Command to generate emacs TAGS file for a directory tree

find . -name '*.[chSs]' -print | etags -

The find command searches for files matching the specified criteria and prints those files to standard output. The '-' argument asks etags to read list of files from standard input.

The part -name '*.[chSs]' -print asks find to print files with extension c, h, S, or s. The first argument is the root of the directory tree in which to search for files.