loops.html

一本完整的描述Unix Shell 编程的工具书的所有范例

CLASS="USERINPUT"
><B
>in [list]</B
></TT
> part of a
	      <I
CLASS="FIRSTTERM"
>for loop</I
> causes the loop to operate
	      on <SPAN
CLASS="TOKEN"
>$@</SPAN
> -- the list of arguments given
	      on the command line to the script. A particularly clever
	      illustration of this is <A
HREF="contributed-scripts.html#PRIMES"
>Example A-16</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX23"
></A
><P
><B
>Example 10-5. Missing <TT
CLASS="USERINPUT"
><B
>in [list]</B
></TT
> in a
		<B
CLASS="COMMAND"
>for</B
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  Invoke this script both with and without arguments,
   4&nbsp;#+ and see what happens.
   5&nbsp;
   6&nbsp;for a
   7&nbsp;do
   8&nbsp; echo -n "$a "
   9&nbsp;done
  10&nbsp;
  11&nbsp;#  The 'in list' missing, therefore the loop operates on '$@'
  12&nbsp;#+ (command-line argument list, including whitespace).
  13&nbsp;
  14&nbsp;echo
  15&nbsp;
  16&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><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 12-49</A
>,
	      <A
HREF="loops.html#SYMLINKS"
>Example 10-10</A
> and <A
HREF="mathc.html#BASE"
>Example 12-43</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="FORLOOPCMD"
></A
><P
><B
>Example 10-6. Generating the [list] in a <B
CLASS="COMMAND"
>for</B
>
	        loop with command substitution</B
></P
><TABLE
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><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 [list].</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="BINGREP"
></A
><P
><B
>Example 10-7. A <A
HREF="textproc.html#GREPREF"
>grep</A
> replacement
	        for binary files</B
></P
><TABLE
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><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"  to exercise this script.
  37&nbsp;
  38&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
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BGCOLOR="#E0E0E0"
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><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 [list] 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
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# findstring.sh:
   3&nbsp;# Find a particular string in 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 taking command-line parameters
  22&nbsp;#+ for $directory and $fstring.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>The output of a <I
CLASS="FIRSTTERM"
>for loop</I
> may be piped to
	      a command or commands.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SYMLINKS"
></A
><P
><B
>Example 10-10. Listing the <A
HREF="external.html#SYMLINKREF"
>symbolic
	        links</A
> in a directory</B
></P
><TABLE
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WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# symlinks.sh: Lists symbolic links in a directory.
   3&nbsp;
   4&nbsp;
   5&nbsp;directory=${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 $directory -type l )
  32&nbsp;#+ will choke on filenames with embedded whitespace.
  33&nbsp;#  Even this will only pick up the first field of each argument.
  34&nbsp;
  35&nbsp;exit 0
  36&nbsp;
  37&nbsp;
  38&nbsp;# Jean Helou proposes the following alternative:
  39&nbsp;
  40&nbsp;echo "symbolic links in directory \"$directory\""
  41&nbsp;# Backup of the current IFS. One can never be too cautious.
  42&nbsp;OLDIFS=$IFS
  43&nbsp;IFS=:
  44&nbsp;
  45&nbsp;for file in $(find $directory -type l -printf "%p$IFS")
  46&nbsp;do     #                              ^^^^^^^^^^^^^^^^
  47&nbsp;       echo "$file"
  48&nbsp;       done|sort</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>The <TT
CLASS="FILENAME"
>stdout</TT
> of a loop may be <A
HREF="io-redirection.html#IOREDIRREF"
>redirected</A
> to a file, as this slight
	      modification to the previous example shows.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SYMLINKS2"
></A
><P
><B
>Example 10-11. Symbolic links in a directory, saved to a file</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# symlinks.sh: Lists symbolic links in a directory.
   3&nbsp;
   4&nbsp;OUTFILE=symlinks.list                         # save file
   5&nbsp;
   6&nbsp;directory=${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; "$OUTFILE"                     # stdout of loop
  18&nbsp;#           ^^^^^^^^^^^^^                       redirected to save file.
  19&nbsp;
  20&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>There is an alternative syntax to a <I
CLASS="FIRSTTERM"
>for
	      loop</I
> that will look very familiar to C
	      programmers. This requires double parentheses.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="FORLOOPC"
></A
><P
><B
>Example 10-12. A C-like <B
CLASS="COMMAND"
>for</B
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Two ways to count up to 10.
   3&nbsp;
   4&nbsp;echo
   5&nbsp;
   6&nbsp;# Standard syntax.
   7&nbsp;for a in 1 2 3 4 5 6 7 8 9 10
   8&nbsp;do
   9&nbsp;  echo -n "$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 26-15</A
>, <A
HREF="arrays.html#TWODIM"
>Example 26-16</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 <B
CLASS="COMMAND"
>efax</B
> 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 'fax' installed).
   3&nbsp;
   4&nbsp;EXPECTED_ARGS=2
   5&nbsp;E_BADARGS=65
   6&nbsp;