sed ex

tyler@mac:~ % tail *.txt
==> a.txt <==
heaveno

==> b.txt <==
heaveno

==> c.txt <==
heaveno

==> v.txt <==
heaveno
tyler@mac:~ % sed -i.backup -e s/heaven/hell/g *.txt
tyler@mac:~ % tail *.txt
==> a.txt <==
hello

==> b.txt <==
hello

==> c.txt <==
hello

==> v.txt <==
hello
tyler@mac:~ % ls
Applications   Dropbox        Pictures       a.txt.backup   c.txt.backup
Desktop        Library        Public         b.txt          v.txt
Documents      Movies         VirtualBox VMs b.txt.backup   v.txt.backup
Downloads      Music          a.txt          c.txt
tyler@mac:~ % rm *.backup