internal.html

一本完整的描述Unix Shell 编程的工具书的所有范例

NAME="EX43"
></A
><P
><B
>Example 11-11. Showing the effect of <B
CLASS="COMMAND"
>eval</B
></B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;y=`eval ls -l`  #  Similar to y=`ls -l`
   4&nbsp;echo $y         #+ but linefeeds removed because "echoed" variable is unquoted.
   5&nbsp;echo
   6&nbsp;echo "$y"       #  Linefeeds preserved when variable is quoted.
   7&nbsp;
   8&nbsp;echo; echo
   9&nbsp;
  10&nbsp;y=`eval df`     #  Similar to y=`df`
  11&nbsp;echo $y         #+ but linefeeds removed.
  12&nbsp;
  13&nbsp;#  When LF's not preserved, it may make it easier to parse output,
  14&nbsp;#+ using utilities such as "awk".
  15&nbsp;
  16&nbsp;echo
  17&nbsp;echo "==========================================================="
  18&nbsp;echo
  19&nbsp;
  20&nbsp;# Now, showing how to "expand" a variable using "eval" . . .
  21&nbsp;
  22&nbsp;for i in 1 2 3 4 5; do
  23&nbsp;  eval value=$i
  24&nbsp;  #  value=$i has same effect. The "eval" is not necessary here.
  25&nbsp;  #  A variable lacking a meta-meaning evaluates to itself --
  26&nbsp;  #+ it can't expand to anything other than its literal self.
  27&nbsp;  echo $value
  28&nbsp;done
  29&nbsp;
  30&nbsp;echo
  31&nbsp;echo "---"
  32&nbsp;echo
  33&nbsp;
  34&nbsp;for i in ls df; do
  35&nbsp;  value=eval $i
  36&nbsp;  #  value=$i has an entirely different effect here.
  37&nbsp;  #  The "eval" evaluates the commands "ls" and "df" . . .
  38&nbsp;  #  The terms "ls" and "df" have a meta-meaning,
  39&nbsp;  #+ since they are interpreted as commands,
  40&nbsp;  #+ rather than just character strings.
  41&nbsp;  echo $value
  42&nbsp;done
  43&nbsp;
  44&nbsp;
  45&nbsp;exit 0</PRE
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><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX44"
></A
><P
><B
>Example 11-12. Forcing a log-off</B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Killing ppp to force a log-off.
   3&nbsp;
   4&nbsp;# Script should be run as root user.
   5&nbsp;
   6&nbsp;killppp="eval kill -9 `ps ax | awk '/ppp/ { print $1 }'`"
   7&nbsp;#                     -------- process ID of ppp -------  
   8&nbsp;
   9&nbsp;$killppp                  # This variable is now a command.
  10&nbsp;
  11&nbsp;
  12&nbsp;# The following operations must be done as root user.
  13&nbsp;
  14&nbsp;chmod 666 /dev/ttyS3      # Restore read+write permissions, or else what?
  15&nbsp;#  Since doing a SIGKILL on ppp changed the permissions on the serial port,
  16&nbsp;#+ we restore permissions to previous state.
  17&nbsp;
  18&nbsp;rm /var/lock/LCK..ttyS3   # Remove the serial port lock file. Why?
  19&nbsp;
  20&nbsp;exit 0
  21&nbsp;
  22&nbsp;# Exercises:
  23&nbsp;# ---------
  24&nbsp;# 1) Have script check whether root user is invoking it.
  25&nbsp;# 2) Do a check on whether the process to be killed
  26&nbsp;#+   is actually running before attempting to kill it.   
  27&nbsp;# 3) Write an alternate version of this script based on 'fuser':
  28&nbsp;#+      if [ fuser -s /dev/modem ]; then . . .</PRE
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><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ROT14"
></A
><P
><B
>Example 11-13. A version of <SPAN
CLASS="QUOTE"
>"rot13"</SPAN
></B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# A version of "rot13" using 'eval'.
   3&nbsp;# Compare to "rot13.sh" example.
   4&nbsp;
   5&nbsp;setvar_rot_13()              # "rot13" scrambling
   6&nbsp;{
   7&nbsp;  local varname=$1 varvalue=$2
   8&nbsp;  eval $varname='$(echo "$varvalue" | tr a-z n-za-m)'
   9&nbsp;}
  10&nbsp;
  11&nbsp;
  12&nbsp;setvar_rot_13 var "foobar"   # Run "foobar" through rot13.
  13&nbsp;echo $var                    # sbbone
  14&nbsp;
  15&nbsp;setvar_rot_13 var "$var"     # Run "sbbone" through rot13.
  16&nbsp;                             # Back to original variable.
  17&nbsp;echo $var                    # foobar
  18&nbsp;
  19&nbsp;# This example by Stephane Chazelas.
  20&nbsp;# Modified by document author.
  21&nbsp;
  22&nbsp;exit 0</PRE
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><HR></DIV
><P
>Rory Winston contributed the following instance of how
	      useful <B
CLASS="COMMAND"
>eval</B
> can be.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EVALEX"
></A
><P
><B
>Example 11-14. Using <B
CLASS="COMMAND"
>eval</B
> to force variable
	        substitution in a Perl script</B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;In the Perl script "test.pl":
   2&nbsp;        ...		
   3&nbsp;        my $WEBROOT = &#60;WEBROOT_PATH&#62;;
   4&nbsp;        ...
   5&nbsp;
   6&nbsp;To force variable substitution try:
   7&nbsp;        $export WEBROOT_PATH=/usr/local/webroot
   8&nbsp;        $sed 's/&#60;WEBROOT_PATH&#62;/$WEBROOT_PATH/' &#60; test.pl &#62; out
   9&nbsp;
  10&nbsp;But this just gives:
  11&nbsp;        my $WEBROOT = $WEBROOT_PATH;
  12&nbsp;
  13&nbsp;However:
  14&nbsp;        $export WEBROOT_PATH=/usr/local/webroot
  15&nbsp;        $eval sed 's%\&#60;WEBROOT_PATH\&#62;%$WEBROOT_PATH%' &#60; test.pl &#62; out
  16&nbsp;#        ====
  17&nbsp;
  18&nbsp;That works fine, and gives the expected substitution:
  19&nbsp;        my $WEBROOT = /usr/local/webroot;
  20&nbsp;
  21&nbsp;
  22&nbsp;### Correction applied to original example by Paulo Marcel Coelho Aragao.</PRE
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><HR></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
>The <B
CLASS="COMMAND"
>eval</B
> command can be
	      risky, and normally should be avoided when there
	      exists a reasonable alternative. An <TT
CLASS="USERINPUT"
><B
>eval
	      $COMMANDS</B
></TT
> executes the contents of
	      <TT
CLASS="REPLACEABLE"
><I
>COMMANDS</I
></TT
>, which may
	      contain such unpleasant surprises as <B
CLASS="COMMAND"
>rm -rf
	      *</B
>. Running an <B
CLASS="COMMAND"
>eval</B
> on
	      unfamiliar code written by persons unknown is living
	      dangerously.</P
></TD
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></DIV
></DD
><DT
><A
NAME="SETREF"
></A
><B
CLASS="COMMAND"
>set</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>set</B
> command changes
	      the value of internal script variables. One use for
	      this is to toggle <A
HREF="options.html#OPTIONSREF"
>option
	      flags</A
> which help determine the behavior of the
	      script. Another application for it is to reset the <A
HREF="variables2.html#POSPARAMREF"
>positional parameters</A
> that
	      a script sees as the result of a command (<TT
CLASS="USERINPUT"
><B
>set
	      `command`</B
></TT
>). The script can then parse the
	      fields of the command output.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX34"
></A
><P
><B
>Example 11-15. Using <B
CLASS="COMMAND"
>set</B
> with positional
	        parameters</B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# script "set-test"
   4&nbsp;
   5&nbsp;# Invoke this script with three command line parameters,
   6&nbsp;# for example, "./set-test one two three".
   7&nbsp;
   8&nbsp;echo
   9&nbsp;echo "Positional parameters before  set \`uname -a\` :"
  10&nbsp;echo "Command-line argument #1 = $1"
  11&nbsp;echo "Command-line argument #2 = $2"
  12&nbsp;echo "Command-line argument #3 = $3"
  13&nbsp;
  14&nbsp;
  15&nbsp;set `uname -a` # Sets the positional parameters to the output
  16&nbsp;               # of the command `uname -a`
  17&nbsp;
  18&nbsp;echo $_        # unknown
  19&nbsp;# Flags set in script.
  20&nbsp;
  21&nbsp;echo "Positional parameters after  set \`uname -a\` :"
  22&nbsp;# $1, $2, $3, etc. reinitialized to result of `uname -a`
  23&nbsp;echo "Field #1 of 'uname -a' = $1"
  24&nbsp;echo "Field #2 of 'uname -a' = $2"
  25&nbsp;echo "Field #3 of 'uname -a' = $3"
  26&nbsp;echo ---
  27&nbsp;echo $_        # ---
  28&nbsp;echo
  29&nbsp;
  30&nbsp;exit 0</PRE
></TD
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><HR></DIV
><P
>Invoking <B
CLASS="COMMAND"
>set</B
> without any options or
	      arguments simply lists all the <A
HREF="othertypesv.html#ENVREF"
>environmental</A
> and other variables
	      that have been initialized.
	      <TABLE
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><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>set</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>AUTHORCOPY=/home/bozo/posts
 BASH=/bin/bash
 BASH_VERSION=$'2.05.8(1)-release'
 ...
 XAUTHORITY=/home/bozo/.Xauthority
 _=/etc/bashrc
 variable22=abc
 variable23=xzy</TT
>
 	      </PRE
></TD
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>
            </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. When no variable follows the
	      <TT
CLASS="OPTION"
>--</TT
>, it <I
CLASS="EMPHASIS"
>unsets</I
>
	      the positional parameters.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SETPOS"
></A
><P
><B
>Example 11-16. Reassigning the positional parameters</B
></P
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><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=$2
  10&nbsp;shift; shift        # Shift past first two positional params.
  11&nbsp;remaining_params="$*"
  12&nbsp;
  13&nbsp;echo
  14&nbsp;echo "first parameter = $first_param"             # one
  15&nbsp;echo "second parameter = $second_param"           # two
  16&nbsp;echo "remaining parameters = $remaining_params"   # three four five
  17&nbsp;
  18&nbsp;echo; echo
  19&nbsp;
  20&nbsp;# Again.
  21&nbsp;set -- $variable
  22&nbsp;first_param=$1
  23&nbsp;second_param=$2
  24&nbsp;echo "first parameter = $first_param"             # one
  25&nbsp;echo "second parameter = $second_param"           # two
  26&nbsp;
  27&nbsp;# ======================================================
  28&nbsp;
  29&nbsp;set --
  30&nbsp;# Unsets positional parameters if no variable specified.
  31&nbsp;
  32&nbsp;first_param=$1
  33&nbsp;second_param=$2
  34&nbsp;echo "first parameter = $first_param"             # (null value)
  35&nbsp;echo "second parameter = $second_param"           # (null value)
  36&nbsp;
  37&nbsp;exit 0</PRE
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><HR></DIV
><P
>See also <A
HREF="loops.html#EX22A"
>Example 10-2</A
> and <A
HREF="extmisc.html#EX33A"
>Example 12-51</A
>.</P
></DD
><DT
><A
NAME="UNSETREF"
></A
><B
CLASS="COMMAND"
>unset</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>unset</B
> command deletes a
	      shell variable, effectively setting it to
	      <I
CLASS="EMPHASIS"
>null</I
>. Note that this command does
	      not affect positional parameters.</P
><P
>	      <TABLE
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><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </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 11-17. <SPAN
CLASS="QUOTE"
>"Unsetting"</SPAN
> a variable</B
></P
><TABLE
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><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;exit 0</PRE
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><HR></DIV
></DD
><DT
><A
NAME="EXPORTREF"
></A
><B
CLASS="COMMAND"
>export</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>export</B
> command makes
	      available variables to all child processes of the
	      running script or shell. <I
CLASS="EMPHASIS"
>Unfortunately, there
	      is no way to</I
> export <I
CLASS="EMPHASIS"
>variables back
	      to the parent process, to the process that called or
	      invoked the script or shell.</I
> 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="EXAMPLE"
><HR><A
NAME="COLTOTALER3"
></A
><P
><B
>Example 11-18. Using <B
CLASS="COMMAND"
>export</B
> to pass a variable to an
	      embedded <A
HREF="awk.html#AWKREF"
>awk</A
> script</B
></P
><TABLE
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><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.