.\"
.\"	aegis - project change supervisor
.\"	Copyright (C) 1993, 1994 Peter Miller.
.\"	All rights reserved.
.\"
.\"	This program is free software; you can redistribute it and/or modify
.\"	it under the terms of the GNU General Public License as published by
.\"	the Free Software Foundation; either version 2 of the License, or
.\"	(at your option) any later version.
.\"
.\"	This program is distributed in the hope that it will be useful,
.\"	but WITHOUT ANY WARRANTY; without even the implied warranty of
.\"	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\"	GNU General Public License for more details.
.\"
.\"	You should have received a copy of the GNU General Public License
.\"	along with this program; if not, write to the Free Software
.\"	Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA.
.\"
.\" MANIFEST: User Guide, The Difference Tools, Using diff and merge
.\"
.bp
.nh 2 "Using diff and merge"
.LP
These two tools are available with most flavours of UNIX,
but often in a very limited form.
One severe limitation is the
.I diff3 (1)
command,
which often can only cope with 200 lines of differences.
The best alternative is to use GNU diff,
with context differences available,
and a far more robust diff3.
.LP
See the earlier
.I Interfacing
section for substitution details.
.nh 3 "diff_command"
.LP
The entry in the
.I config
file looks like this:
.E(
diff_command =
  "set +e; diff -c $original \e
   $input > $output; test $? -le 1";
.E)
.LP
This needs a little explanation:
.br
\(bu
This command is always executed with the shell's
.B -e
option enabled,
causing the shell to exit on the first error.
The "set +e" turns this off.
.br
\(bu
The
.I diff (1)
command exits with a status of 0 if the files are identical,
and a status of 1 if they differ.
Any other status means something horrible happened.
The "test" command is used to change this to the exit status aegis expects.
.nh 3 "merge_command"
.LP
The entry in the
.I config
file looks like this:
.E(
merge_command =
  "(diff3 -e $MostRecent $original  \e
   $input | sed -e '/^w$$/d' -e     \e
   '/^q$$/d'; echo '1,$$p' ) | ed - \e
   $MostRecent > $output";
.E)
.LP
This needs a lot of explanation.
.br
\(bu
The
.I diff3 (1)
command is used to produce an edit script that will
incorporate into $MostRecent,
all the changes between $original and $input.
.br
\(bu
The
.I sed (1)
command is used to remove the "write" and "quit" commands
from the generated edit script.
.br
\(bu
The
.I ed (1)
command is used to apply the generated edit script to
the $MostRecent file,
and print the results on the standard output,
which are redirected into the $output file.