special-chars.html

Shall高级编程

></DD
><DT
><A
NAME="BACKTICKSREF"
></A
><SPAN
CLASS="TOKEN"
>`</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="commandsub.html#COMMANDSUBREF"
>command substitution</A
>. </B
>The <B
CLASS="COMMAND"
>`command`</B
> construct makes
		available the output of <B
CLASS="COMMAND"
>command</B
>
		for assignment to a variable. This is also known as
		<A
HREF="commandsub.html#BACKQUOTESREF"
>backquotes</A
> or
		backticks.</P
></DIV
></DD
><DT
><A
NAME="COLON0REF"
></A
><SPAN
CLASS="TOKEN"
>:</SPAN
></DT
><DD
><P
><A
NAME="NULLREF"
></A
></P
><DIV
CLASS="FORMALPARA"
><P
><B
>null command [colon]. </B
>This is the shell equivalent of a
		<SPAN
CLASS="QUOTE"
>"NOP"</SPAN
> (<TT
CLASS="REPLACEABLE"
><I
>no op</I
></TT
>, a
		do-nothing operation). It may be considered a synonym for
		the shell builtin <A
HREF="internal.html#TRUEREF"
>true</A
>. The
		<SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>:</SPAN
>"</SPAN
> command is itself a
		<I
CLASS="FIRSTTERM"
>Bash</I
> <A
HREF="internal.html#BUILTINREF"
>builtin</A
>, and its <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
> is
		<SPAN
CLASS="QUOTE"
>"true"</SPAN
> (<SPAN
CLASS="RETURNVALUE"
>0</SPAN
>).</P
></DIV
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;:
   2&nbsp;echo $?   # 0</PRE
></TD
></TR
></TABLE
></P
><P
>Endless loop:</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;while :
   2&nbsp;do
   3&nbsp;   operation-1
   4&nbsp;   operation-2
   5&nbsp;   ...
   6&nbsp;   operation-n
   7&nbsp;done
   8&nbsp;
   9&nbsp;# Same as:
  10&nbsp;#    while true
  11&nbsp;#    do
  12&nbsp;#      ...
  13&nbsp;#    done</PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Placeholder in if/then test:</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if condition
   2&nbsp;then :   # Do nothing and branch ahead
   3&nbsp;else
   4&nbsp;   take-some-action
   5&nbsp;fi</PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Provide a placeholder where a binary operation is
	      expected, see <A
HREF="operations.html#ARITHOPS"
>Example 8-2</A
> and <A
HREF="parameter-substitution.html#DEFPARAM"
>default parameters</A
>.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;: ${username=`whoami`}
   2&nbsp;# ${username=`whoami`}   Gives an error without the leading :
   3&nbsp;#                        unless "username" is a command or builtin...</PRE
></TD
></TR
></TABLE
></P
><P
>Provide a placeholder where a command is expected in a
	      <A
HREF="here-docs.html#HEREDOCREF"
>here document</A
>. See <A
HREF="here-docs.html#ANONHEREDOC"
>Example 18-10</A
>.</P
><P
>Evaluate string of variables using
		<A
HREF="parameter-substitution.html#PARAMSUBREF"
>parameter substitution</A
>
		(as in <A
HREF="parameter-substitution.html#EX6"
>Example 9-16</A
>).

	    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;: ${HOSTNAME?} ${USER?} ${MAIL?}
   2&nbsp;#  Prints error message
   3&nbsp;#+ if one or more of essential environmental variables not set.</PRE
></TD
></TR
></TABLE
></P
><P
><B
CLASS="COMMAND"
><A
HREF="parameter-substitution.html#EXPREPL1"
>Variable expansion / substring
	      replacement</A
></B
>.</P
><P
>In combination with the <SPAN
CLASS="TOKEN"
>&#62;</SPAN
> <A
HREF="io-redirection.html#IOREDIRREF"
>redirection operator</A
>,
	      truncates a file to zero length, without changing its
	      permissions. If the file did not previously exist,
	      creates it.

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;: &#62; data.xxx   # File "data.xxx" now empty.	      
   2&nbsp;
   3&nbsp;# Same effect as   cat /dev/null &#62;data.xxx
   4&nbsp;# However, this does not fork a new process, since ":" is a builtin.</PRE
></TD
></TR
></TABLE
>
              See also <A
HREF="textproc.html#EX12"
>Example 15-15</A
>.</P
><P
>In combination with the <SPAN
CLASS="TOKEN"
>&#62;&#62;</SPAN
>
	      redirection operator, has no effect on a pre-existing
	      target file (<TT
CLASS="USERINPUT"
><B
>: &#62;&#62; target_file</B
></TT
>).
	      If the file did not previously exist, creates it.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>This applies to regular files, not pipes,
	      symlinks, and certain special files.</P
></TD
></TR
></TABLE
></DIV
><P
>May be used to begin a comment line, although this is not
	      recommended. Using <SPAN
CLASS="TOKEN"
>#</SPAN
> 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
	      <SPAN
CLASS="TOKEN"
>:</SPAN
>.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;: This is a comment that generates an error, ( if [ $x -eq 3] ).</PRE
></TD
></TR
></TABLE
>
	    </P
><P
>The <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>:</SPAN
>"</SPAN
> also serves as a field
	      separator, in <TT
CLASS="FILENAME"
>/etc/passwd</TT
>, and in the <A
HREF="variables2.html#PATHREF"
>$PATH</A
> variable.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $PATH</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games</TT
></PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="NOTREF"
></A
><SPAN
CLASS="TOKEN"
>!</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>reverse (or negate) the sense of
	  a test or exit status [bang]. </B
>The <SPAN
CLASS="TOKEN"
>!</SPAN
> operator inverts the <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
>
		of the command to which it is applied (see
		<A
HREF="exit-status.html#NEGCOND"
>Example 6-2</A
>). It also inverts
		the meaning of a test operator. This can, for
		example, change the sense of <I
CLASS="FIRSTTERM"
>equal</I
>
		( <A
HREF="comparison-ops.html#EQUALSIGNREF"
>=</A
>
		) to <I
CLASS="FIRSTTERM"
>not-equal</I
> ( != ). The
		<SPAN
CLASS="TOKEN"
>!</SPAN
> operator is a Bash <A
HREF="internal.html#KEYWORDREF"
>keyword</A
>.</P
></DIV
><P
>In a different context, the <SPAN
CLASS="TOKEN"
>!</SPAN
>
	        also appears in <A
HREF="ivr.html#IVRREF"
>indirect variable
		references</A
>.</P
><P
>In yet another context, from the <I
CLASS="FIRSTTERM"
>command
		line</I
>, the <SPAN
CLASS="TOKEN"
>!</SPAN
> invokes the
		Bash <I
CLASS="FIRSTTERM"
>history mechanism</I
> (see <A
HREF="histcommands.html"
>Appendix J</A
>). Note that within a script,
		the history mechanism is disabled.</P
></DD
><DT
><A
NAME="ASTERISKREF"
></A
><SPAN
CLASS="TOKEN"
>*</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>wild card [asterisk]. </B
>The <SPAN
CLASS="TOKEN"
>*</SPAN
> character serves as a <SPAN
CLASS="QUOTE"
>"wild
	        card"</SPAN
> for filename expansion in
		<A
HREF="globbingref.html"
>globbing</A
>. By itself,
		it matches every filename in a given directory.</P
></DIV
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo *</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>abs-book.sgml add-drive.sh agram.sh alias.sh</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
><A
NAME="ASTERISKREF2"
></A
></P
><P
>The <SPAN
CLASS="TOKEN"
>*</SPAN
> also represents any number
	      (or zero) characters in a <A
HREF="regexp.html#REGEXREF"
>regular expression</A
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>*</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="operations.html#AROPS1"
>arithmetic operator</A
>. </B
>In the context of arithmetic operations, the
	        <SPAN
CLASS="TOKEN"
>*</SPAN
> denotes multiplication.</P
></DIV
><P
>A double asterisk, <SPAN
CLASS="TOKEN"
>**</SPAN
>, is the <A
HREF="operations.html#EXPONENTIATIONREF"
>exponentiation
	        operator</A
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>?</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>test operator. </B
>Within certain expressions, the <SPAN
CLASS="TOKEN"
>?</SPAN
> indicates
	        a test for a condition.</P
></DIV
><P
><A
NAME="CSTRINARY"
></A
></P
><P
>In a <A
HREF="dblparens.html"
>double
		parentheses construct</A
>, the <SPAN
CLASS="TOKEN"
>?</SPAN
>
		can serve as an element of a C-style
		<I
CLASS="FIRSTTERM"
>trinary</I
> operator,
		<TT
CLASS="VARNAME"
>?:</TT
>.

		<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;(( var0 = var1&#60;98?9:21 ))
   2&nbsp;#                ^ ^
   3&nbsp;
   4&nbsp;# if [ "$var1" -lt 98 ]
   5&nbsp;# then
   6&nbsp;#   var0=9
   7&nbsp;# else
   8&nbsp;#   var0=21
   9&nbsp;# fi</PRE
></TD
></TR
></TABLE
></P
><P
>In a <A
HREF="parameter-substitution.html#PARAMSUBREF"
>parameter
	        substitution</A
> expression, the <SPAN
CLASS="TOKEN"
>?</SPAN
>
		<A
HREF="parameter-substitution.html#QERRMSG"
>tests whether a variable has been
		set</A
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>?</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>wild card. </B
><A
NAME="QUEXWC"
></A
>The <SPAN
CLASS="TOKEN"
>?</SPAN
> character
		serves as a single-character <SPAN
CLASS="QUOTE"
>"wild card"</SPAN
>
		for filename expansion in <A
HREF="globbingref.html"
>globbing</A
>, as well as <A
HREF="regexp.html#QUEXREGEX"
>representing one character</A
>
		in an <A
HREF="regexp.html#EXTREGEX"
>extended regular
		expression</A
>.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>$</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="variables.html#VARSUBN"
>Variable
              substitution</A
> (contents of a variable). </B
>	       <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;var1=5
   2&nbsp;var2=23skidoo
   3&nbsp;
   4&nbsp;echo $var1     # 5
   5&nbsp;echo $var2     # 23skidoo</PRE
></TD
></TR
></TABLE
>
	       </P
></DIV
><P
><A
NAME="VARPREFIXREF"
></A
></P
><P
>A <SPAN
CLASS="TOKEN"
>$</SPAN
> prefixing a variable name
		indicates the <I
CLASS="FIRSTTERM"
>value</I
> the variable
		holds.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>$</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>end-of-line. </B
>In a <A
HREF="regexp.html#REGEXREF"
>regular expression</A
>, a
	        <SPAN
CLASS="QUOTE"
>"$"</SPAN
> addresses the end of a line of text.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>${}</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="parameter-substitution.html#PARAMSUBREF"
>Parameter
              substitution</A
>. </B
></P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>$*</SPAN
>, <SPAN
CLASS="TOKEN"
>$@</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="variables2.html#APPREF"
>positional
	    parameters</A
>. </B
></P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>$?</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>exit status variable. </B
>The <A
HREF="exit-status.html#EXSREF"
>$? variable</A
>
	        holds the <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
>
		of a command, a <A
HREF="functions.html#FUNCTIONREF"
>function</A
>,
		or of the script itself.</P
></DIV
></DD
><DT
><A
NAME="PROCESSIDREF"
></A
><SPAN
CLASS="TOKEN"
>$$</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>process ID variable. </B
>The <A
HREF="variables2.html#PROCCID"
>$$ variable</A
>
	        holds the <I
CLASS="FIRSTTERM"
>process ID</I
>
		  <A
NAME="AEN949"
HREF="#FTN.AEN949"
>[2]</A
>  
                of the script in which it appears.</P
></DIV
></DD
><DT
><A
NAME="PARENSREF"
></A
><SPAN
CLASS="TOKEN"
>()</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>command group. </B
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;(a=hello; echo $a)</PRE
></TD
></TR
></TABLE
></P
></DIV
><DIV
CLASS="IMPORTANT"
><TABLE
CLASS="IMPORTANT"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/important.png"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>A listing of commands within
	      <TT
CLASS="REPLACEABLE"
><I
>parentheses</I
></TT
> starts a <A
HREF="subshells.html#SUBSHELLSREF"
>subshell</A
>.</P
><P
>Variables inside parentheses, within the subshell, are not
	      visible to the rest of the script. The parent process,
	      the script, <A
HREF="subshells.html#PARVIS"
>cannot read variables
	      created in the child process</A
>, the subshell.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;a=123
   2&nbsp;( a=321; )	      
   3&nbsp;
   4&nbsp;echo "a = $a"   # a = 123
   5&nbsp;# "a" within parentheses acts like a local variable.</PRE
></TD