abs-book.sgml

一本完整的描述Unix Shell 编程的工具书的所有范例

	<varlistentry><term><token>.</token></term>
	  <indexterm>
	    <primary>.</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>.</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>character match</primary>
	  </indexterm>	  
	  <indexterm>
	    <primary>match single character</primary>
	  </indexterm>	  

	  <listitem>
	    <formalpara><title><quote>dot</quote> character match</title>
	      <para>When <link linkend="regexdot">matching
	        characters</link>, as part of a <link
	        linkend="regexref">regular expression</link>, a
	        <quote>dot</quote> matches a single character.</para>
	    </formalpara>

	    </listitem>
	</varlistentry>


	<varlistentry>
	  <term><token>"</token></term>
	  <listitem><formalpara><title><link linkend="dblquo">partial
	    quoting</link> [double quote]</title>
	      <para><emphasis>"STRING"</emphasis> preserves (from
	      interpretation) most of the special characters within
	      <emphasis>STRING</emphasis>. See also <xref
	      linkend="quoting">.</para>
	    </formalpara> </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>'</token></term>
	  <listitem><formalpara><title><link linkend="snglquo">full
	    quoting</link> [single quote]</title>
	      <para><emphasis>'STRING'</emphasis> preserves all special
	      characters within <emphasis>STRING</emphasis>. This is a
	      stronger form of quoting than using <token>"</token>.
	      See also <xref linkend="quoting">.</para>
	    </formalpara> </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>,</token></term>
	  <listitem><formalpara><title><link linkend="commaop">comma
	    operator</link></title>
	      <para>The <command>comma operator</command> links together a
	        series of arithmetic operations. All are evaluated, but only
		the last one is returned.
               <programlisting>let "t2 = ((a = 9, 15 / 3))"  # Set "a = 9" and "t2 = 15 / 3"</programlisting>
	      </para> 
	    </formalpara> </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>\</token></term>
	  <listitem><formalpara><title><link linkend="escp">escape</link> [backslash]</title>
	      <para>A quoting mechanism for single characters.</para>
	    </formalpara>
	      
	      <para><userinput>\X</userinput>
		<quote>escapes</quote> the character
		<emphasis>X</emphasis>. This has the effect of
		<quote>quoting</quote> <emphasis>X</emphasis>, equivalent
		to <emphasis>'X'</emphasis>.  The <token>\</token> may
		be used to quote <token>"</token> and <token>'</token>,
		so they are expressed literally.</para>
	      <para>See <xref linkend="quoting"> for an in-depth explanation
	        of escaped characters.</para>
	    </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>/</token></term>
	  <listitem><formalpara><title>Filename path separator [forward slash]</title>
	      <para>Separates the components of a filename (as in
	        <filename>/home/bozo/projects/Makefile</filename>).</para>
	    </formalpara>
	    <para>This is also the division <link
	      linkend="arops1">arithmetic operator</link>.</para>
	    </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>`</token></term>
	  <listitem><formalpara><title><link
	  linkend="commandsubref">command substitution</link></title>
	      <para>The <command>`command`</command> construct makes
		available the output of <command>command</command>
		for assignment to a variable. This is also known as
		<link linkend="backquotesref">backquotes</link> or
		backticks.</para></formalpara> </listitem>
	</varlistentry>

	<varlistentry><term><token>:</token></term>
	  <indexterm>
	    <primary>:</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>:</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>null command</primary>
	  </indexterm>	  
	  <indexterm>
	    <primary>true</primary>
	  </indexterm>	  
	  <indexterm>
	    <primary>endless loop</primary>
	  </indexterm>	  

	  <listitem>
	    <para><anchor id="nullref"></para>
	    <formalpara><title>null command [colon]</title>
	      <para>This is the shell equivalent of a
		<quote>NOP</quote> (<replaceable>no op</replaceable>, a
		do-nothing operation). It may be considered a synonym for
		the shell builtin <link linkend="trueref">true</link>. The
		<quote><token>:</token></quote> command is itself a
		Bash <link linkend="builtinref">builtin</link>, and its
		<link linkend="exitstatusref">exit status</link> is
		<quote>true</quote> (<returnvalue>0</returnvalue>).</para>
	    </formalpara>

	    <para><programlisting>:
echo $?   # 0</programlisting></para>


	    <para>Endless loop:</para>

	    <para><programlisting>
while :
do
   operation-1
   operation-2
   ...
   operation-n
done

# Same as:
#    while true
#    do
#      ...
#    done</programlisting>
	    </para>

	    <para>Placeholder in if/then test:</para>

	    <para><programlisting>
if condition
then :   # Do nothing and branch ahead
else
   take-some-action
fi</programlisting>
	    </para>

	    <para>Provide a placeholder where a binary operation is
	      expected, see <xref linkend="arithops"> and <link
	      linkend="defparam">default parameters</link>.</para>

	    <para><programlisting>: ${username=`whoami`}
# ${username=`whoami`}   Gives an error without the leading :
#                        unless "username" is a command or builtin...</programlisting>
</para>

	    <para>Provide a placeholder where a command is expected in a
	      <link linkend="heredocref">here document</link>. See <xref
	      linkend="anonheredoc">.</para>

	    <para>Evaluate string of variables using
		<link linkend="paramsubref">parameter substitution</link>
		(as in <xref linkend="ex6">).

	    <programlisting>: ${HOSTNAME?} ${USER?} ${MAIL?}
#  Prints error message
#+ if one or more of essential environmental variables not set.</programlisting>
</para>
            
            <para><command><link linkend="exprepl1">Variable expansion / substring
	      replacement</link></command>.</para>
	    
	    <para>In combination with the <token>&gt;</token> <link
	      linkend="ioredirref">redirection operator</link>,
	      truncates a file to zero length, without changing its
	      permissions. If the file did not previously exist,
	      creates it.

	      <programlisting>: > data.xxx   # File "data.xxx" now empty.	      

# Same effect as   cat /dev/null >data.xxx
# However, this does not fork a new process, since ":" is a builtin.</programlisting>
              See also <xref linkend="ex12">.</para>

	    <para>In combination with the <token>&gt;&gt;</token>
	      redirection operator, has no effect on a pre-existing
	      target file (<userinput>: &gt;&gt; target_file</userinput>).
	      If the file did not previously exist, creates it.</para>

	    <note><para>This applies to regular files, not pipes,
	      symlinks, and certain special files.</para></note>


	    <para>May be used to begin a comment line, although this is not
	      recommended. Using <token>#</token> for a comment turns
	      off error checking for the remainder of that line, so
	      almost anything may appear in a comment. However,
	      this is not the case with
	      <token>:</token>.
	      <programlisting>: This is a comment that generates an error, ( if [ $x -eq 3] ).</programlisting>
	    </para>

	    <para>The <quote><token>:</token></quote> also serves as a field
	      separator, in <filename>/etc/passwd</filename>, and in the <link
	      linkend="pathref">$PATH</link> variable.
	      <screen><prompt>bash$ </prompt><userinput>echo $PATH</userinput>
<computeroutput>/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games</computeroutput></screen>
	    </para>



	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>!</token></term>
	  <indexterm>
	    <primary>!</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>!</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>not</primary>
	    <secondary>logical</secondary>
	  </indexterm>	  
	  <indexterm>
	    <primary>not</primary>
	  </indexterm>	  

	  <listitem>
	  <para><anchor id="notref"></para>
	  <formalpara><title>reverse (or negate) the sense of
	  a test or exit status [bang]</title>

	      <para>The <token>!</token> operator inverts the <link
		linkend="exitstatusref">exit status</link>
		of the command to which it is applied (see
		<xref linkend="negcond">). It also inverts
		the meaning of a test operator. This can, for
		example, change the sense of <quote>equal</quote>
		( <link linkend="equalsignref">=</link>
		) to <quote>not-equal</quote> ( != ). The
		<token>!</token> operator is a Bash <link
		linkend="keywordref">keyword</link>.</para>

	    </formalpara>
	      <para>In a different context, the <token>!</token>
	        also appears in <link linkend="ivrref">indirect variable
		references</link>.</para>

	      <para>In yet another context, from the <emphasis>command
		line</emphasis>, the <token>!</token> invokes the
		Bash <emphasis>history mechanism</emphasis> (see <xref
		linkend="histcommands">). Note that within a script,
		the history mechanism is disabled.</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 [asterisk]</title>

	      <para>The <token>*</token> character serves as a <quote>wild
	        card</quote> for filename expansion in
		<link linkend="globbingref">globbing</link>. By itself,
		it matches every filename in a given directory.</para>
	    </formalpara>

	  <para>
	      <screen><prompt>bash$ </prompt><userinput>echo *</userinput>
<computeroutput>abs-book.sgml add-drive.sh agram.sh alias.sh</computeroutput>
	      </screen>
	    </para>
		
	      <para>The <token>*</token> also represents any number
	      (or zero) characters in a <link
		linkend="regexref">regular expression</link>.</para>

	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>*</token></term>
	  <indexterm>
	    <primary>*</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>*</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>multiplication</primary>
	    <secondary>exponentiation</secondary>
	  </indexterm>	  
	  <indexterm>
	    <primary>arithmetic operator</primary>
	  </indexterm>	  

	  <listitem><formalpara><title><link linkend="arops1">arithmetic operator</link></title>

	      <para>In the context of arithmetic operations, the
	        <token>*</token> denotes multiplication.</para>

	    </formalpara>

	      <para>A double asterisk, <token>**</token>, is the <link
	        linkend="exponentiationref">exponentiation
	        operator</link>.</para>

	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term><token>?</token></term>
	  <indexterm>
	    <primary>?</primary>
	  </indexterm>
	  <indexterm>
	    <primary>special character</primary>
	    <secondary>?</secondary>
	  </indexterm>
	  <indexterm>
	    <primary>test</primary>
	    <secondary>operator</secondary>
	  </indexterm>	  
	  <indexterm>
	    <primary>test token</primary>
	  </indexterm>	  

	  <listitem><formalpara><title>test operator</title>
	      <para>Within certain expressions, the <token>?</token> indicates
	        a test for a condition.</para>
	    </formalpara>

	      <para>In a <link linkend="dblparens">double
	        parentheses construct</link>, the <token>?</token> serves
		as a C-style trinary operator. See <xref
		linkend="cvars">.</para>