internal.html

Shall高级编程

   7                     # (Double quotes and spaces make it more readable.)
   8 echo "11 + 5 = $a"  # 16
   9 
  10 let "a <<= 3"       # Equivalent to  let "a = a << 3"
  11 echo "\"\$a\" (=16) left-shifted 3 places = $a"
  12                     # 128
  13 
  14 let "a /= 4"        # Equivalent to  let "a = a / 4"
  15 echo "128 / 4 = $a" # 32
  16 
  17 let "a -= 5"        # Equivalent to  let "a = a - 5"
  18 echo "32 - 5 = $a"  # 27
  19 
  20 let "a *=  10"      # Equivalent to  let "a = a * 10"
  21 echo "27 * 10 = $a" # 270
  22 
  23 let "a %= 8"        # Equivalent to  let "a = a % 8"
  24 echo "270 modulo 8 = $a  (270 / 8 = 33, remainder $a)"
  25                     # 6
  26 
  27 echo
  28 
  29&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="EVALREF"
></A
><B
CLASS="COMMAND"
>eval</B
></DT
><DD
><P
><TT
CLASS="USERINPUT"
><B
>eval arg1 [arg2] ... [argN]</B
></TT
></P
><P
>Combines the arguments in an expression or list of
	      expressions and <I
CLASS="FIRSTTERM"
>evaluates</I
> them. Any
	      variables contained within the expression are expanded. The
	      result translates into a command. This can be useful for
	      code generation from the command line or within a script.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>process=xterm</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>show_process="eval ps ax | grep $process"</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>$show_process</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>1867 tty1     S      0:02 xterm
 2779 tty1     S      0:00 xterm
 2886 pts/1    S      0:00 grep xterm</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
><A
NAME="EVALFORCED"
></A
></P
><P
>Each invocation of <I
CLASS="FIRSTTERM"
>eval</I
> forces
        a re-<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>evaluation</I
></SPAN
> of its arguments.
	<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;a='$b'
   2&nbsp;b='$c'
   3&nbsp;c=d
   4&nbsp;
   5&nbsp;echo $a             # $b
   6&nbsp;                    # First level.
   7&nbsp;eval echo $a        # $c
   8&nbsp;                    # Second level.
   9&nbsp;eval eval echo $a   # d
  10&nbsp;                    # Third level.
  11&nbsp;
  12&nbsp;# Thank you, E. Choroba.</PRE
></TD
></TR
></TABLE
></P
><P
><A
NAME="EVALEFF"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX43"
></A
><P
><B
>Example 14-11. Showing the effect of <I
CLASS="FIRSTTERM"
>eval</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Exercising "eval" ...
   3&nbsp;
   4&nbsp;y=`eval ls -l`  #  Similar to y=`ls -l`
   5&nbsp;echo $y         #+ but linefeeds removed because "echoed" variable is unquoted.
   6&nbsp;echo
   7&nbsp;echo "$y"       #  Linefeeds preserved when variable is quoted.
   8&nbsp;
   9&nbsp;echo; echo
  10&nbsp;
  11&nbsp;y=`eval df`     #  Similar to y=`df`
  12&nbsp;echo $y         #+ but linefeeds removed.
  13&nbsp;
  14&nbsp;#  When LF's not preserved, it may make it easier to parse output,
  15&nbsp;#+ using utilities such as "awk".
  16&nbsp;
  17&nbsp;echo
  18&nbsp;echo "==========================================================="
  19&nbsp;echo
  20&nbsp;
  21&nbsp;
  22&nbsp;# Now, showing how to do something useful with "eval" . . .
  23&nbsp;# (Thank you, E. Choroba!)
  24&nbsp;
  25&nbsp;version=3.4     #  Can we split the version into major and minor
  26&nbsp;                #+ part in one command?
  27&nbsp;echo "version = $version"
  28&nbsp;eval major=${version/./;minor=}     #  Replaces '.' in version by ';minor='
  29&nbsp;                                    #  The substitution yields '3; minor=4'
  30&nbsp;                                    #+ so eval does minor=4, major=3
  31&nbsp;echo Major: $major, minor: $minor   #  Major: 3, minor: 4</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ECHOPARAMS"
></A
><P
><B
>Example 14-12. <I
CLASS="FIRSTTERM"
>Echoing</I
> the
	        <I
CLASS="FIRSTTERM"
>command-line parameters</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# echo-params.sh
   3&nbsp;
   4&nbsp;# Call this script with a few command line parameters.
   5&nbsp;# For example:
   6&nbsp;#     sh echo-params.sh first second third fourth fifth
   7&nbsp;
   8&nbsp;params=$#              # Number of command-line parameters.
   9&nbsp;param=1                # Start at first command-line param.
  10&nbsp;
  11&nbsp;while [ "$param" -le "$params" ]
  12&nbsp;do
  13&nbsp;  echo -n "Command line parameter "
  14&nbsp;  echo -n \$$param     #  Gives only the *name* of variable.
  15&nbsp;#         ^^^          #  $1, $2, $3, etc.
  16&nbsp;                       #  Why?
  17&nbsp;                       #  \$ escapes the first "$"
  18&nbsp;                       #+ so it echoes literally,
  19&nbsp;                       #+ and $param dereferences "$param" . . .
  20&nbsp;                       #+ . . . as expected.
  21&nbsp;  echo -n " = "
  22&nbsp;  eval echo \$$param   #  Gives the *value* of variable.
  23&nbsp;# ^^^^      ^^^        #  The "eval" forces the *evaluation*
  24&nbsp;                       #+ of \$$
  25&nbsp;                       #+ as an indirect variable reference.
  26&nbsp;
  27&nbsp;(( param ++ ))         # On to the next.
  28&nbsp;done
  29&nbsp;
  30&nbsp;exit $?
  31&nbsp;
  32&nbsp;# =================================================
  33&nbsp;
  34&nbsp;$ sh echo-params.sh first second third fourth fifth
  35&nbsp;Command line parameter $1 = first
  36&nbsp;Command line parameter $2 = second
  37&nbsp;Command line parameter $3 = third
  38&nbsp;Command line parameter $4 = fourth
  39&nbsp;Command line parameter $5 = fifth</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX44"
></A
><P
><B
>Example 14-13. Forcing a log-off</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><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;#  Note:
  21&nbsp;#  Depending on the hardware and even the kernel version,
  22&nbsp;#+ the modem port on your machine may be different --
  23&nbsp;#+ /dev/ttyS1 or /dev/ttyS2.
  24&nbsp;
  25&nbsp;exit 0
  26&nbsp;
  27&nbsp;# Exercises:
  28&nbsp;# ---------
  29&nbsp;# 1) Have script check whether root user is invoking it.
  30&nbsp;# 2) Do a check on whether the process to be killed
  31&nbsp;#+   is actually running before attempting to kill it.   
  32&nbsp;# 3) Write an alternate version of this script based on 'fuser':
  33&nbsp;#+      if [ fuser -s /dev/modem ]; then . . .</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ROT14"
></A
><P
><B
>Example 14-14. A version of <I
CLASS="FIRSTTERM"
>rot13</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><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
></TD
></TR
></TABLE
><HR></DIV
><P
>Rory Winston contributed the following instance of how
	      useful <I
CLASS="FIRSTTERM"
>eval</I
> can be.</P
><P
><A
NAME="EVALX0"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EVALEX"
></A
><P
><B
>Example 14-15. Using <I
CLASS="FIRSTTERM"
>eval</I
> to force variable
	        substitution in a <I
CLASS="FIRSTTERM"
>Perl</I
> script</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><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
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="EVALRISK"
></A
></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
>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
></TR
></TABLE
></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/options. 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 14-16. Using <I
CLASS="FIRSTTERM"
>set</I
> with positional
	        parameters</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><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 $_        # ---
  28&nbsp;echo
  29&nbsp;
  30&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>More fun with positional parameters.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REVPOSPARAMS"
></A
><P
><B
>Example 14-17. Reversing the positional parameters</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# revposparams.sh: Reverse positional parameters.
   3&nbsp;# Script by Dan Jacobson, with stylistic revisions by document author.
   4&nbsp;
   5&nbsp;
   6&nbsp;set a\ b c d\ e;
   7&nbsp;#     ^      ^     Spaces escaped 
   8&nbsp;#       ^ ^        Spaces not escaped
   9&nbsp;OIFS=$IFS; IFS=:;
  10&nbsp;#              ^   Saving old IFS and setting new one.
  11&nbsp;
  12&nbsp;echo
  13&nbsp;
  14&nbsp;until [ $# -eq 0 ]
  15&nbsp;do          #      Step through positional parameters.
  16&nbsp;  echo "### k0 = "$k""     # Before
  17&nbsp;  k=$1:$k;  #      Append each pos param to loop variable.
  18&nbsp;#     ^
  19&nbsp;  echo "### k = "$k""      # After
  20&nbsp;  echo
  21&nbsp;  shift;
  22&nbsp;done
  23&nbsp;
  24&nbsp;set $k  #  Set new positional parameters.
  25&nbsp;echo -
  26&nbsp;echo $# #  Count of positional parameters.
  27&nbsp;echo -
  28&nbsp;echo
  29&nbsp;
  30&nbsp;for i   #  Omitting the "in list" sets the variable -- i --
  31&nbsp;        #+ to the positional parameters.
  32&nbsp;do
  33&nbsp;  echo $i  # Display new positional parameters.
  34&nbsp;done
  35&nbsp;
  36&nbsp;IFS=$OIFS  # Restore IFS.
  37&nbsp;
  38&nbsp;#  Question:
  39&nbsp;#  Is it necessary to set an new IFS, internal field separator,
  40&nbsp;#+ in order for this script to work properly?
  41&nbsp;#  What happens if you don't? Try it.
  42&nbsp;#  And, why use the new IFS -- a colon -- in line 17,
  43&nbsp;#+ to append to the loop variable?
  44&nbsp;#  What is the purpose of this?
  45&nbsp;
  46&nbsp;exit 0
  47&nbsp;
  48&nbsp;$ ./revposparams.sh
  49&nbsp;
  50&nbsp;### k0 = 
  51&nbsp;### k = a b
  52&nbsp;
  53&nbsp;### k0 = a b
  54&nbsp;### k = c a b
  55&nbsp;
  56&nbsp;### k0 = c a b
  57&nbsp;### k = d e c a b
  58&nbsp;
  59&nbsp;-
  60&nbsp;3
  61&nbsp;-
  62&nbsp;
  63&nbsp;d e
  64&nbsp;c
  65&nbsp;a b</PRE
></TD
></TR
></TABLE
><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.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><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