387 lines
6.9 KiB
HTML
387 lines
6.9 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>Using case statements</TITLE
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Bash Guide for Beginners"
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TITLE="Conditional statements"
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HREF="chap_07.html"><LINK
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TITLE="More advanced if usage"
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HREF="sect_07_02.html"><LINK
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TITLE="Summary"
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HREF="sect_07_04.html"></HEAD
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>Bash Guide for Beginners</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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><A
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HREF="sect_07_02.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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VALIGN="bottom"
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>Chapter 7. Conditional statements</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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HREF="sect_07_04.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="sect_07_03"
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></A
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>7.3. Using case statements</H1
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_07_03_01"
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></A
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>7.3.1. Simplified conditions</H2
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><P
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>Nested <B
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CLASS="command"
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>if</B
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> statements might be nice, but as soon as you are confronted with a couple of different possible actions to take, they tend to confuse. For the more complex conditionals, use the <B
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CLASS="command"
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>case</B
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> syntax:</P
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><P
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><B
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CLASS="command"
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>case <TT
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CLASS="function"
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>EXPRESSION</TT
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> in <TT
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CLASS="function"
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>CASE1</TT
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>) COMMAND-LIST;; <TT
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CLASS="function"
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>CASE2</TT
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>) COMMAND-LIST;; ... <TT
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CLASS="function"
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>CASEN</TT
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>) COMMAND-LIST;; esac</B
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> </P
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><P
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>Each case is an expression matching a pattern. The commands in the <B
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CLASS="command"
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>COMMAND-LIST</B
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> for the first match are executed. The <SPAN
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CLASS="QUOTE"
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>"|"</SPAN
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> symbol is used for separating multiple patterns, and the <SPAN
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CLASS="QUOTE"
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>")"</SPAN
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> operator terminates a pattern list. Each case plus its according commands are called a <EM
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>clause</EM
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>. Each clause must be terminated with <SPAN
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CLASS="QUOTE"
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>";;"</SPAN
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>. Each <B
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CLASS="command"
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>case</B
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> statement is ended with the <B
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CLASS="command"
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>esac</B
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> statement.</P
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><P
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>In the example, we demonstrate use of cases for sending a more selective warning message with the <TT
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CLASS="filename"
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>disktest.sh</TT
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> script:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> <TT
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CLASS="prompt"
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>anny ~/testdir></TT
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> <B
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CLASS="command"
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>cat <TT
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CLASS="filename"
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>disktest.sh</TT
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></B
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>
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#!/bin/bash
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# This script does a very simple test for checking disk space.
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space=`df -h | awk '{print $5}' | grep % | grep -v Use | sort -n | tail -1 | cut -d "%" -f1 -`
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case $space in
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[1-6]*)
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Message="All is quiet."
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;;
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[7-8]*)
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Message="Start thinking about cleaning out some stuff. There's a partition that is $space % full."
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;;
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9[1-8])
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Message="Better hurry with that new disk... One partition is $space % full."
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;;
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99)
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Message="I'm drowning here! There's a partition at $space %!"
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;;
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*)
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Message="I seem to be running with an nonexistent amount of disk space..."
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;;
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esac
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echo $Message | mail -s "disk report `date`" anny
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<TT
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CLASS="prompt"
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>anny ~/testdir></TT
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>
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You have new mail.
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<TT
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CLASS="prompt"
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>anny ~/testdir></TT
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> <B
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CLASS="command"
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>tail <TT
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CLASS="parameter"
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><I
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>-16</I
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></TT
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> <TT
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CLASS="filename"
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>/var/spool/mail/anny</TT
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></B
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>
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From anny@octarine Tue Jan 14 22:10:47 2003
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Return-Path: <anny@octarine>
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Received: from octarine (localhost [127.0.0.1])
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by octarine (8.12.5/8.12.5) with ESMTP id h0ELAlBG020414
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for <anny@octarine>; Tue, 14 Jan 2003 22:10:47 +0100
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Received: (from anny@localhost)
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by octarine (8.12.5/8.12.5/Submit) id h0ELAltn020413
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for anny; Tue, 14 Jan 2003 22:10:47 +0100
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Date: Tue, 14 Jan 2003 22:10:47 +0100
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From: Anny <anny@octarine>
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Message-Id: <200301142110.h0ELAltn020413@octarine>
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To: anny@octarine
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Subject: disk report Tue Jan 14 22:10:47 CET 2003
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Start thinking about cleaning out some stuff. There's a partition that is 87 % full.
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<TT
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CLASS="prompt"
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>anny ~/testdir></TT
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>
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>Of course you could have opened your mail program to check the results; this is just to demonstrate that the script sends a decent mail with <SPAN
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CLASS="QUOTE"
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>"To:"</SPAN
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>, <SPAN
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CLASS="QUOTE"
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>"Subject:"</SPAN
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> and <SPAN
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CLASS="QUOTE"
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>"From:"</SPAN
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> header lines.</P
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><P
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>Many more examples using <B
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CLASS="command"
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>case</B
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> statements can be found in your system's init script directory. The startup scripts use <B
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CLASS="command"
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>start</B
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> and <B
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CLASS="command"
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>stop</B
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> cases to run or stop system processes. A theoretical example can be found in the next section.</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_07_03_02"
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></A
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>7.3.2. Initscript example</H2
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><P
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>Initscripts often make use of <B
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CLASS="command"
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>case</B
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> statements for starting, stopping and querying system services. This is an excerpt of the script that starts <SPAN
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CLASS="application"
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>Anacron</SPAN
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>, a daemon that runs commands periodically with a frequency specified in days.</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> case "$1" in
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start)
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start
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;;
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stop)
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stop
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;;
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status)
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status anacron
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;;
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restart)
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stop
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start
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;;
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condrestart)
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if test "x`pidof anacron`" != x; then
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stop
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start
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fi
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;;
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*)
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echo $"Usage: $0 {start|stop|restart|condrestart|status}"
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exit 1
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esac
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>The tasks to execute in each case, such as stopping and starting the daemon, are defined in functions, which are partially sourced from the <TT
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CLASS="filename"
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>/etc/rc.d/init.d/functions</TT
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> file. See <A
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HREF="chap_11.html"
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>Chapter 11</A
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> for more explanation.</P
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></DIV
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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CELLPADDING="0"
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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HREF="sect_07_02.html"
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ACCESSKEY="P"
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>Prev</A
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VALIGN="top"
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HREF="index.html"
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>Home</A
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>Next</A
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WIDTH="33%"
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>More advanced if usage</TD
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><TD
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WIDTH="34%"
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VALIGN="top"
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WIDTH="33%"
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VALIGN="top"
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>Summary</TD
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> |