gotchas.html

一本完整的描述Unix Shell 编程的工具书的所有范例

   6 chmod 755 $0   # Change back to execute permission.
   7                # The 'unix2dos' command removes execute permission.
   8 
   9 ./$0           # Script tries to run itself again.
  10                # But it won't work as a DOS file.
  11 
  12 echo "There"
  13 
  14&nbsp;exit 0</PRE
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>
      </P
><P
>A shell script headed by <TT
CLASS="USERINPUT"
><B
>#!/bin/sh</B
></TT
>
	will not run in full Bash-compatibility mode. Some Bash-specific
	functions might be disabled. Scripts that need complete
	access to all the Bash-specific extensions should start with
	<TT
CLASS="USERINPUT"
><B
>#!/bin/bash</B
></TT
>.</P
><P
><A
HREF="here-docs.html#INDENTEDLS"
>Putting whitespace in front of
	the terminating limit string</A
> of a <A
HREF="here-docs.html#HEREDOCREF"
>here document</A
> will cause unexpected
	behavior in a script.</P
><P
><A
NAME="PARCHILDPROBREF"
></A
></P
><P
>A script may not <B
CLASS="COMMAND"
>export</B
> variables back
	to its <A
HREF="internal.html#FORKREF"
>parent process</A
>, the shell,
	or to the environment. Just as we learned in biology, a child
	process can inherit from a parent, but not vice versa.
	  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;WHATEVER=/home/bozo
   2&nbsp;export WHATEVER
   3&nbsp;exit 0</PRE
></TD
></TR
></TABLE
>
          <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><B
CLASS="COMMAND"
>echo $WHATEVER</B
>
 <TT
CLASS="COMPUTEROUTPUT"
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
></PRE
></TD
></TR
></TABLE
>
        Sure enough, back at the command prompt, $WHATEVER remains unset. 		  
	  </P
><P
>Setting and manipulating variables in a <A
HREF="subshells.html#SUBSHELLSREF"
>subshell</A
>, then attempting
        to use those same variables outside the scope of the subshell will
	result an unpleasant surprise.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SUBPIT"
></A
><P
><B
>Example 31-2. Subshell Pitfalls</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Pitfalls of variables in a subshell.
   3&nbsp;
   4&nbsp;outer_variable=outer
   5&nbsp;echo
   6&nbsp;echo "outer_variable = $outer_variable"
   7&nbsp;echo
   8&nbsp;
   9&nbsp;(
  10&nbsp;# Begin subshell
  11&nbsp;
  12&nbsp;echo "outer_variable inside subshell = $outer_variable"
  13&nbsp;inner_variable=inner  # Set
  14&nbsp;echo "inner_variable inside subshell = $inner_variable"
  15&nbsp;outer_variable=inner  # Will value change globally?
  16&nbsp;echo "outer_variable inside subshell = $outer_variable"
  17&nbsp;
  18&nbsp;# Will 'exporting' make a difference?
  19&nbsp;#    export inner_variable
  20&nbsp;#    export outer_variable
  21&nbsp;# Try it and see.
  22&nbsp;
  23&nbsp;# End subshell
  24&nbsp;)
  25&nbsp;
  26&nbsp;echo
  27&nbsp;echo "inner_variable outside subshell = $inner_variable"  # Unset.
  28&nbsp;echo "outer_variable outside subshell = $outer_variable"  # Unchanged.
  29&nbsp;echo
  30&nbsp;
  31&nbsp;exit 0
  32&nbsp;
  33&nbsp;# What happens if you uncomment lines 19 and 20?
  34&nbsp;# Does it make a difference?</PRE
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><HR></DIV
><P
><A
NAME="BADREAD0"
></A
></P
><P
><A
HREF="special-chars.html#PIPEREF"
>Piping</A
>
	<B
CLASS="COMMAND"
>echo</B
> output to a <A
HREF="internal.html#READREF"
>read</A
> may produce unexpected
	results.  In this scenario, the <B
CLASS="COMMAND"
>read</B
>
	acts as if it were running in a subshell. Instead, use
	the <A
HREF="internal.html#SETREF"
>set</A
> command (as in <A
HREF="internal.html#SETPOS"
>Example 11-16</A
>).</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="BADREAD"
></A
><P
><B
>Example 31-3. Piping the output of <B
CLASS="COMMAND"
>echo</B
> to a <B
CLASS="COMMAND"
>read</B
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;#  badread.sh:
   3&nbsp;#  Attempting to use 'echo and 'read'
   4&nbsp;#+ to assign variables non-interactively.
   5&nbsp;
   6&nbsp;a=aaa
   7&nbsp;b=bbb
   8&nbsp;c=ccc
   9&nbsp;
  10&nbsp;echo "one two three" | read a b c
  11&nbsp;# Try to reassign a, b, and c.
  12&nbsp;
  13&nbsp;echo
  14&nbsp;echo "a = $a"  # a = aaa
  15&nbsp;echo "b = $b"  # b = bbb
  16&nbsp;echo "c = $c"  # c = ccc
  17&nbsp;# Reassignment failed.
  18&nbsp;
  19&nbsp;# ------------------------------
  20&nbsp;
  21&nbsp;# Try the following alternative.
  22&nbsp;
  23&nbsp;var=`echo "one two three"`
  24&nbsp;set -- $var
  25&nbsp;a=$1; b=$2; c=$3
  26&nbsp;
  27&nbsp;echo "-------"
  28&nbsp;echo "a = $a"  # a = one
  29&nbsp;echo "b = $b"  # b = two
  30&nbsp;echo "c = $c"  # c = three 
  31&nbsp;# Reassignment succeeded.
  32&nbsp;
  33&nbsp;# ------------------------------
  34&nbsp;
  35&nbsp;#  Note also that an echo to a 'read' works within a subshell.
  36&nbsp;#  However, the value of the variable changes *only* within the subshell.
  37&nbsp;
  38&nbsp;a=aaa          # Starting all over again.
  39&nbsp;b=bbb
  40&nbsp;c=ccc
  41&nbsp;
  42&nbsp;echo; echo
  43&nbsp;echo "one two three" | ( read a b c;
  44&nbsp;echo "Inside subshell: "; echo "a = $a"; echo "b = $b"; echo "c = $c" )
  45&nbsp;# a = one
  46&nbsp;# b = two
  47&nbsp;# c = three
  48&nbsp;echo "-----------------"
  49&nbsp;echo "Outside subshell: "
  50&nbsp;echo "a = $a"  # a = aaa
  51&nbsp;echo "b = $b"  # b = bbb
  52&nbsp;echo "c = $c"  # c = ccc
  53&nbsp;echo
  54&nbsp;
  55&nbsp;exit 0</PRE
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><HR></DIV
><P
>In fact, as Anthony Richardson points out, piping to
        <I
CLASS="EMPHASIS"
>any</I
> loop can cause a similar problem.</P
><P
>	
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Loop piping troubles.
   2&nbsp;#  This example by Anthony Richardson,
   3&nbsp;#+ with addendum by Wilbert Berendsen.
   4&nbsp;
   5&nbsp;
   6&nbsp;foundone=false
   7&nbsp;find $HOME -type f -atime +30 -size 100k |
   8&nbsp;while true
   9&nbsp;do
  10&nbsp;   read f
  11&nbsp;   echo "$f is over 100KB and has not been accessed in over 30 days"
  12&nbsp;   echo "Consider moving the file to archives."
  13&nbsp;   foundone=true
  14&nbsp;   # ------------------------------------
  15&nbsp;   echo "Subshell level = $BASH_SUBSHELL"
  16&nbsp;   # Subshell level = 1
  17&nbsp;   # Yes, we're inside a subshell.
  18&nbsp;   # ------------------------------------
  19&nbsp;done
  20&nbsp;   
  21&nbsp;#  foundone will always be false here since it is
  22&nbsp;#+ set to true inside a subshell
  23&nbsp;if [ $foundone = false ]
  24&nbsp;then
  25&nbsp;   echo "No files need archiving."
  26&nbsp;fi
  27&nbsp;
  28&nbsp;# =====================Now, here is the correct way:=================
  29&nbsp;
  30&nbsp;foundone=false
  31&nbsp;for f in $(find $HOME -type f -atime +30 -size 100k)  # No pipe here.
  32&nbsp;do
  33&nbsp;   echo "$f is over 100KB and has not been accessed in over 30 days"
  34&nbsp;   echo "Consider moving the file to archives."
  35&nbsp;   foundone=true
  36&nbsp;done
  37&nbsp;   
  38&nbsp;if [ $foundone = false ]
  39&nbsp;then
  40&nbsp;   echo "No files need archiving."
  41&nbsp;fi
  42&nbsp;
  43&nbsp;# ==================And here is another alternative==================
  44&nbsp;
  45&nbsp;#  Places the part of the script that reads the variables
  46&nbsp;#+ within a code block, so they share the same subshell.
  47&nbsp;#  Thank you, W.B.
  48&nbsp;
  49&nbsp;find $HOME -type f -atime +30 -size 100k | {
  50&nbsp;     foundone=false
  51&nbsp;     while read f
  52&nbsp;     do
  53&nbsp;       echo "$f is over 100KB and has not been accessed in over 30 days"
  54&nbsp;       echo "Consider moving the file to archives."
  55&nbsp;       foundone=true
  56&nbsp;     done
  57&nbsp;
  58&nbsp;     if ! $foundone
  59&nbsp;     then
  60&nbsp;       echo "No files need archiving."
  61&nbsp;     fi
  62&nbsp;}</PRE
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>
      </P
><P
>        A related problem occurs when trying to write the
	<TT
CLASS="FILENAME"
>stdout</TT
> of a <B
CLASS="COMMAND"
>tail -f</B
>
	piped to <A
HREF="textproc.html#GREPREF"
>grep</A
>.
	  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;tail -f /var/log/messages | grep "$ERROR_MSG" &#62;&#62; error.log
   2&nbsp;# The "error.log" file will not have anything written to it.</PRE
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>
      </P
><P
>--</P
><P
>Using <SPAN
CLASS="QUOTE"
>"suid"</SPAN
> commands within scripts is risky,
	as it may compromise system security.
	  <A
NAME="AEN15214"
HREF="#FTN.AEN15214"
>[1]</A
>
      </P
><P
>Using shell scripts for CGI programming may be problematic. Shell
        script variables are not <SPAN
CLASS="QUOTE"
>"typesafe"</SPAN
>, and this can cause
	undesirable behavior as far as CGI is concerned. Moreover, it is
	difficult to <SPAN
CLASS="QUOTE"
>"cracker-proof"</SPAN
> shell scripts.</P
><P
>Bash does not handle the <A
HREF="internal.html#DOUBLESLASHREF"
>double slash
        (<SPAN
CLASS="TOKEN"
>//</SPAN
>) string</A
> correctly.</P
><P
>Bash scripts written for Linux or BSD systems may need
	fixups to run on a commercial UNIX (or Apple OSX) machine. Such
	scripts often employ GNU commands and filters which have greater
	functionality than their generic UNIX counterparts. This is
	particularly true of such text processing utilites as <A
HREF="textproc.html#TRREF"
>tr</A
>.</P
><TABLE
BORDER="0"
WIDTH="100%"
CELLSPACING="0"
CELLPADDING="0"
CLASS="EPIGRAPH"
><TR
><TD
WIDTH="45%"
>&nbsp;</TD
><TD
WIDTH="45%"
ALIGN="LEFT"
VALIGN="TOP"
><I
><P
><I
>Danger is near thee --</I
></P
><P
><I
>Beware, beware, beware, beware.</I
></P
><P
><I
>Many brave hearts are asleep in the deep.</I
></P
><P
><I
>So beware --</I
></P
><P
><I
>Beware.</I
></P
></I
></TD
></TR
><TR
><TD
WIDTH="45%"
>&nbsp;</TD
><TD
WIDTH="45%"
ALIGN="RIGHT"
VALIGN="TOP"
><I
><SPAN
CLASS="ATTRIBUTION"
>A.J. Lamb and H.W. Petrie</SPAN
></I
></TD
></TR
></TABLE
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN15214"
HREF="gotchas.html#AEN15214"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Setting the <I
CLASS="EMPHASIS"
>suid</I
> permission on
	    the script itself has no effect.</P
></TD
></TR
></TABLE
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CLASS="NAVFOOTER"
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