internal.html

Shall高级编程

 BASH_VERSION=$'2.05.8(1)-release'
 ...
 XAUTHORITY=/home/bozo/.Xauthority
 _=/etc/bashrc
 variable22=abc
 variable23=xzy</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
            </P
><P
>Using <B
CLASS="COMMAND"
>set</B
> with the <TT
CLASS="OPTION"
>--</TT
>
	      option explicitly assigns the contents of a variable to
	      the positional parameters. If no variable follows the
	      <TT
CLASS="OPTION"
>--</TT
> it <I
CLASS="FIRSTTERM"
>unsets</I
>
	      the positional parameters.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SETPOS"
></A
><P
><B
>Example 14-18. Reassigning the positional parameters</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;variable="one two three four five"
   4&nbsp;
   5&nbsp;set -- $variable
   6&nbsp;# Sets positional parameters to the contents of "$variable".
   7&nbsp;
   8&nbsp;first_param=$1
   9&nbsp;second_param=$2
  10&nbsp;shift; shift        # Shift past first two positional params.
  11&nbsp;# shift 2             also works.
  12&nbsp;remaining_params="$*"
  13&nbsp;
  14&nbsp;echo
  15&nbsp;echo "first parameter = $first_param"             # one
  16&nbsp;echo "second parameter = $second_param"           # two
  17&nbsp;echo "remaining parameters = $remaining_params"   # three four five
  18&nbsp;
  19&nbsp;echo; echo
  20&nbsp;
  21&nbsp;# Again.
  22&nbsp;set -- $variable
  23&nbsp;first_param=$1
  24&nbsp;second_param=$2
  25&nbsp;echo "first parameter = $first_param"             # one
  26&nbsp;echo "second parameter = $second_param"           # two
  27&nbsp;
  28&nbsp;# ======================================================
  29&nbsp;
  30&nbsp;set --
  31&nbsp;# Unsets positional parameters if no variable specified.
  32&nbsp;
  33&nbsp;first_param=$1
  34&nbsp;second_param=$2
  35&nbsp;echo "first parameter = $first_param"             # (null value)
  36&nbsp;echo "second parameter = $second_param"           # (null value)
  37&nbsp;
  38&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>See also <A
HREF="loops.html#EX22A"
>Example 10-2</A
> and <A
HREF="extmisc.html#EX33A"
>Example 15-54</A
>.</P
></DD
><DT
><A
NAME="UNSETREF"
></A
><B
CLASS="COMMAND"
>unset</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>unset</B
> command deletes a
	      shell variable, effectively setting it to
	      <I
CLASS="FIRSTTERM"
>null</I
>. Note that this command does
	      not affect positional parameters.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>unset PATH</B
></TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $PATH</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>&#13;</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
></PRE
></TD
></TR
></TABLE
>
	    </P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="UNS"
></A
><P
><B
>Example 14-19. <SPAN
CLASS="QUOTE"
>"Unsetting"</SPAN
> a variable</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# unset.sh: Unsetting a variable.
   3&nbsp;
   4&nbsp;variable=hello                       # Initialized.
   5&nbsp;echo "variable = $variable"
   6&nbsp;
   7&nbsp;unset variable                       # Unset.
   8&nbsp;                                     # Same effect as:  variable=
   9&nbsp;echo "(unset) variable = $variable"  # $variable is null.
  10&nbsp;
  11&nbsp;if [ -z "$variable" ]                # Try a string-length test.
  12&nbsp;then
  13&nbsp;  echo "\$variable has zero length."
  14&nbsp;fi
  15&nbsp;
  16&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="EXPORTREF"
></A
><B
CLASS="COMMAND"
>export</B
></DT
><DD
><P
><A
NAME="EXPORTREF2"
></A
></P
><P
>The <B
CLASS="COMMAND"
>export</B
>

	  
	    <A
NAME="AEN8304"
HREF="#FTN.AEN8304"
>[2]</A
>
	  
	      command makes available variables to all child processes
	      of the running script or shell. One important use
	      of the <B
CLASS="COMMAND"
>export</B
> command is in <A
HREF="files.html#FILESREF1"
>startup files</A
>, to initialize
	      and make accessible <A
HREF="othertypesv.html#ENVREF"
>environmental
	      variables</A
> to subsequent user processes.</P
><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
>Unfortunately, <A
HREF="gotchas.html#PARCHILDPROBREF"
>	     there is no way to export variables back to the parent
	     process</A
>, to the process that called or invoked the
	     script or shell.</P
></TD
></TR
></TABLE
></DIV
><P
><A
NAME="EXPORTAWK"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="COLTOTALER3"
></A
><P
><B
>Example 14-20. Using <I
CLASS="FIRSTTERM"
>export</I
> to pass a variable to an
	      embedded <I
CLASS="FIRSTTERM"
>awk</I
> script</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  Yet another version of the "column totaler" script (col-totaler.sh)
   4&nbsp;#+ that adds up a specified column (of numbers) in the target file.
   5&nbsp;#  This uses the environment to pass a script variable to 'awk' . . .
   6&nbsp;#+ and places the awk script in a variable.
   7&nbsp;
   8&nbsp;
   9&nbsp;ARGS=2
  10&nbsp;E_WRONGARGS=65
  11&nbsp;
  12&nbsp;if [ $# -ne "$ARGS" ] # Check for proper no. of command line args.
  13&nbsp;then
  14&nbsp;   echo "Usage: `basename $0` filename column-number"
  15&nbsp;   exit $E_WRONGARGS
  16&nbsp;fi
  17&nbsp;
  18&nbsp;filename=$1
  19&nbsp;column_number=$2
  20&nbsp;
  21&nbsp;#===== Same as original script, up to this point =====#
  22&nbsp;
  23&nbsp;export column_number
  24&nbsp;# Export column number to environment, so it's available for retrieval.
  25&nbsp;
  26&nbsp;
  27&nbsp;# -----------------------------------------------
  28&nbsp;awkscript='{ total += $ENVIRON["column_number"] }
  29&nbsp;END { print total }'
  30&nbsp;# Yes, a variable can hold an awk script.
  31&nbsp;# -----------------------------------------------
  32&nbsp;
  33&nbsp;# Now, run the awk script.
  34&nbsp;awk "$awkscript" "$filename"
  35&nbsp;
  36&nbsp;# Thanks, Stephane Chazelas.
  37&nbsp;
  38&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>It is possible to initialize and export
		  variables in the same operation, as in <B
CLASS="COMMAND"
>export
		  var1=xxx</B
>.</P
><P
>However, as Greg Keraunen points out, in certain
		  situations this may have a different effect than
		  setting a variable, then exporting it.</P
><P
>	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>export var=(a b); echo ${var[0]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>(a b)</TT
>
 
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>var=(a b); export var; echo ${var[0]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>a</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="DECLARE2REF"
></A
><B
CLASS="COMMAND"
>declare</B
>, <B
CLASS="COMMAND"
>typeset</B
></DT
><DD
><P
>The <A
HREF="declareref.html"
>declare</A
> and
	      <A
HREF="declareref.html"
>typeset</A
> commands specify
	      and/or restrict properties of variables.</P
></DD
><DT
><A
NAME="READONLYREF"
></A
><B
CLASS="COMMAND"
>readonly</B
></DT
><DD
><P
>Same as <A
HREF="declareref.html"
>declare -r</A
>,
	      sets a variable as read-only, or, in effect, as a
	      constant. Attempts to change the variable fail with
	      an error message. This is the shell analog of the
	      <I
CLASS="FIRSTTERM"
>C</I
> language <B
CLASS="COMMAND"
>const</B
>
	      type qualifier.</P
></DD
><DT
><A
NAME="GETOPTSX"
></A
><B
CLASS="COMMAND"
>getopts</B
></DT
><DD
><P
>This powerful tool parses command-line arguments passed
	      to the script. This is the Bash analog of the <A
HREF="extmisc.html#GETOPTY"
>getopt</A
> external command and the
	      <I
CLASS="FIRSTTERM"
>getopt</I
> library function familiar to
	      <I
CLASS="FIRSTTERM"
>C</I
> programmers. It permits passing
	      and concatenating multiple options

	      <A
NAME="AEN8391"
HREF="#FTN.AEN8391"
>[3]</A
>

	      and associated arguments to a script (for
	      example <TT
CLASS="USERINPUT"
><B
>scriptname -abc -e
	      /usr/local</B
></TT
>).</P
><P
><A
NAME="GETOPTSOPT"
></A
></P
><P
>The <B
CLASS="COMMAND"
>getopts</B
> construct uses two implicit
	      variables. <TT
CLASS="VARNAME"
>$OPTIND</TT
> is the argument
	      pointer (<I
CLASS="WORDASWORD"
>OPTion INDex</I
>)
	      and <TT
CLASS="VARNAME"
>$OPTARG</TT
> (<I
CLASS="WORDASWORD"
>OPTion
	      ARGument</I
>) the (optional) argument attached
	      to an option. A colon following the option name in the
	      declaration tags that option as having an associated
	      argument.</P
><P
>A <B
CLASS="COMMAND"
>getopts</B
> construct usually comes
	      packaged in a <A
HREF="loops.html#WHILELOOPREF"
>while
	      loop</A
>, which processes the options and
	      arguments one at a time, then increments the implicit
	      <TT
CLASS="VARNAME"
>$OPTIND</TT
> variable to step to the
	      next.</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
>		<OL
TYPE="1"
><LI
><P
>The arguments passed from the command line to
		      the script must be preceded by a
		      minus (<TT
CLASS="OPTION"
>-</TT
>). It is the
		      prefixed <TT
CLASS="OPTION"
>-</TT
> that lets
		      <B
CLASS="COMMAND"
>getopts</B
> recognize command-line
		      arguments as <I
CLASS="FIRSTTERM"
>options</I
>.
		      In fact, <B
CLASS="COMMAND"
>getopts</B
> will not process
		      arguments without the prefixed <TT
CLASS="OPTION"
>-</TT
>,
		      and will terminate option processing at the first
		      argument encountered lacking them.</P
></LI
><LI
><P
>The <B
CLASS="COMMAND"
>getopts</B
> template
		      differs slightly from the standard <B
CLASS="COMMAND"
>while</B
>
		      loop, in that it lacks condition brackets.</P
></LI
><LI
><P
>The <B
CLASS="COMMAND"
>getopts</B
> construct replaces
		     the deprecated <A
HREF="extmisc.html#GETOPTY"
>getopt</A
>
		     external command.</P
></LI
></OL
>
	      </P
></TD
></TR
></TABLE
></DIV
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;while getopts ":abcde:fg" Option
   2&nbsp;# Initial declaration.
   3&nbsp;# a, b, c, d, e, f, and g are the options (flags) expected.
   4&nbsp;# The : after option 'e' shows it will have an argument passed with it.
   5&nbsp;do
   6&nbsp;  case $Option in
   7&nbsp;    a ) # Do something with variable 'a'.
   8&nbsp;    b ) # Do something with variable 'b'.
   9&nbsp;    ...
  10&nbsp;    e)  # Do something with 'e', and also with $OPTARG,
  11&nbsp;        # which is the associated argument passed with option 'e'.
  12&nbsp;    ...
  13&nbsp;    g ) # Do something with variable 'g'.
  14&nbsp;  esac
  15&nbsp;done
  16&nbsp;shift $(($OPTIND - 1))
  17&nbsp;# Move argument pointer to next.
  18&nbsp;
  19&nbsp;# All this is not nearly as complicated as it looks &#60;grin&#62;.</PRE
></TD
></TR
></TABLE
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX33"
></A
><P
><B
>Example 14-21. Using <I
CLASS="FIRSTTERM"
>getopts</I
> to read the
	        options/arguments passed to a script</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Exercising getopts and OPTIND