If you regularly edit a certain group of files, you can define them
as a fileset. This lets you perform certain operations, such as
query-replace, and shell commands on all the files at
once. To make use of filesets, you must first add the expression
(filesets-init) to your init file (see Init File). This
adds a ‘Filesets’ menu to the menu bar.
The simplest way to define a fileset is by adding files to it one at a time. To add a file to fileset name, visit the file and type M-x filesets-add-buffer <RET> name <RET>. If there is no fileset name, this creates a new one, which initially contains only the current file. The command M-x filesets-remove-buffer removes the current file from a fileset.
You can also edit the list of filesets directly, with M-x filesets-edit (or by choosing ‘Edit Filesets’ from the ‘Filesets’ menu). The editing is performed in a Customize buffer (see Easy Customization). Normally, a fileset is a simple list of files, but you can also define a fileset as a regular expression matching file names. Some examples of these more complicated filesets are shown in the Customize buffer. Remember to select ‘Save for future sessions’ if you want to use the same filesets in future Emacs sessions.
You can use the command M-x filesets-open to visit all the files in a fileset, and M-x filesets-close to close them. Use M-x filesets-run-cmd to run a shell command on all the files in a fileset. These commands are also available from the ‘Filesets’ menu, where each existing fileset is represented by a submenu.
See Version Control, for a different concept of filesets: groups of files bundled together for version control operations. Filesets of that type are unnamed, and do not persist across Emacs sessions.