Quickest way to rename files, without retyping directory path

If your shell supported Brace Expansion (works with csh, tcsh, ksh, zsh, bash, mksh, lksh, pdksh, yash with brace-expand enabled by calling yash --brace-expand or set in interative shell with set -o brace-expand, or fish):

mv ~/folder/subfolder/file.{txt,sh}

You can also use rename (part of the util-linux package).

rename .txt .sh ~/folder/subfolder/file.txt

See the rename man page for more details.

All the above are good. This would also work :

( cd ~/folder/subfolder && mv file.txt file.sh )