abs-book.sgml

一本完整的描述Unix Shell 编程的工具书的所有范例

              <para>In a <link linkend="paramsubref">parameter
	        substitution</link> expression, the <token>?</token>
		<link linkend="qerrmsg">tests whether a variable has been
		set</link>.</para>

	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>?</token></term>
	  <indexterm>
	    <primary>?</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>?</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>wild card</primary>
	    <secondary>globbing</secondary>
	  </indexterm>	  
	  <indexterm>
	    <primary>regular expression</primary>
	  </indexterm>	  

	  <listitem><formalpara><title>wild card</title>

	      <para>The <token>?</token> character serves as a
		single-character <quote>wild card</quote> for filename
		expansion in <link linkend="globbingref">globbing</link>,
		as well as <link linkend="quexregex">representing one
		character</link> in an <link linkend="extregex">extended
		regular expression</link>.</para>

	    </formalpara>

	  </listitem>
	</varlistentry>

	<varlistentry><term><token>$</token></term>
	  <indexterm>
	    <primary>$</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>$</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>variable substitution</primary>
	  </indexterm>	  
	  <listitem>
            <formalpara><title><link linkend="varsubn">Variable
              substitution</link> (contents of a variable)</title>
             <para>
	       <programlisting>var1=5
var2=23skidoo

echo $var1     # 5
echo $var2     # 23skidoo</programlisting>
	       </para>
	       </formalpara>

	       <para>A <token>$</token> prefixing a variable name
		indicates the <emphasis>value</emphasis> the variable
		holds.</para>

	  </listitem>
	</varlistentry>  

	<varlistentry><term><token>$</token></term>
	  <indexterm>
	    <primary>$</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>$</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>regular expression</primary>
	  </indexterm>	  
	  <indexterm>
	    <primary>end of line</primary>
	  </indexterm>	  

	  <listitem>
	    <formalpara><title>end-of-line</title>
	      <para>In a <link linkend="regexref">regular expression</link>, a
	        <quote>$</quote> addresses the end of a line of text.</para>
	    </formalpara>

	    </listitem>
	</varlistentry>

	<varlistentry><term><token>${}</token></term>
	  <indexterm>
	    <primary>${}</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>${}</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>parameter substitution</primary>
	  </indexterm>	  
	  <listitem>
            <formalpara><title><link linkend="paramsubref">Parameter
              substitution</link></title>
             <para></para></formalpara>
	  </listitem>
	</varlistentry>  

	<varlistentry>
	  <term><token>$*</token></term>
	  <term><token>$@</token></term>
	  <indexterm>
	    <primary>$*</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>$*</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>$@</primary>
	    <secondary>positional parameters</secondary>
	  </indexterm>	  
	  <indexterm>
	    <primary>$@</primary>
	  </indexterm>	  

	  <listitem><formalpara><title><link linkend="appref">positional
	    parameters</link></title>
	      <para></para>
	    </formalpara>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>$?</token></term>
	  <indexterm>
	    <primary>$?</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>?</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>exit status</primary>
	    <secondary>variable</secondary>
	  </indexterm>	  
	  <indexterm>
	    <primary>exit status</primary>
	  </indexterm>	  

	  <listitem><formalpara><title>exit status variable</title>
	      <para>The <link linkend="exsref">$? variable</link>
	        holds the <link linkend="exitstatusref">exit status</link>
		of a command, a <link linkend="functionref">function</link>,
		or of the script itself.</para>
	    </formalpara>

	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>$$</token></term>
	  <indexterm>
	    <primary>$$</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>$$</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>process ID</primary>
	    <secondary>variable</secondary>
	  </indexterm>	  
	  <indexterm>
	    <primary>process ID</primary>
	  </indexterm>	  

	  <listitem><formalpara><title>process ID variable</title>
	      <para>The <link linkend="proccid">$$ variable</link>
	        holds the <emphasis>process ID</emphasis> of the script in
		which it appears.</para>
	    </formalpara>

	  </listitem>
	</varlistentry>


	<varlistentry>
	  <term><token>()</token></term>

	  <listitem><formalpara><title>command group</title>
	      <para><programlisting>(a=hello; echo $a)</programlisting></para>
	    </formalpara>

	  <important>
	  
	    <para>A listing of commands within
	      <replaceable>parentheses</replaceable> starts a <link
	      linkend="subshellsref">subshell</link>.</para>

	    <para>Variables inside parentheses, within the subshell, are not
	      visible to the rest of the script. The parent process,
	      the script, <link linkend="parvis">cannot read variables
	      created in the child process</link>, the subshell.
	      <programlisting>a=123
( a=321; )	      

echo "a = $a"   # a = 123
# "a" within parentheses acts like a local variable.</programlisting></para>
	  </important>


	  <formalpara><title>array initialization</title>
	    <para><programlisting>Array=(element1 element2 element3)</programlisting></para>
	    </formalpara>
	  </listitem>

	</varlistentry>

	<varlistentry>
	  <indexterm>
	    <primary>{xxx,yyy,zzz..}</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>{}</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>brace expansion</primary>
	  </indexterm>	  
	  <term><token>{xxx,yyy,zzz,...}</token></term>
	  <listitem><formalpara><title>Brace expansion</title>
	      <para><anchor id="braceexpref"><programlisting>cat {file1,file2,file3} > combined_file
# Concatenates the files file1, file2, and file3 into combined_file.


cp file22.{txt,backup}
# Copies "file22.txt" to "file22.backup"
</programlisting></para>
	    </formalpara>
	  <para>A command may act upon a comma-separated list of file specs within
	  <replaceable>braces</replaceable>.

	     <footnote><para>The shell does the <emphasis>brace
	       expansion</emphasis>. The command itself acts upon the
	       <emphasis>result</emphasis> of the expansion.</para></footnote>
	  
	  Filename expansion (<link linkend="globbingref">globbing</link>)
	  applies to the file specs between the braces.</para>

	  <caution>
	    <para>No spaces allowed within the braces
	    <emphasis>unless</emphasis> the spaces are quoted or escaped.</para>
	    
	    <para><userinput>echo {file1,file2}\ :{\ A," B",' C'}</userinput></para>
	    <para><computeroutput>file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C</computeroutput></para>

	   </caution>

	  </listitem>
	</varlistentry>


	<varlistentry>
	  <term><token>{}</token></term>
	  <indexterm>
	    <primary>{}</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>{}</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>block of code</primary>
	  </indexterm>	  

	  <listitem>
	  <para><anchor id="codeblockref"></para>
	  <formalpara><title>Block of code [curly brackets]</title>
	      <para>Also referred to as an <quote>inline group</quote>,
		this construct, in effect, creates an anonymous
		function. However, unlike a <link
		linkend="functionref">function</link>, the variables
		in a code block remain visible to the remainder of the
		script.</para></formalpara>

	      <para> <screen><prompt>bash$ </prompt><userinput>{ local a;
	      a=123; }</userinput>
<computeroutput>bash: local: can only be used in a
function</computeroutput>
	      </screen> </para>

	      <para><programlisting>a=123
{ a=321; }
echo "a = $a"   # a = 321   (value inside code block)

# Thanks, S.C.</programlisting></para>


	    <para>The code block enclosed in braces may have <link
	      linkend="ioredirref">I/O redirected</link> to and from
	      it.</para>

	    <example id="ex8">
	      <title>Code blocks and I/O redirection</title>
	      <programlisting>&ex8;</programlisting>
	    </example>

	    <example id="rpmcheck">
	      <title>Saving the results of a code block to a file</title>
	      <programlisting>&rpmcheck;</programlisting>
	    </example>

	    <note><para>Unlike a command group within (parentheses),
	      as above, a code block enclosed by {braces} will
	      <emphasis>not</emphasis> normally launch a <link
	      linkend="subshellsref">subshell</link>.

		<footnote><para>Exception: a code block in braces as
		  part of a pipe <emphasis>may</emphasis> run as a
		  <link linkend="subshellsref">subshell</link>.

		<programlisting>ls | { read firstline; read secondline; }
#  Error. The code block in braces runs as a subshell,
#+ so the output of "ls" cannot be passed to variables within the block.
echo "First line is $firstline; second line is $secondline"  # Will not work.

# Thanks, S.C.</programlisting></para></footnote>

	      </para></note>

	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>{} \;</token></term>
	  <listitem>

	  <formalpara><title>pathname</title>
	      <para>Mostly used in <link linkend="findref">find</link>
		constructs.  This is <emphasis>not</emphasis> a shell
		<link linkend="builtinref">builtin</link>.</para>
	    </formalpara>

	      <note><para>The <quote><token>;</token></quote> ends
		the <option>-exec</option> option of a
		<command>find</command> command sequence.  It needs
		to be escaped to protect it from interpretation by the
		shell.</para></note>

	  </listitem>
	</varlistentry>


	<varlistentry><term><token>[ ]</token></term>
	  <indexterm>
	    <primary>[]</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>[ ]</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>test</primary>
	  </indexterm>	  
	  <listitem>
            <formalpara><title>test</title>
             <para></para></formalpara>
	     <para><anchor id="leftbracket"><link
	       linkend="ifthen">Test</link> expression between <command>[
	       ]</command>.  Note that <command>[</command> is part of
	       the shell builtin <command>test</command> (and a synonym
	       for it), <emphasis>not</emphasis> a link to the external
	       command <filename