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<TITLE>MH & nmh: Using Exit Status</TITLE>
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<H1>Using Exit Status</H1>
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<P>
When a UNIX command runs, it can return a numeric status value to the
program that started it.
The status can tell the calling program whether the command succeeded
or failed.
Many (but not all) UNIX commands return a status of zero if everything
was okay or nonzero (1, 2, etc.) if something went wrong.
Almost all MH programs return a status.
<A NAME="index2"></A>
The Bourne shell puts the exit status of each process it runs into its
question mark (<TT>?</TT>) variable; the C shell uses its <I>status</I> variable.
So, to see the exit status of an MH command in a particular situation,
run the MH command -- then type <TT>echo $?</TT> to the Bourne shell or
<TT>echo $status</TT> to the C shell.
<P>
Of course, you usually don't have to display the exit status in this
way, because the Bourne shell provides a couple of ways to use the exit
status of one command as a condition of further execution.
After a shell program
runs a UNIX command, it can test the exit status to see if there was a problem.
<P>
<A NAME="index3"></A>
Here's a simple example: a Bourne shell script called <I>newmsgs</I>.
It uses the shell's <I>if</I> structure to run <I>inc</I>,
then test <I>inc</I>'s exit status and branch.
If <I>inc</I> brings in some messages, <I>newmsgs</I> runs the
<I>show</I> command to show the current message (the first new message).
Otherwise, the program prints a note that there were no messages to show.
(You can also get this simple script from this book's online archive.
It's at
<A HREF="../download/split/mh/bin/newmsgs">
<I>download/split/mh/bin/newmsgs</I></A>.)
<A NAME="index4"></A>
<PRE>
% <B>cat newmsgs</B>
#! /bin/sh
if inc
then
show
else
echo "Sorry; no new message to show now."
fi
</PRE>
A more UNIX-like shell script wouldn't tell you that there were no
messages; the <TT>no mail to incorporate</TT>
message from <I>inc</I> would be enough.
You could rewrite <I>newmsgs</I> to use the shell's <TT>&&</TT> operator:
<PRE>
#! /bin/sh
inc && show
</PRE>
There's more information about <TT>&&</TT> in the
Section <A HREF="mhanthsh.htm#MaAlanFu">Making Aliases and Functions</A>.
The shells also have a <TT>||</TT> operator that runs the second command
if the first command returns a nonzero status.
<P>
<P ALIGN=CENTER>
[<A HREF="tocs/jump.htm">Table of Contents</A>] [<A HREF="indexes/map.htm">Index</A>]
[<A HREF="comsub.htm">Previous: Shell Command Substitution</A>]
[<A HREF="hanarg.htm">Next: Looping Through a List of Arguments</A>]
</P>
<HR>
<STRONG>Revised by Jerry Peek.</STRONG>
<EM>Last change $Date: 1999/10/10 05:14:05 $</EM>
<P>
This file is from the third edition of the book <I>MH & xmh: Email
for Users & Programmers</I>, ISBN 1-56592-093-7, by Jerry Peek.
Copyright © 1991, 1992, 1995 by O'Reilly & Associates, Inc.
This file is freely available; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation. For more information, see
<A HREF="../copying.htm">the file <I>copying.htm</I></A>.
<P>
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Suggestions are welcome:
<A HREF="http://www.jpeek.com/">Jerry Peek</A>
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