Bash
Bash (the Bourne Again SHell) is a Unix shell written for the GNU project. A shell is a command line interpreter that provides an interface to a computer system. The Bash shell is the standard shell used with GNU/Linux systems, and it provides a very high level of functionality. Importantly, it has been ported to Microsoft Windows within the Cygwin POSIX emulation environment for Windows.
Examples
Remove a trailing _Commentary from the end of a filename (excluding the file extension)
for a in *.pdf; do mv "$a" "${a/_Commentary.pdf/.pdf}"; done
Change filenames to all lowercase
for i in *.pdf; do new=`echo $i | tr A-Z a-z`; mv $i $new; done