special-chars.html

一本完整的描述Unix Shell 编程的工具书的所有范例

></TD
></TR
></TABLE
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>previous working directory. </B
>A <B
CLASS="COMMAND"
>cd -</B
> command changes to the
		previous working directory. This uses the
		<A
HREF="variables2.html#OLDPWD"
>$OLDPWD</A
> <A
HREF="othertypesv.html#ENVREF"
>environmental variable</A
>.</P
></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Do not confuse the <SPAN
CLASS="QUOTE"
>"-"</SPAN
> used in this
		sense with the <SPAN
CLASS="QUOTE"
>"-"</SPAN
> redirection
		operator just discussed. The interpretation of the
		<SPAN
CLASS="QUOTE"
>"-"</SPAN
> depends on the context in which it
		appears.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Minus. </B
>Minus sign in an <A
HREF="operations.html#AROPS1"
>arithmetic
	        operation</A
>.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>=</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Equals. </B
><A
HREF="varassignment.html#EQREF"
>Assignment operator</A
>
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;a=28
   2&nbsp;echo $a   # 28</PRE
></TD
></TR
></TABLE
></P
></DIV
><P
>In a <A
HREF="comparison-ops.html#EQUALSIGNREF"
>different context</A
>,
	      the <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>=</SPAN
>"</SPAN
> is a <A
HREF="comparison-ops.html#SCOMPARISON1"
>string comparison</A
>
	      operator.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>+</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Plus. </B
>Addition  <A
HREF="operations.html#AROPS1"
>arithmetic
	        operator</A
>.</P
></DIV
><P
>In a <A
HREF="regexp.html#PLUSREF"
>different context</A
>,
	      the <SPAN
CLASS="TOKEN"
>+</SPAN
> is a <A
HREF="regexp.html"
>Regular
	      Expression</A
> operator.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>+</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Option. </B
>Option flag for a command or filter.</P
></DIV
><P
>Certain commands and <A
HREF="internal.html#BUILTINREF"
>builtins</A
> use the
	      <SPAN
CLASS="TOKEN"
>+</SPAN
> to enable certain options and the
	      <SPAN
CLASS="TOKEN"
>-</SPAN
> to disable them.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>%</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="operations.html#MODULOREF"
>modulo</A
>. </B
>Modulo (remainder of a division) <A
HREF="operations.html#AROPS1"
>arithmetic
	        operation</A
>.</P
></DIV
><P
>In a <A
HREF="parameter-substitution.html#PCTPATREF"
>different context</A
>,
	      the <SPAN
CLASS="TOKEN"
>%</SPAN
> is a <A
HREF="parameter-substitution.html#PSUB2"
>pattern
	      matching</A
> operator.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>~</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>home directory [tilde]. </B
>This corresponds to the <A
HREF="variables2.html#HOMEDIRREF"
>$HOME</A
> internal variable.

	      <I
CLASS="EMPHASIS"
>~bozo</I
> is bozo's home directory,
		and <B
CLASS="COMMAND"
>ls ~bozo</B
> lists the contents of it.
		<SPAN
CLASS="TOKEN"
>~/</SPAN
> is the current user's home directory,
		and <B
CLASS="COMMAND"
>ls ~/</B
> lists the contents of it.

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~bozo</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/home/bozo</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/home/bozo</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~/</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/home/bozo/</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~:</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/home/bozo:</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~nonexistent-user</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>~nonexistent-user</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>~+</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>current working directory. </B
>This corresponds to the <A
HREF="variables2.html#PWDREF"
>$PWD</A
> internal variable.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>~-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>previous working directory. </B
>This corresponds to the <A
HREF="variables2.html#OLDPWD"
>$OLDPWD</A
> internal variable.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>=~</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="bashver3.html#REGEXMATCHREF"
>regular
	  expression match</A
>. </B
>This operator was introduced with <A
HREF="bashver3.html#BASH3REF"
>version 3</A
> of Bash.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>^</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>beginning-of-line. </B
>In a <A
HREF="regexp.html#REGEXREF"
>regular expression</A
>, a
	        <SPAN
CLASS="QUOTE"
>"^"</SPAN
> addresses the beginning of a line of text.</P
></DIV
></DD
><DT
>Control Characters</DT
><DD
><P
><A
NAME="CONTROLCHARREF"
></A
></P
><DIV
CLASS="FORMALPARA"
><P
><B
> change the behavior of the
	        terminal or text display. </B
>A control character is a <B
CLASS="KEYCAP"
>CONTROL</B
>
	      + <B
CLASS="KEYCAP"
>key</B
> combination.</P
></DIV
><P
>Control characters are not normally useful inside a
	    script.</P
><UL
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-B</B
></TT
></P
><P
>Backspace (nondestructive).</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-C</B
></TT
></P
><P
>Break. Terminate a foreground job.</P
></LI
><LI
><P
><A
NAME="CTLDREF"
></A
></P
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-D</B
></TT
></P
><P
>Log out from a shell (similar to
		  <A
HREF="exit-status.html#EXITCOMMANDREF"
>exit</A
>).</P
><P
><SPAN
CLASS="QUOTE"
>"EOF"</SPAN
> (end of file). This also
		  terminates input from <TT
CLASS="FILENAME"
>stdin</TT
>.</P
><P
>When typing text on the console or in an
                  <I
CLASS="FIRSTTERM"
>xterm</I
> window,
		  <TT
CLASS="USERINPUT"
><B
>Ctl-D</B
></TT
> erases the character under the
		  cursor. When there are no characters present,
		  <TT
CLASS="USERINPUT"
><B
>Ctl-D</B
></TT
> logs out of the session, as
		  expected. In an xterm window, this has the effect of closing
		  the window.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-G</B
></TT
></P
><P
><SPAN
CLASS="QUOTE"
>"BEL"</SPAN
> (beep). On some old-time teletype
		  terminals, this would actually ring a bell.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-H</B
></TT
></P
><P
><SPAN
CLASS="QUOTE"
>"Rubout"</SPAN
> (destructive backspace). Erases
		  characters the cursor backs over while backspacing.</P
><P
>		<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Embedding Ctl-H in a string.
   3&nbsp;
   4&nbsp;a="^H^H"                  # Two Ctl-H's (backspaces).
   5&nbsp;echo "abcdef"             # abcdef
   6&nbsp;echo -n "abcdef$a "       # abcd f
   7&nbsp;#  Space at end  ^              ^ Backspaces twice.
   8&nbsp;echo -n "abcdef$a"        # abcdef
   9&nbsp;#  No space at end                Doesn't backspace (why?).
  10&nbsp;                          # Results may not be quite as expected.
  11&nbsp;echo; echo</PRE
></TD
></TR
></TABLE
>
                </P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-I</B
></TT
></P
><P
>Horizontal tab.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-J</B
></TT
></P
><P
>Newline (line feed). In a script, may also be expressed
		  in octal notation -- '\012' or in hexadecimal -- '\x0a'.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-K</B
></TT
></P
><P
>Vertical tab.</P
><P
>When typing text on the console or in an
                  <I
CLASS="EMPHASIS"
>xterm</I
> window,
		  <TT
CLASS="USERINPUT"
><B
>Ctl-K</B
></TT
> erases from the character
		  under the cursor to end of line.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-L</B
></TT
></P
><P
>Formfeed (clear the terminal screen). This has
		  the same effect as the <A
HREF="terminalccmds.html#CLEARREF"
>clear</A
> command.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-M</B
></TT
></P
><P
>Carriage return.</P
><P
>		<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Thank you, Lee Maschmeyer, for this example.
   3&nbsp;
   4&nbsp;read -n 1 -s -p $'Control-M leaves cursor at beginning of this line. Press Enter. \x0d'
   5&nbsp;                                  # Of course, '0d' is the hex equivalent of Control-M.
   6&nbsp;echo &#62;&#38;2   #  The '-s' makes anything typed silent,
   7&nbsp;           #+ so it is necessary to go to new line explicitly.
   8&nbsp;
   9&nbsp;read -n 1 -s -p $'Control-J leaves cursor on next line. \x0a'
  10&nbsp;           #  '0a' is the hex equivalent of Control-J, linefeed.
  11&nbsp;echo &#62;&#38;2
  12&nbsp;
  13&nbsp;###
  14&nbsp;
  15&nbsp;read -n 1 -s -p $'And Control-K\x0bgoes straight down.'
  16&nbsp;echo &#62;&#38;2   #  Control-K is vertical tab.
  17&nbsp;
  18&nbsp;# A better example of the effect of a vertical tab is:
  19&nbsp;
  20&nbsp;var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a'
  21&nbsp;echo "$var"
  22&nbsp;#  This works the same way as the above example. However:
  23&nbsp;echo "$var" | col
  24&nbsp;#  This causes the right end of the line to be higher than the left end.
  25&nbsp;#  It also explains why we started and ended with a line feed --
  26&nbsp;#+ to avoid a garbled screen.
  27&nbsp;
  28&nbsp;# As Lee Maschmeyer explains:
  29&nbsp;# --------------------------
  30&nbsp;#  In the [first vertical tab example] . . . the vertical tab
  31&nbsp;#+ makes the printing go straight down without a carriage return.
  32&nbsp;#  This is true only on devices, such as the Linux console,
  33&nbsp;#+ that can't go "backward."
  34&nbsp;#  The real purpose of VT is to go straight UP, not down.
  35&nbsp;#  It can be used to print superscripts on a printer.
  36&nbsp;#  The col utility can be used to emulate the proper behavior of VT.
  37&nbsp;
  38&nbsp;exit 0</PRE
></TD
></TR
></TABLE
>
		</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-Q</B
></TT
></P
><P
>Resume (XON).</P
><P
>This resumes <TT
CLASS="FILENAME"
>stdin</TT
> in a terminal.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-S</B
></TT
></P
><P
>Suspend (XOFF).</P
><P
>This freezes <TT
CLASS="FILENAME"
>stdin</TT
> in a terminal.
		  (Use Ctl-Q to restore input.)</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-U</B
></TT
></P
><P
>Erase a line of input, from the cursor backward to
		  beginning of line. In some settings,
		  <TT
CLASS="USERINPUT"
><B
>Ctl-U</B
></TT
> erases the entire
		  line of input, <I
CLASS="EMPHASIS"
>regardless of cursor
		  position</I
>.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-V</B
></TT
></P
><P
>When inputting text, <TT
CLASS="USERINPUT"
><B
>Ctl-V</B
></TT
>
		  permits inserting control characters. For example, the
		  following two are equivalent:
		    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;echo -e '\x0a'
   2&nbsp;echo &#60;Ctl-V&#62;&#60;Ctl-J&#62;</PRE
></TD
></TR
></TABLE
></P
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-V</B
></TT
> is primarily useful from
		within a text editor.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-W</B
></TT
></P
><P
>When typing text on the console or in an xterm window,
		  <TT
CLASS="USERINPUT"
><B
>Ctl-W</B
></TT
> erases from the character
		  under the cursor backwards to the first instance of
		  whitespace. In some settings, <TT
CLASS="USERINPUT"
><B
>Ctl-W</B
></TT
>
		  erases backwards to first non-alphanumeric character.</P
></LI
><LI
><P
><TT
CLASS="USERINPUT"
><B
>Ctl-Z</B
></TT
></P
><P
>Pause a foreground job.</P
></LI
></UL
></DD
><DT
>Whitespace</DT
><DD
><P
><A
NAME="WHITESPACEREF"
></A
></P
><DIV
CLASS="FORMALPARA"
><P
><B
>functions as a separator, separating commands or variables. </B
>Whitespace consists of either
		<I
CLASS="FIRSTTERM"
>spaces</I
>,
		<I
CLASS="FIRSTTERM"
>tabs</I
>, <I
CLASS="FIRSTTERM"
>blank
		lines</I
>, or any combination thereof.

		  <A
NAME="AEN1676"
HREF="#FTN.AEN1676"
>[3]</A
>

		In some contexts, such as <A
HREF="gotchas.html#WSBAD"
>variable
		assignment</A
>, whitespace