abs-book.sgml

一本完整的描述Unix Shell 编程的工具书的所有范例

      script.</para>

    <para><token>#!</token> can be omitted if the script consists only
      of a set of generic system commands, using no internal
      shell directives.  The second example, above, requires the
      initial <token>#!</token>, since the variable assignment line,
      <userinput>lines=50</userinput>, uses a shell-specific construct.
      Note again that <userinput>#!/bin/sh</userinput> invokes the default
      shell interpreter, which defaults to <filename>/bin/bash</filename>
      on a Linux machine.</para>


      <tip>
      <para>This tutorial encourages a modular approach
	to constructing a script. Make note of and collect
	<quote>boilerplate</quote> code snippets that might be useful
	in future scripts. Eventually you can build quite an extensive
	library of nifty routines. As an example, the following script
	prolog tests whether the script has been invoked with the correct
	number of parameters.</para>

	<para><programlisting>E_WRONG_ARGS=65
script_parameters="-a -h -m -z"
#                  -a = all, -h = help, etc.

if [ $# -ne $Number_of_expected_args ]
then
  echo "Usage: `basename $0` $script_parameters"
  # `basename $0` is the script's filename.
  exit $E_WRONG_ARGS
fi</programlisting>
      </para>

      <para>Many times, you will write a script that carries out one
        particular task. The first script in this chapter is an
        example of this. Later, it might occur to you to generalize
        the script to do other, similar tasks. Replacing the literal
        (<quote>hard-wired</quote>) constants by variables is a step in
        that direction, as is replacing repetitive code blocks by <link
        linkend="functionref">functions</link>.</para>

      </tip>



    <sect1 id="invoking">
      <title>Invoking the script</title>

      <para>Having written the script, you can invoke it by <userinput>sh
	scriptname</userinput>,

	  <footnote><para>Caution: invoking a Bash script by <userinput>sh
	  scriptname</userinput> turns off Bash-specific extensions, and the
	  script may therefore fail to execute.</para></footnote>
	
	or alternatively <userinput>bash scriptname</userinput>. (Not
	recommended is using <userinput>sh &lt;scriptname</userinput>,
	since this effectively disables reading from
	<filename>stdin</filename> within the script.)	Much more
	convenient is to make the script itself directly executable with
	a <link linkend="chmodref">chmod</link>.

	<variablelist>
	  <varlistentry>
	    <term>Either:</term> <listitem>
	      <para><userinput>chmod 555 scriptname</userinput> (gives
	      everyone read/execute permission)
	        <footnote><para>A script needs <emphasis>read</emphasis>, as
		well as execute permission for it to run, since the shell
		needs to be able to read it.</para></footnote>
	      </para>
	    </listitem>
	  </varlistentry> <varlistentry>
	    <term>or</term> <listitem>
	      <para><userinput>chmod +rx scriptname</userinput> (gives
	      everyone read/execute permission)</para> <para><userinput>chmod
	      u+rx scriptname</userinput> (gives only the
		script owner read/execute permission)</para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </para>

      <para>Having made the script executable, you may now test it by
	<userinput>./scriptname</userinput>.
	
	  <footnote><para>Why not simply invoke the script with
	  <userinput>scriptname</userinput>? If the directory you
	  are in (<link linkend="pwdref">$PWD</link>) is where
	  <emphasis>scriptname</emphasis> is located, why doesn't
	  this work? This fails because, for security reasons, the
	  current directory is not by default included in a user's <link
	  linkend="pathref">$PATH</link>. It is therefore necessary to
	  explicitly invoke the script in the current directory with
	  a <userinput>./scriptname</userinput>.</para></footnote>

	If it begins with a <quote>sha-bang</quote> line, invoking the
	script calls the correct command interpreter to run it.</para>

      <para>As a final step, after testing and debugging,
        you would likely want to move it to <filename
        class="directory">/usr/local/bin</filename> (as root, of
        course), to make the script available to yourself and all
        other users as a system-wide executable.  The script could
        then be invoked by simply typing <command>scriptname</command>
        <keycap>[ENTER]</keycap> from the command line.</para>

    </sect1> <!-- Invoking the script -->


    <sect1 id="prelimexer">
      <title>Preliminary Exercises</title>

      <orderedlist>

        <listitem>
	<para>System administrators often write scripts to automate common
	  tasks. Give several instances where such scripts would be
	  useful.</para>
	</listitem>  

	<listitem>  
	<para>Write a script that upon invocation shows the
	  <link linkend="dateref">time and date</link>, <link
	  linkend="whoref">lists all logged-in users</link>, and gives
	  the system <link linkend="uptimeref">uptime</link>. The script
	  then <link linkend="ioredirref">saves this information</link>
	  to a logfile.</para>
	</listitem>

      </orderedlist>

    </sect1> <!-- Preliminary Exercises -->

  </chapter> <!-- Starting Off With a Sha-Bang -->


  </part> <!-- Part 1 (Introduction) -->



  <part label="Part 2" id="part2">
    <title>Basics</title>


  <chapter id="special-chars">
    <title>Special Characters</title>

      <variablelist id="scharlist">
        <title><anchor id="scharlist1">Special Characters Found In
          Scripts and Elsewhere</title>

	<varlistentry><term><token>#</token></term>
	  <indexterm>
	    <primary>#</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>#</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>comment</primary>
	  </indexterm>	  
	  <listitem>
	    <formalpara><title>Comments</title>
	      <para>Lines beginning with a <token>#</token>
	      (<link linkend="magnumref">with the exception of
	      <token>#!</token></link>) are comments.</para>
	    </formalpara>

	    <para><programlisting># This line is a comment.</programlisting></para>

	    <para>Comments may also occur following the end of a command.</para>
	    
	    <para><programlisting>echo "A comment will follow." # Comment here.
#                            ^ Note whitespace before #</programlisting></para>

	    <para>Comments may also follow <link
	    linkend="whitespaceref">whitespace</link> at the beginning
	    of a line.</para>

	    <para><programlisting>	# A tab precedes this comment.</programlisting></para>

	    <caution><para>A command may not follow a comment on the
	      same line. There is no method of terminating the comment,
	      in order for <quote>live code</quote> to begin on the same
	      line. Use a new line for the next command.</para></caution>

	    <note><para>Of course, an escaped <token>#</token> in an
	    <command>echo</command> statement does
	    <emphasis>not</emphasis> begin a comment. Likewise, a
	    <token>#</token> appears in <link linkend="psub2">certain parameter
	    substitution constructs</link> and in <link linkend="numconstants">
	    numerical constant expressions</link>.

	    <programlisting>echo "The # here does not begin a comment."
echo 'The # here does not begin a comment.'
echo The \# here does not begin a comment.
echo The # here begins a comment.

echo ${PATH#*:}       # Parameter substitution, not a comment.
echo $(( 2#101011 ))  # Base conversion, not a comment.

# Thanks, S.C.</programlisting>

	  The standard <link linkend="quotingref">quoting and
	  escape</link> characters (&quot; ' \) escape the #.
	  </para></note>

	  <para>Certain <link linkend="psorex1">pattern matching
	    operations</link> also use the <token>#</token>.</para>

	  </listitem>
	</varlistentry>

	<varlistentry><term><token>;</token></term>
	  <indexterm>
	    <primary>;</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>;</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>separator</primary>
	  </indexterm>	  
	  
	  <listitem>
	    <formalpara><title>Command separator [semicolon]</title>
	      <para>Permits putting two or more commands on the same
	        line.</para>
	    </formalpara>

	    <para>
	      <programlisting>echo hello; echo there


if [ -x "$filename" ]; then    # Note that "if" and "then" need separation.
                               # Why?
  echo "File $filename exists."; cp $filename $filename.bak
else
  echo "File $filename not found."; touch $filename
fi; echo "File test complete."</programlisting>
</para>
	    
	    <para>Note that the <quote><token>;</token></quote> sometimes
	      needs to be <link linkend="escp">escaped</link>.</para>

	    </listitem>
	</varlistentry>

	<varlistentry><term><token>;;</token></term>
	  <indexterm>
	    <primary>;;</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>case</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>;;</primary>
	  </indexterm>	  
	  
	  <listitem>
	    <formalpara><title>Terminator in a <link
	      linkend="caseesac1">case</link> option [double semicolon]</title>
	      <para><anchor id="doublesemicolon"></para>
	    </formalpara>

	    <para><programlisting>case "$variable" in
abc)  echo "\$variable = abc" ;;
xyz)  echo "\$variable = xyz" ;;
esac</programlisting></para>

	    </listitem>
	</varlistentry>


	<varlistentry><term><token>.</token></term>
	  <indexterm>
	    <primary>.</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>.</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>dot command</primary>
	  </indexterm>	  
	  <indexterm>
	    <primary>source</primary>
	  </indexterm>	  

	  <listitem>
	    <para><anchor id="dotref"></para>
	    <formalpara><title><quote>dot</quote> command [period]</title>
	      <para>Equivalent to <link
		linkend="sourceref">source</link> (see
		<xref linkend="ex38">). This is a bash <link
		linkend="builtinref">builtin</link>.</para>
	    </formalpara>

	    </listitem>
	</varlistentry>


	<varlistentry><term><token>.</token></term>
	  <indexterm>
	    <primary>.</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>.</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>filename</primary>
	  </indexterm>	  
	  <indexterm>
	    <primary>part of a filename</primary>
	  </indexterm>	  

	  <listitem>
	    <formalpara><title><quote>dot</quote>, as a component of a filename</title>
              <para>When working with filenames, a dot is the prefix
		of a <quote>hidden</quote> file, a file that an
		<link linkend="lsref">ls</link> will not normally show.
	        <screen><prompt>bash$ </prompt><userinput>touch .hidden-file</userinput>
<prompt>bash$ </prompt><userinput>ls -l</userinput>	      
<computeroutput>total 10
 -rw-r--r--    1 bozo      4034 Jul 18 22:04 data1.addressbook
 -rw-r--r--    1 bozo      4602 May 25 13:58 data1.addressbook.bak
 -rw-r--r--    1 bozo       877 Dec 17  2000 employment.addressbook</computeroutput>


<prompt>bash$ </prompt><userinput>ls -al</userinput>	      
<computeroutput>total 14
 drwxrwxr-x    2 bozo  bozo      1024 Aug 29 20:54 ./
 drwx------   52 bozo  bozo      3072 Aug 29 20:51 ../
 -rw-r--r--    1 bozo  bozo      4034 Jul 18 22:04 data1.addressbook
 -rw-r--r--    1 bozo  bozo      4602 May 25 13:58 data1.addressbook.bak
 -rw-r--r--    1 bozo  bozo       877 Dec 17  2000 employment.addressbook
 -rw-rw-r--    1 bozo  bozo         0 Aug 29 20:54 .hidden-file</computeroutput>
	        </screen>
	      </para>
	    </formalpara>

	      <para>When considering directory names, <emphasis>a single
		dot</emphasis> represents the current working directory,
		and <emphasis>two dots</emphasis> denote the parent
		directory.</para>

	      <para>
	        <screen>
<prompt>bash$ </prompt><userinput>pwd</userinput>
<computeroutput>/home/bozo/projects</computeroutput>

<prompt>bash$ </prompt><userinput>cd .</userinput>
<prompt>bash$ </prompt><userinput>pwd</userinput>
<computeroutput>/home/bozo/projects</computeroutput>

<prompt>bash$ </prompt><userinput>cd ..</userinput>
<prompt>bash$ </prompt><userinput>pwd</userinput>
<computeroutput>/home/bozo/</computeroutput>
	        </screen>
	      </para>

	      <para>The <emphasis>dot</emphasis> often appears as the
	        destination (directory) of a file movement command.</para>
	      
	      <para>
	        <screen>
<prompt>bash$ </prompt><userinput>cp /home/bozo/current_work/junk/* .</userinput>
	        </screen>
	      </para>

	  </listitem>
	</varlistentry>