filename_extension_manipulation.md

The following script ensures that all the files that match a specific pattern have ".pgp" extension. This will not touch a matching file which already has ".pgp" extension. Note that if a non ".pgp" extension already exists, it still appends ".pgp".

#!/usr/bin/ksh
for filename in pattern*
do
    test "${filename}" = "${filename%.pgp}" && mv "$filename" "${filename}.pgp"
done

This can be easily tweaked to change one extension to another. The below script changes extension to ".gpg" on all ".pgp" files that match a pattern.

#!/usr/bin/ksh
for filename in pattern*
do
    test "${filename}" != "${filename%.pgp}" && mv "$filename" "${filename%.pgp}.gpg"
done

The above two scripts can be combined when the goal is to change the extension to ".gpg" on all ".pgp" files that match a pattern along with adding ".gpg" extension on extensionless files.

#!/usr/bin/ksh
for filename in pattern*
do
  echo "Changing .pgp extension, if any, to .gpg : ${filename}"

  modifiedfilename="${filename}"
  test "${filename}" != "${filename%.pgp}" && mv "$filename" "${filename%.pgp}.gpg"  && modifiedfilename="${filename%.pgp}.gpg"

  echo "Append .gpg extension if not present already: ${modifiedfilename}"

  test "${modifiedfilename}" = "${modifiedfilename%.gpg}" && mv "$modifiedfilename" "${modifiedfilename}.gpg" && modifiedfilename="${modifiedfilename}.gpg"

  echo "Final name: ${modifiedfilename}"
done

shell's parameter expansion

• % in below scripts match shortest from the end of the string.
• %% matches longest from the end
• # shortest from the beginning
• ## longest from the beginning

See: https://praveenlobo.com/blog/shell-scripts-to-manipulate-the-file-name-extensions/