BASH file mass rename with counter

This does what you ask:

n=1; for f in *.txt; do mv "$f" "CO_$((n++))_$f"; done

How it works

• n=1

  This initializes the variable n to 1.

• for f in *.txt; do

  This starts a loop over all files in the current directory whose names end with .txt.

• mv "$f" "CO_$((n++))_$f"

  This renames the files to have the CO_ prefix with n as the counter. The ++ symbol tells bash to increment the variable n.

• done

  This signals the end of the loop.

Improvement

This version uses printf which allows greater control over how the number will be formatted:

n=1; for f in *.txt; do mv "$f" "$(printf "CO_%02i_%s" "$n" "$f")"; ((n++)); done

In particular, the %02i format will put a leading zero before the number when n is still in single digits.