Rename utility

Perl utility on Arch Linux

pamac install perl-rename
#
perl-rename --version
perl-rename 1.16

Change extension

perl-rename 's/.txt$/.doc$/' *.txt

... recursive

find . -type f -exec perl-rename 's/$/.eml/' {} +

Debian

apt install rename

rename --version
# /usr/bin/rename using File::Rename version 2.01

find . -type f -exec rename 's/$/.eml/' {} +