io-redirection.html

Shall高级编程

>
        </P
><P
>For a more detailed introduction to I/O redirection see
	  <A
HREF="ioredirintro.html"
>Appendix E</A
>.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN16560"
></A
>19.1. 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 19-1. 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 19-2. 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 19-3. 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 19-4. 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.AEN16482"
HREF="io-redirection.html#AEN16482"
>[1]</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
		 <I
CLASS="FIRSTTERM"
>C</I
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN16494"
HREF="io-redirection.html#AEN16494"
>[2]</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://www.geocrawler.com/archives/3/342/1996/1/0/1939805/"
TARGET="_top"
>	       SUBJECT: RE:  File descriptor 5 is held open</A
>).
	       Best leave this particular fd alone.</P
></TD
></TR
></TABLE
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="here-docs.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="redircb.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Here Documents</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="part5.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Redirecting Code Blocks</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>