter expansion" proposed by bakunin, "arithmetic expansion" could help. Its syntax is (( variable /...

A few things:



The first is: any command involving sed .... | sed ... is (99.99999% of the times) wrong: sed is perfectly capable of handling several commands in a script with loops, branches...

$ < file.txt sed 's/.*[^0-9]\([0-9][0-9]*\).txt$/\1/'
4567
789
099
012
13or$ sed 's/.*[^0-9]\([0-9][0-9]*\).txt$/\1/' file.txt
4567
789
099
012
13

Andrew