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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="IO-REDIRECTION"
></A
>Chapter 20. I/O Redirection</H1
><P
><A
NAME="IOREDIRREF"
></A
></P
><P
>There are always three default <I
CLASS="FIRSTTERM"
>files</I
>
         <A
NAME="AEN17884"
HREF="#FTN.AEN17884"
>[1]</A
>
	open, <TT
CLASS="FILENAME"
>stdin</TT
> (the keyboard),
	<TT
CLASS="FILENAME"
>stdout</TT
> (the screen), and
	<TT
CLASS="FILENAME"
>stderr</TT
> (error messages output to the
	screen).  These, and any other open files, can be redirected.
	Redirection simply means capturing output from a file, command,
	program, script, or even code block within a script (see <A
HREF="special-chars.html#EX8"
>Example 3-1</A
> and <A
HREF="special-chars.html#RPMCHECK"
>Example 3-2</A
>) and sending it as
	input to another file, command, program, or script.</P
><P
><A
NAME="FDREF"
></A
>Each open file gets assigned a file descriptor.	

               <A
NAME="AEN17894"
HREF="#FTN.AEN17894"
>[2]</A
>

	The file descriptors for <TT
CLASS="FILENAME"
>stdin</TT
>,
	<TT
CLASS="FILENAME"
>stdout</TT
>, and <TT
CLASS="FILENAME"
>stderr</TT
> are
	0, 1, and 2, respectively.  For opening additional files, there
	remain descriptors 3 to 9. It is sometimes useful to assign one of
	these additional file descriptors to <TT
CLASS="FILENAME"
>stdin</TT
>,
	<TT
CLASS="FILENAME"
>stdout</TT
>, or <TT
CLASS="FILENAME"
>stderr</TT
>
	as a temporary duplicate link.
	       <A
NAME="AEN17906"
HREF="#FTN.AEN17906"
>[3]</A
>
	This simplifies restoration to normal after complex redirection
	and reshuffling (see <A
HREF="io-redirection.html#REDIR1"
>Example 20-1</A
>).</P
><P
><A
NAME="IOREDIRECTIONREF"
></A
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;   COMMAND_OUTPUT &#62;
   2&nbsp;      # Redirect stdout to a file.
   3&nbsp;      # Creates the file if not present, otherwise overwrites it.
   4&nbsp;
   5&nbsp;      ls -lR &#62; dir-tree.list
   6&nbsp;      # Creates a file containing a listing of the directory tree.
   7&nbsp;
   8&nbsp;   : &#62; filename
   9&nbsp;      # The &#62; truncates file "filename" to zero length.
  10&nbsp;      # If file not present, creates zero-length file (same effect as 'touch').
  11&nbsp;      # The : serves as a dummy placeholder, producing no output.
  12&nbsp;
  13&nbsp;   &#62; filename    
  14&nbsp;      # The &#62; truncates file "filename" to zero length.
  15&nbsp;      # If file not present, creates zero-length file (same effect as 'touch').
  16&nbsp;      # (Same result as ": &#62;", above, but this does not work with some shells.)
  17&nbsp;
  18&nbsp;   COMMAND_OUTPUT &#62;&#62;
  19&nbsp;      # Redirect stdout to a file.
  20&nbsp;      # Creates the file if not present, otherwise appends to it.
  21&nbsp;
  22&nbsp;
  23&nbsp;      # Single-line redirection commands (affect only the line they are on):
  24&nbsp;      # --------------------------------------------------------------------
  25&nbsp;
  26&nbsp;   1&#62;filename
  27&nbsp;      # Redirect stdout to file "filename."
  28&nbsp;   1&#62;&#62;filename
  29&nbsp;      # Redirect and append stdout to file "filename."
  30&nbsp;   2&#62;filename
  31&nbsp;      # Redirect stderr to file "filename."
  32&nbsp;   2&#62;&#62;filename
  33&nbsp;      # Redirect and append stderr to file "filename."
  34&nbsp;   &#38;&#62;filename
  35&nbsp;      # Redirect both stdout and stderr to file "filename."
  36&nbsp;      # This operator is now functional, as of Bash 4, final release.
  37&nbsp;
  38&nbsp;   M&#62;N
  39&nbsp;     # "M" is a file descriptor, which defaults to 1, if not explicitly set.
  40&nbsp;     # "N" is a filename.
  41&nbsp;     # File descriptor "M" is redirect to file "N."
  42&nbsp;   M&#62;&#38;N
  43&nbsp;     # "M" is a file descriptor, which defaults to 1, if not set.
  44&nbsp;     # "N" is another file descriptor.
  45&nbsp;
  46&nbsp;      #==============================================================================
  47&nbsp;
  48&nbsp;      # Redirecting stdout, one line at a time.
  49&nbsp;      LOGFILE=script.log
  50&nbsp;
  51&nbsp;      echo "This statement is sent to the log file, \"$LOGFILE\"." 1&#62;$LOGFILE
  52&nbsp;      echo "This statement is appended to \"$LOGFILE\"." 1&#62;&#62;$LOGFILE
  53&nbsp;      echo "This statement is also appended to \"$LOGFILE\"." 1&#62;&#62;$LOGFILE
  54&nbsp;      echo "This statement is echoed to stdout, and will not appear in \"$LOGFILE\"."
  55&nbsp;      # These redirection commands automatically "reset" after each line.
  56&nbsp;
  57&nbsp;
  58&nbsp;
  59&nbsp;      # Redirecting stderr, one line at a time.
  60&nbsp;      ERRORFILE=script.errors
  61&nbsp;
  62&nbsp;      bad_command1 2&#62;$ERRORFILE       #  Error message sent to $ERRORFILE.
  63&nbsp;      bad_command2 2&#62;&#62;$ERRORFILE      #  Error message appended to $ERRORFILE.
  64&nbsp;      bad_command3                    #  Error message echoed to stderr,
  65&nbsp;                                      #+ and does not appear in $ERRORFILE.
  66&nbsp;      # These redirection commands also automatically "reset" after each line.
  67&nbsp;      #=======================================================================</PRE
></TD
></TR
></TABLE
><P
><A
NAME="IOREDIRECTIONREF1"
></A
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;   2&#62;&#38;1
   2&nbsp;      # Redirects stderr to stdout.
   3&nbsp;      # Error messages get sent to same place as standard output.
   4&nbsp;        &#62;&#62;filename 2&#62;&#38;1
   5&nbsp;            bad_command &#62;&#62;filename 2&#62;&#38;1
   6&nbsp;            # Appends both stdout and stderr to the file "filename" ...
   7&nbsp;        2&#62;&#38;1 | [command(s)]
   8&nbsp;            bad_command 2&#62;&#38;1 | awk '{print $5}'   # found
   9&nbsp;            # Sends stderr through a pipe.
  10&nbsp;            # |&#38; was added to Bash 4 as an abbreviation for 2&#62;&#38;1 |.
  11&nbsp;
  12&nbsp;   i&#62;&#38;j
  13&nbsp;      # Redirects file descriptor <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>i</I
></SPAN
> to <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>j</I
></SPAN
>.
  14&nbsp;      # All output of file pointed to by <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>i</I
></SPAN
> gets sent to file pointed to by <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>j</I
></SPAN
>.
  15&nbsp;
  16&nbsp;   &#62;&#38;j
  17&nbsp;      # Redirects, by default, file descriptor <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>1</I
></SPAN
> (stdout) to <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>j</I
></SPAN
>.
  18&nbsp;      # All stdout gets sent to file pointed to by <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>j</I
></SPAN
>.</PRE
></TD
></TR
></TABLE
><P
><A
NAME="IOREDIRECTIONREF2"
></A
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;   0&#60; FILENAME
   2&nbsp;    &#60; FILENAME
   3&nbsp;      # Accept input from a file.
   4&nbsp;      # Companion command to <SPAN
CLASS="QUOTE"
>"&#62;"</SPAN
>, and often used in combination with it.
   5&nbsp;      #
   6&nbsp;      # grep search-word &#60;filename
   7&nbsp;
   8&nbsp;
   9&nbsp;   [j]&#60;&#62;filename
  10&nbsp;      #  Open file "filename" for reading and writing,
  11&nbsp;      #+ and assign file descriptor "j" to it.
  12&nbsp;      #  If "filename" does not exist, create it.
  13&nbsp;      #  If file descriptor "j" is not specified, default to fd 0, stdin.
  14&nbsp;      #
  15&nbsp;      #  An application of this is writing at a specified place in a file. 
  16&nbsp;      echo 1234567890 &#62; File    # Write string to "File".
  17&nbsp;      exec 3&#60;&#62; File             # Open "File" and assign fd 3 to it.
  18&nbsp;      read -n 4 &#60;&#38;3             # Read only 4 characters.
  19&nbsp;      echo -n . &#62;&#38;3             # Write a decimal point there.
  20&nbsp;      exec 3&#62;&#38;-                 # Close fd 3.
  21&nbsp;      cat File                  # ==&#62; 1234.67890
  22&nbsp;      #  Random access, by golly.
  23&nbsp;
  24&nbsp;
  25&nbsp;
  26&nbsp;   |
  27&nbsp;      # Pipe.
  28&nbsp;      # General purpose process and command chaining tool.
  29&nbsp;      # Similar to <SPAN
CLASS="QUOTE"
>"&#62;"</SPAN
>, but more general in effect.
  30&nbsp;      # Useful for chaining commands, scripts, files, and programs together.
  31&nbsp;      cat *.txt | sort | uniq &#62; result-file
  32&nbsp;      # Sorts the output of all the .txt files and deletes duplicate lines,
  33&nbsp;      # finally saves results to <SPAN
CLASS="QUOTE"
>"result-file"</SPAN
>.</PRE
></TD
></TR
></TABLE
><P
>Multiple instances of input and output redirection
        and/or pipes can be combined in a single command
        line.

       <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;command &#60; input-file &#62; output-file
   2&nbsp;# Or the equivalent:
   3&nbsp;&#60; input-file command &#62; output-file   # Although this is non-standard.
   4&nbsp;
   5&nbsp;command1 | command2 | command3 &#62; output-file</PRE
></TD
></TR
></TABLE
>
        See <A
HREF="filearchiv.html#DERPM"
>Example 16-31</A
> and <A
HREF="contributed-scripts.html#FIFO"
>Example A-14</A
>.</P
><P
>Multiple output streams may be redirected to one file.

         <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;ls -yz &#62;&#62; command.log 2&#62;&#38;1
   2&nbsp;#  Capture result of illegal options "yz" in file "command.log."
   3&nbsp;#  Because stderr is redirected to the file,
   4&nbsp;#+ any error messages will also be there.
   5&nbsp;
   6&nbsp;#  Note, however, that the following does *not* give the same result.
   7&nbsp;ls -yz 2&#62;&#38;1 &#62;&#62; command.log
   8&nbsp;#  Outputs an error message, but does not write to file.
   9&nbsp;#  More precisely, the command output (in this case, null)
  10&nbsp;#+ writes to the file, but the error message goes only to stdout.
  11&nbsp;
  12&nbsp;#  If redirecting both stdout and stderr,
  13&nbsp;#+ the order of the commands makes a difference.</PRE
></TD
></TR
></TABLE
></P
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="CFD"
></A
>Closing File Descriptors</B
></P
><DL
><DT
><SPAN
CLASS="TOKEN"
>n&#60;&#38;-</SPAN
></DT
><DD
><P
>Close input file descriptor
	    <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>0&#60;&#38;-</SPAN
>, <SPAN
CLASS="TOKEN"
>&#60;&#38;-</SPAN
></DT
><DD
><P
>Close <TT
CLASS="FILENAME"
>stdin</TT
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>n&#62;&#38;-</SPAN
></DT
><DD
><P
>Close output file descriptor <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>1&#62;&#38;-</SPAN
>, <SPAN
CLASS="TOKEN"
>&#62;&#38;-</SPAN
></DT
><DD
><P
>Close <TT
CLASS="FILENAME"
>stdout</TT
>.</P
></DD
></DL
></DIV
><P
><A
NAME="FDREF2"
></A
></P
><P
>Child processes inherit open file descriptors. This is
          why pipes work. To prevent an fd from being inherited, close it.
	    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Redirecting only stderr to a pipe.
   2&nbsp;
   3&nbsp;exec 3&#62;&#38;1                              # Save current "value" of stdout.
   4&nbsp;ls -l 2&#62;&#38;1 &#62;&#38;3 3&#62;&#38;- | grep bad 3&#62;&#38;-    # Close fd 3 for 'grep' (but not 'ls').
   5&nbsp;#              ^^^^   ^^^^
   6&nbsp;exec 3&#62;&#38;-                              # Now close it for the remainder of the script.
   7&nbsp;
   8&nbsp;# Thanks, S.C.</PRE
></TD
></TR
></TABLE
>
        </P
><P
>For a more detailed introduction to I/O redirection see
	  <A
HREF="ioredirintro.html"
>Appendix F</A
>.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN17974"
></A
>20.1. Using <I
CLASS="FIRSTTERM"
>exec</I
></H1
><P
><A
NAME="USINGEXECREF"
></A
></P
><P
>An <B
CLASS="COMMAND"
>exec &#60;filename</B
> command redirects
	  <TT
CLASS="FILENAME"
>stdin</TT
> to a file. From that point on, all
	  <TT
CLASS="FILENAME"
>stdin</TT
> comes from that file, rather than
	  its normal source (usually keyboard input). This provides a
	  method of reading a file line by line and possibly parsing
	  each line of input using <A
HREF="sedawk.html#SEDREF"
>sed</A
>
	  and/or <A
HREF="awk.html#AWKREF"
>awk</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REDIR1"
></A
><P
><B
>Example 20-1. Redirecting <TT
CLASS="FILENAME"
>stdin</TT
> using
	  <I
CLASS="FIRSTTERM"
>exec</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Redirecting stdin using 'exec'.
   3&nbsp;
   4&nbsp;
   5&nbsp;exec 6&#60;&#38;0          # Link file descriptor #6 with stdin.
   6&nbsp;                   # Saves stdin.
   7&nbsp;
   8&nbsp;exec &#60; data-file   # stdin replaced by file "data-file"
   9&nbsp;
  10&nbsp;read a1            # Reads first line of file "data-file".
  11&nbsp;read a2            # Reads second line of file "data-file."
  12&nbsp;
  13&nbsp;echo
  14&nbsp;echo "Following lines read from file."
  15&nbsp;echo "-------------------------------"
  16&nbsp;echo $a1
  17&nbsp;echo $a2
  18&nbsp;
  19&nbsp;echo; echo; echo
  20&nbsp;
  21&nbsp;exec 0&#60;&#38;6 6&#60;&#38;-
  22&nbsp;#  Now restore stdin from fd #6, where it had been saved,
  23&nbsp;#+ and close fd #6 ( 6&#60;&#38;- ) to free it for other processes to use.
  24&nbsp;#
  25&nbsp;# &#60;&#38;6 6&#60;&#38;-    also works.
  26&nbsp;
  27&nbsp;echo -n "Enter data  "
  28&nbsp;read b1  # Now "read" functions as expected, reading from normal stdin.
  29&nbsp;echo "Input read from stdin."
  30&nbsp;echo "----------------------"
  31&nbsp;echo "b1 = $b1"
  32&nbsp;
  33&nbsp;echo
  34&nbsp;
  35&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Similarly, an <B
CLASS="COMMAND"
>exec &#62;filename</B
>
	  command redirects <TT
CLASS="FILENAME"
>stdout</TT
> to a designated
	  file.  This sends all command output that would normally go
	  to <TT
CLASS="FILENAME"
>stdout</TT
> to that file.</P
><DIV
CLASS="IMPORTANT"
><TABLE
CLASS="IMPORTANT"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/important.png"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>	  <B
CLASS="COMMAND"
>exec N &#62; filename</B
> affects the entire
	  script or <I
CLASS="FIRSTTERM"
>current shell</I
>. Redirection in
	  the <A
HREF="special-chars.html#PROCESSIDREF"
>PID</A
> of the script or shell
	  from that point on has changed. However . . .
        </P
><P
>	  <B
CLASS="COMMAND"
>N &#62; filename</B
> affects only the newly-forked process,
	  not the entire script or shell.
        </P
><P
>Thank you, Ahmed Darwish, for pointing this out.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REASSIGNSTDOUT"
></A
><P
><B
>Example 20-2. Redirecting <TT
CLASS="FILENAME"
>stdout</TT
> using
	  <I
CLASS="FIRSTTERM"
>exec</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# reassign-stdout.sh
   3&nbsp;
   4&nbsp;LOGFILE=logfile.txt
   5&nbsp;
   6&nbsp;exec 6&#62;&#38;1           # Link file descriptor #6 with stdout.
   7&nbsp;                    # Saves stdout.
   8&nbsp;
   9&nbsp;exec &#62; $LOGFILE     # stdout replaced with file "logfile.txt".
  10&nbsp;
  11&nbsp;# ----------------------------------------------------------- #
  12&nbsp;# All output from commands in this block sent to file $LOGFILE.
  13&nbsp;
  14&nbsp;echo -n "Logfile: "
  15&nbsp;date
  16&nbsp;echo "-------------------------------------"
  17&nbsp;echo
  18&nbsp;
  19&nbsp;echo "Output of \"ls -al\" command"
  20&nbsp;echo
  21&nbsp;ls -al
  22&nbsp;echo; echo
  23&nbsp;echo "Output of \"df\" command"
  24&nbsp;echo
  25&nbsp;df
  26&nbsp;
  27&nbsp;# ----------------------------------------------------------- #
  28&nbsp;
  29&nbsp;exec 1&#62;&#38;6 6&#62;&#38;-      # Restore stdout and close file descriptor #6.
  30&nbsp;
  31&nbsp;echo
  32&nbsp;echo "== stdout now restored to default == "
  33&nbsp;echo
  34&nbsp;ls -al
  35&nbsp;echo
  36&nbsp;
  37&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="UPPERCONV"
></A
><P
><B
>Example 20-3. Redirecting both <TT
CLASS="FILENAME"
>stdin</TT
> and
	  <TT
CLASS="FILENAME"
>stdout</TT
> in the same script with
	  <I
CLASS="FIRSTTERM"
>exec</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# upperconv.sh
   3&nbsp;# Converts a specified input file to uppercase.
   4&nbsp;
   5&nbsp;E_FILE_ACCESS=70
   6&nbsp;E_WRONG_ARGS=71
   7&nbsp;
   8&nbsp;if [ ! -r "$1" ]     # Is specified input file readable?
   9&nbsp;then
  10&nbsp;  echo "Can't read from input file!"
  11&nbsp;  echo "Usage: $0 input-file output-file"
  12&nbsp;  exit $E_FILE_ACCESS
  13&nbsp;fi                   #  Will exit with same error
  14&nbsp;                     #+ even if input file ($1) not specified (why?).
  15&nbsp;
  16&nbsp;if [ -z "$2" ]
  17&nbsp;then
  18&nbsp;  echo "Need to specify output file."
  19&nbsp;  echo "Usage: $0 input-file output-file"
  20&nbsp;  exit $E_WRONG_ARGS
  21&nbsp;fi
  22&nbsp;
  23&nbsp;
  24&nbsp;exec 4&#60;&#38;0
  25&nbsp;exec &#60; $1            # Will read from input file.
  26&nbsp;
  27&nbsp;exec 7&#62;&#38;1
  28&nbsp;exec &#62; $2            # Will write to output file.
  29&nbsp;                     # Assumes output file writable (add check?).
  30&nbsp;
  31&nbsp;# -----------------------------------------------
  32&nbsp;    cat - | tr a-z A-Z   # Uppercase conversion.
  33&nbsp;#   ^^^^^                # Reads from stdin.
  34&nbsp;#           ^^^^^^^^^^   # Writes to stdout.
  35&nbsp;# However, both stdin and stdout were redirected.
  36&nbsp;# Note that the 'cat' can be omitted.
  37&nbsp;# -----------------------------------------------
  38&nbsp;
  39&nbsp;exec 1&#62;&#38;7 7&#62;&#38;-       # Restore stout.
  40&nbsp;exec 0&#60;&#38;4 4&#60;&#38;-       # Restore stdin.
  41&nbsp;
  42&nbsp;# After restoration, the following line prints to stdout as expected.
  43&nbsp;echo "File \"$1\" written to \"$2\" as uppercase conversion."
  44&nbsp;
  45&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>I/O redirection is a clever way of avoiding the dreaded <A
HREF="subshells.html#PARVIS"
>inaccessible variables within a subshell</A
>
      problem.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="AVOIDSUBSHELL"
></A
><P
><B
>Example 20-4. Avoiding a subshell</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# avoid-subshell.sh
   3&nbsp;# Suggested by Matthew Walker.
   4&nbsp;
   5&nbsp;Lines=0
   6&nbsp;
   7&nbsp;echo
   8&nbsp;
   9&nbsp;cat myfile.txt | while read line;
  10&nbsp;                 do {
  11&nbsp;                   echo $line
  12&nbsp;                   (( Lines++ ));  #  Incremented values of this variable
  13&nbsp;                                   #+ inaccessible outside loop.
  14&nbsp;                                   #  Subshell problem.
  15&nbsp;                 }
  16&nbsp;                 done
  17&nbsp;
  18&nbsp;echo "Number of lines read = $Lines"     # 0
  19&nbsp;                                         # Wrong!
  20&nbsp;
  21&nbsp;echo "------------------------"
  22&nbsp;
  23&nbsp;
  24&nbsp;exec 3&#60;&#62; myfile.txt
  25&nbsp;while read line &#60;&#38;3
  26&nbsp;do {
  27&nbsp;  echo "$line"
  28&nbsp;  (( Lines++ ));                   #  Incremented values of this variable
  29&nbsp;                                   #+ accessible outside loop.
  30&nbsp;                                   #  No subshell, no problem.
  31&nbsp;}
  32&nbsp;done
  33&nbsp;exec 3&#62;&#38;-
  34&nbsp;
  35&nbsp;echo "Number of lines read = $Lines"     # 8
  36&nbsp;
  37&nbsp;echo
  38&nbsp;
  39&nbsp;exit 0
  40&nbsp;
  41&nbsp;# Lines below not seen by script.
  42&nbsp;
  43&nbsp;$ cat myfile.txt
  44&nbsp;
  45&nbsp;Line 1.
  46&nbsp;Line 2.
  47&nbsp;Line 3.
  48&nbsp;Line 4.
  49&nbsp;Line 5.
  50&nbsp;Line 6.
  51&nbsp;Line 7.
  52&nbsp;Line 8.</PRE
></TD
></TR
></TABLE
><HR></DIV
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN17884"
HREF="io-redirection.html#AEN17884"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>By convention in UNIX and Linux, data streams
	 and peripherals (<A
HREF="devproc.html#DEVFILEREF"
>device files</A
>)
	 are treated as files, in a fashion analogous to ordinary
	 files.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN17894"
HREF="io-redirection.html#AEN17894"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><A
NAME="FDREF1"
></A
>A <I
CLASS="FIRSTTERM"
>file
		 descriptor</I
> is simply a number that
		 the operating system assigns to an open file
		 to keep track of it.  Consider it a simplified
		 type of file pointer. It is analogous
		 to a <I
CLASS="FIRSTTERM"
>file handle</I
> in
		 <B
CLASS="COMMAND"
>C</B
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN17906"
HREF="io-redirection.html#AEN17906"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Using <TT
CLASS="REPLACEABLE"
><I
>file
	       descriptor 5</I
></TT
> might cause problems.
	       When Bash creates a child process, as with <A
HREF="internal.html#EXECREF"
>exec</A
>, the child inherits
	       fd 5 (see Chet Ramey's archived e-mail, <A
HREF="http://groups.google.com/group/gnu.bash.bug/browse_thread/thread/13955daafded3b5c/18c17050087f9f37"
TARGET="_top"
>	       SUBJECT: RE:  File descriptor 5 is held open</A
>).
	       Best leave this particular fd alone.</P
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