loops.html

Shall高级编程

  16&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="LOOPCS"
></A
></P
><P
>It is possible to use <A
HREF="commandsub.html#COMMANDSUBREF"
>command substitution</A
>
	      to generate the <TT
CLASS="USERINPUT"
><B
>[list]</B
></TT
> in a
	      <I
CLASS="FIRSTTERM"
>for loop</I
>. See also <A
HREF="extmisc.html#EX53"
>Example 15-52</A
>,
	      <A
HREF="loops.html#SYMLINKS"
>Example 10-10</A
> and <A
HREF="mathc.html#BASE"
>Example 15-46</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="FORLOOPCMD"
></A
><P
><B
>Example 10-6. Generating the <TT
CLASS="USERINPUT"
><B
>[list]</B
></TT
> in
	      a <I
CLASS="FIRSTTERM"
>for</I
> loop with command substitution</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;#  for-loopcmd.sh: for-loop with [list]
   3&nbsp;#+ generated by command substitution.
   4&nbsp;
   5&nbsp;NUMBERS="9 7 3 8 37.53"
   6&nbsp;
   7&nbsp;for number in `echo $NUMBERS`  # for number in 9 7 3 8 37.53
   8&nbsp;do
   9&nbsp;  echo -n "$number "
  10&nbsp;done
  11&nbsp;
  12&nbsp;echo 
  13&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Here is a somewhat more complex example of using command
	      substitution to create the <TT
CLASS="USERINPUT"
><B
>[list]</B
></TT
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="BINGREP"
></A
><P
><B
>Example 10-7. A <I
CLASS="FIRSTTERM"
>grep</I
> replacement
	        for binary files</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# bin-grep.sh: Locates matching strings in a binary file.
   3&nbsp;
   4&nbsp;# A "grep" replacement for binary files.
   5&nbsp;# Similar effect to "grep -a"
   6&nbsp;
   7&nbsp;E_BADARGS=65
   8&nbsp;E_NOFILE=66
   9&nbsp;
  10&nbsp;if [ $# -ne 2 ]
  11&nbsp;then
  12&nbsp;  echo "Usage: `basename $0` search_string filename"
  13&nbsp;  exit $E_BADARGS
  14&nbsp;fi
  15&nbsp;
  16&nbsp;if [ ! -f "$2" ]
  17&nbsp;then
  18&nbsp;  echo "File \"$2\" does not exist."
  19&nbsp;  exit $E_NOFILE
  20&nbsp;fi  
  21&nbsp;
  22&nbsp;
  23&nbsp;IFS=$'\012'       # Per suggestion of Anton Filippov.
  24&nbsp;                  # was:  IFS="\n"
  25&nbsp;for word in $( strings "$2" | grep "$1" )
  26&nbsp;# The "strings" command lists strings in binary files.
  27&nbsp;# Output then piped to "grep", which tests for desired string.
  28&nbsp;do
  29&nbsp;  echo $word
  30&nbsp;done
  31&nbsp;
  32&nbsp;# As S.C. points out, lines 23 - 30 could be replaced with the simpler
  33&nbsp;#    strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]'
  34&nbsp;
  35&nbsp;
  36&nbsp;#  Try something like  "./bin-grep.sh mem /bin/ls"
  37&nbsp;#+ to exercise this script.
  38&nbsp;
  39&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>More of the same.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="USERLIST"
></A
><P
><B
>Example 10-8. Listing all users on the system</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# userlist.sh
   3&nbsp;
   4&nbsp;PASSWORD_FILE=/etc/passwd
   5&nbsp;n=1           # User number
   6&nbsp;
   7&nbsp;for name in $(awk 'BEGIN{FS=":"}{print $1}' &#60; "$PASSWORD_FILE" )
   8&nbsp;# Field separator = :    ^^^^^^
   9&nbsp;# Print first field              ^^^^^^^^
  10&nbsp;# Get input from password file               ^^^^^^^^^^^^^^^^^
  11&nbsp;do
  12&nbsp;  echo "USER #$n = $name"
  13&nbsp;  let "n += 1"
  14&nbsp;done  
  15&nbsp;
  16&nbsp;
  17&nbsp;# USER #1 = root
  18&nbsp;# USER #2 = bin
  19&nbsp;# USER #3 = daemon
  20&nbsp;# ...
  21&nbsp;# USER #30 = bozo
  22&nbsp;
  23&nbsp;exit 0
  24&nbsp;
  25&nbsp;#  Exercise:
  26&nbsp;#  --------
  27&nbsp;#  How is it that an ordinary user (or a script run by same)
  28&nbsp;#+ can read /etc/passwd?
  29&nbsp;#  Isn't this a security hole? Why or why not?</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>A final example of the <TT
CLASS="USERINPUT"
><B
>[list]</B
></TT
>
	      resulting from command substitution.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="FINDSTRING"
></A
><P
><B
>Example 10-9. Checking all the binaries in a directory for
	      authorship</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# findstring.sh:
   3&nbsp;# Find a particular string in the binaries in a specified directory.
   4&nbsp;
   5&nbsp;directory=/usr/bin/
   6&nbsp;fstring="Free Software Foundation"  # See which files come from the FSF.
   7&nbsp;
   8&nbsp;for file in $( find $directory -type f -name '*' | sort )
   9&nbsp;do
  10&nbsp;  strings -f $file | grep "$fstring" | sed -e "s%$directory%%"
  11&nbsp;  #  In the "sed" expression,
  12&nbsp;  #+ it is necessary to substitute for the normal "/" delimiter
  13&nbsp;  #+ because "/" happens to be one of the characters filtered out.
  14&nbsp;  #  Failure to do so gives an error message (try it).
  15&nbsp;done  
  16&nbsp;
  17&nbsp;exit 0
  18&nbsp;
  19&nbsp;#  Exercise (easy):
  20&nbsp;#  ---------------
  21&nbsp;#  Convert this script to take command-line parameters
  22&nbsp;#+ for $directory and $fstring.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="LOOPREDIR"
></A
></P
><P
>The output of a <I
CLASS="FIRSTTERM"
>for loop</I
> may
	      be piped to a command or commands.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SYMLINKS"
></A
><P
><B
>Example 10-10. Listing the <I
CLASS="FIRSTTERM"
>symbolic
	        links</I
> in a directory</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# symlinks.sh: Lists symbolic links in a directory.
   3&nbsp;
   4&nbsp;
   5&nbsp;directory=${1-`pwd`}
   6&nbsp;#  Defaults to current working directory,
   7&nbsp;#+ if not otherwise specified.
   8&nbsp;#  Equivalent to code block below.
   9&nbsp;# ----------------------------------------------------------
  10&nbsp;# ARGS=1                 # Expect one command-line argument.
  11&nbsp;#
  12&nbsp;# if [ $# -ne "$ARGS" ]  # If not 1 arg...
  13&nbsp;# then
  14&nbsp;#   directory=`pwd`      # current working directory
  15&nbsp;# else
  16&nbsp;#   directory=$1
  17&nbsp;# fi
  18&nbsp;# ----------------------------------------------------------
  19&nbsp;
  20&nbsp;echo "symbolic links in directory \"$directory\""
  21&nbsp;
  22&nbsp;for file in "$( find $directory -type l )"   # -type l = symbolic links
  23&nbsp;do
  24&nbsp;  echo "$file"
  25&nbsp;done | sort                                  # Otherwise file list is unsorted.
  26&nbsp;#  Strictly speaking, a loop isn't really necessary here,
  27&nbsp;#+ since the output of the "find" command is expanded into a single word.
  28&nbsp;#  However, it's easy to understand and illustrative this way.
  29&nbsp;
  30&nbsp;#  As Dominik 'Aeneas' Schnitzer points out,
  31&nbsp;#+ failing to quote  $( find $directory -type l )
  32&nbsp;#+ will choke on filenames with embedded whitespace.
  33&nbsp;#  Even this will only pick up the first field of each argument.
  34&nbsp;
  35&nbsp;exit 0
  36&nbsp;
  37&nbsp;
  38&nbsp;# --------------------------------------------------------
  39&nbsp;# Jean Helou proposes the following alternative:
  40&nbsp;
  41&nbsp;echo "symbolic links in directory \"$directory\""
  42&nbsp;# Backup of the current IFS. One can never be too cautious.
  43&nbsp;OLDIFS=$IFS
  44&nbsp;IFS=:
  45&nbsp;
  46&nbsp;for file in $(find $directory -type l -printf "%p$IFS")
  47&nbsp;do     #                              ^^^^^^^^^^^^^^^^
  48&nbsp;       echo "$file"
  49&nbsp;       done|sort
  50&nbsp;
  51&nbsp;# And, James "Mike" Conley suggests modifying Helou's code thusly:
  52&nbsp;
  53&nbsp;OLDIFS=$IFS
  54&nbsp;IFS='' # Null IFS means no word breaks
  55&nbsp;for file in $( find $directory -type l )
  56&nbsp;do
  57&nbsp;  echo $file
  58&nbsp;  done | sort
  59&nbsp;
  60&nbsp;#  This works in the "pathological" case of a directory name having
  61&nbsp;#+ an embedded colon.
  62&nbsp;#  "This also fixes the pathological case of the directory name having
  63&nbsp;#+  a colon (or space in earlier example) as well."
  64&nbsp;</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>The <TT
CLASS="FILENAME"
>stdout</TT
> of a loop may be <A
HREF="io-redirection.html#IOREDIRREF"
>redirected</A
> to a file, as this slight
	      modification to the previous example shows.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SYMLINKS2"
></A
><P
><B
>Example 10-11. Symbolic links in a directory, saved to a file</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# symlinks.sh: Lists symbolic links in a directory.
   3&nbsp;
   4&nbsp;OUTFILE=symlinks.list                         # save file
   5&nbsp;
   6&nbsp;directory=${1-`pwd`}
   7&nbsp;#  Defaults to current working directory,
   8&nbsp;#+ if not otherwise specified.
   9&nbsp;
  10&nbsp;
  11&nbsp;echo "symbolic links in directory \"$directory\"" &#62; "$OUTFILE"
  12&nbsp;echo "---------------------------" &#62;&#62; "$OUTFILE"
  13&nbsp;
  14&nbsp;for file in "$( find $directory -type l )"    # -type l = symbolic links
  15&nbsp;do
  16&nbsp;  echo "$file"
  17&nbsp;done | sort &#62;&#62; "$OUTFILE"                     # stdout of loop
  18&nbsp;#           ^^^^^^^^^^^^^                       redirected to save file.
  19&nbsp;
  20&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="LOOPCSTYLE"
></A
></P
><P
>There is an alternative syntax to a <I
CLASS="FIRSTTERM"
>for
	      loop</I
> that will look very familiar to C
	      programmers. This requires <A
HREF="dblparens.html#DBLPARENSREF"
>double parentheses</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="FORLOOPC"
></A
><P
><B
>Example 10-12. A C-style <I
CLASS="FIRSTTERM"
>for</I
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Two ways to count up to 10.
   3&nbsp;
   4&nbsp;echo
   5&nbsp;
   6&nbsp;# Standard syntax.
   7&nbsp;for a in 1 2 3 4 5 6 7 8 9 10
   8&nbsp;do
   9&nbsp;  echo -n "$a "
  10&nbsp;done  
  11&nbsp;
  12&nbsp;echo; echo
  13&nbsp;
  14&nbsp;# +==========================================+
  15&nbsp;
  16&nbsp;# Now, let's do the same, using C-like syntax.
  17&nbsp;
  18&nbsp;LIMIT=10
  19&nbsp;
  20&nbsp;for ((a=1; a &#60;= LIMIT ; a++))  # Double parentheses, and "LIMIT" with no "$".
  21&nbsp;do
  22&nbsp;  echo -n "$a "
  23&nbsp;done                           # A construct borrowed from 'ksh93'.
  24&nbsp;
  25&nbsp;echo; echo
  26&nbsp;
  27&nbsp;# +=========================================================================+
  28&nbsp;
  29&nbsp;# Let's use the C "comma operator" to increment two variables simultaneously.
  30&nbsp;
  31&nbsp;for ((a=1, b=1; a &#60;= LIMIT ; a++, b++))  # The comma chains together operations.
  32&nbsp;do
  33&nbsp;  echo -n "$a-$b "
  34&nbsp;done
  35&nbsp;
  36&nbsp;echo; echo
  37&nbsp;
  38&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>See also <A
HREF="arrays.html#QFUNCTION"
>Example 26-16</A
>, <A
HREF="arrays.html#TWODIM"
>Example 26-17</A
>, and <A
HREF="contributed-scripts.html#COLLATZ"
>Example A-6</A
>.</P
><P
>---</P
><P
>Now, a <I
CLASS="FIRSTTERM"
>for loop</I
> used in a
	      <SPAN
CLASS="QUOTE"
>"real-life"</SPAN
> context.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX24"
></A
><P
><B
>Example 10-13. Using <I
CLASS="FIRSTTERM"
>efax</I
> in batch mode</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Faxing (must have 'efax' package installed).
   3&nbsp;
   4&nbsp;EXPECTED_ARGS=2
   5&nbsp;E_BADARGS=65
   6&nbsp;MODEM_PORT="/dev/ttyS2"   # May be different on your machine.
   7&nbsp;#                ^^^^^      PCMCIA modem card default port.
   8&nbsp;
   9&nbsp;if [ $# -ne $EXPECTED_ARGS ]
  10&nbsp;# Check for proper number of command line args.
  11&nbsp;then
  12&nbsp;   echo "Usage: `basename $0` phone# text-file"
  13&nbsp;   exit $E_BADARGS
  14&nbsp;fi
  15&nbsp;
  16&nbsp;
  17&nbsp;if [ ! -f "$2" ]
  18&nbsp;then
  19&nbsp;  echo "File $2 is not a text file."
  20&nbsp;  #     File is not a regular file, or does not exist.
  21&nbsp;  exit $E_BADARGS
  22&nbsp;fi
  23&nbsp;  
  24&nbsp;
  25&nbsp;fax make $2              #  Create fax-formatted files from text files.
  26&nbsp;
  27&nbsp;for file in $(ls $2.0*)  #  Concatenate the converted files.
  28&nbsp;                         #  Uses wild card (filename "globbing")
  29&nbsp;			 #+ in variable list.
  30&nbsp;do
  31&nbsp;  fil="$fil $file"
  32&nbsp;done  
  33&nbsp;
  34&nbsp;efax -d "$MODEM_PORT"  -t "T$1" $fil   # Finally, do the work.
  35&nbsp;# Trying adding  -o1  if above line fails.
  36&nbsp;
  37&nbsp;
  38&nbsp;#  As S.C. points out, the for-loop can be eliminated with
  39&nbsp;#     efax -d /dev/ttyS2 -o1 -t "T$1" $2.0*
  40&nbsp;#+ but it's not quite as instructive [grin].
  41&nbsp;
  42&nbsp;exit $?   # Also, efax sends diagnostic messages to stdout.</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="WHILELOOPREF"
></A
><B
CLASS="COMMAND"
>while</B
></DT
><DD
><P
>This construct tests for a condition at the top of a
	      loop, and keeps looping as long as that condition
	      is true (returns a <SPAN
CLASS="RETURNVALUE"
>0</SPAN
> <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
>).  In contrast
	      to a <A
HREF="loops.html#FORLOOPREF1"
>for loop</A
>, a
	      <I
CLASS="FIRSTTERM"
>while loop</I
> finds use in situations
	      where the number of loop repetitions is not known
	      beforehand.</P
><P
><P
><B
CLASS="COMMAND"
>while</B
>  [<TT
CLASS="REPLACEABLE"
><I
> condition </I
></TT
>]<BR>  do <BR>  <TT
CLASS="REPLACEABLE"
><I
>燾ommand(s)</I
></TT
>... <BR>  done </P
></P
><P
>The bracket construct in a <I
CLASS="FIRSTTERM"
>while
	      loop</I
> is nothing more than our old friend,
	      the <A
HREF="tests.html#TESTCONSTRUCTS1"
>test brackets</A
>
	      used in an <I
CLASS="FIRSTTERM"
>if/then</I
> test. In fact,
	      a <I
CLASS="FIRSTTERM"
>while loop</I
> can legally use the
	      more versatile <A
HREF="tests.html#DBLBRACKETS"
>double brackets
	      construct</A
> (while [[ condition ]]).</P
><P
><A
NAME="WHILENEEDSEMI"
></A
></P
><P
><A
HREF="loops.html#NEEDSEMICOLON"
>As is the case with
	      <I
CLASS="FIRSTTERM"
>for loops</I
></A
>, placing the
	      <I
CLASS="FIRSTTERM"
>do</I
> on the same line as the condition
	      test requires a semicolon.</P
><P
><P
><B
CLASS="COMMAND"
>while</B
>  [<TT
CLASS="REPLACEABLE"
><I
> condition </I
></TT
>]  ;   do </P
></P
><P
>Note that the <I
CLASS="FIRSTTERM"
>test brackets</I