sdiff
With sdiff, you can merge two files interactively based on a
side-by-side `-y' format comparison (see section Showing Differences Side by Side). Use
`-o file' or `--output=file' to specify where to
put the merged text. See section Invoking sdiff, for more details on the
options to sdiff.
Another way to merge files interactively is to use the Emacs Lisp
package emerge. See section `emerge' in The GNU Emacs Manual, for more information.
sdiff options.