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