File: redircb.html

package info (click to toggle)
abs-guide 10-4
links: PTS, VCS
area: main
in suites: bookworm, bullseye, forky, sid, trixie
size: 6,952 kB
sloc: sh: 14,129; makefile: 81
file content (658 lines) | stat: -rw-r--r-- 13,734 bytes
parent folder | download | duplicates (4)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML
><HEAD
><TITLE
>Redirecting Code Blocks</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
"><LINK
REL="HOME"
TITLE="Advanced Bash-Scripting Guide"
HREF="index.html"><LINK
REL="UP"
TITLE="I/O Redirection"
HREF="io-redirection.html"><LINK
REL="PREVIOUS"
TITLE="I/O Redirection"
HREF="io-redirection.html"><LINK
REL="NEXT"
TITLE="Applications"
HREF="redirapps.html"><META
HTTP-EQUIV="Content-Style-Type"
CONTENT="text/css"><LINK
REL="stylesheet"
HREF="common/kde-common.css"
TYPE="text/css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=iso-8859-1"><META
HTTP-EQUIV="Content-Language"
CONTENT="en"><LINK
REL="stylesheet"
HREF="common/kde-localised.css"
TYPE="text/css"
TITLE="KDE-English"><LINK
REL="stylesheet"
HREF="common/kde-default.css"
TYPE="text/css"
TITLE="KDE-Default"></HEAD
><BODY
CLASS="SECT1"
BGCOLOR="#FFFFFF"
TEXT="#000000"
LINK="#AA0000"
VLINK="#AA0055"
ALINK="#AA0000"
STYLE="font-family: sans-serif;"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="io-redirection.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
>Chapter 20. I/O Redirection</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="redirapps.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="REDIRCB"
></A
>20.2. Redirecting Code Blocks</H1
><P
><A
NAME="REDIRREF"
></A
>Blocks of code, such as <A
HREF="loops.html#WHILELOOPREF"
>while</A
>, <A
HREF="loops.html#UNTILLOOPREF"
>until</A
>, and <A
HREF="loops.html#FORLOOPREF1"
>for</A
> loops, even <A
HREF="tests.html#IFTHEN"
>if/then</A
> test blocks can also incorporate
	  redirection of <TT
CLASS="FILENAME"
>stdin</TT
>.  Even a function may
	  use this form of redirection (see <A
HREF="functions.html#REALNAME"
>Example 24-11</A
>).
	  The <SPAN
CLASS="TOKEN"
>&#60;</SPAN
> operator at the end of the code block
	  accomplishes this.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REDIR2"
></A
><P
><B
>Example 20-5. Redirected <I
CLASS="FIRSTTERM"
>while</I
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# redir2.sh
   3&nbsp;
   4&nbsp;if [ -z "$1" ]
   5&nbsp;then
   6&nbsp;  Filename=names.data       # Default, if no filename specified.
   7&nbsp;else
   8&nbsp;  Filename=$1
   9&nbsp;fi  
  10&nbsp;#+ Filename=${1:-names.data}
  11&nbsp;#  can replace the above test (parameter substitution).
  12&nbsp;
  13&nbsp;count=0
  14&nbsp;
  15&nbsp;echo
  16&nbsp;
  17&nbsp;while [ "$name" != Smith ]  # Why is variable $name in quotes?
  18&nbsp;do
  19&nbsp;  read name                 # Reads from $Filename, rather than stdin.
  20&nbsp;  echo $name
  21&nbsp;  let "count += 1"
  22&nbsp;done &#60;"$Filename"           # Redirects stdin to file $Filename. 
  23&nbsp;#    ^^^^^^^^^^^^
  24&nbsp;
  25&nbsp;echo; echo "$count names read"; echo
  26&nbsp;
  27&nbsp;exit 0
  28&nbsp;
  29&nbsp;#  Note that in some older shell scripting languages,
  30&nbsp;#+ the redirected loop would run as a subshell.
  31&nbsp;#  Therefore, $count would return 0, the initialized value outside the loop.
  32&nbsp;#  Bash and ksh avoid starting a subshell *whenever possible*,
  33&nbsp;#+ so that this script, for example, runs correctly.
  34&nbsp;#  (Thanks to Heiner Steven for pointing this out.)
  35&nbsp;
  36&nbsp;#  However . . .
  37&nbsp;#  Bash *can* sometimes start a subshell in a PIPED "while-read" loop,
  38&nbsp;#+ as distinct from a REDIRECTED "while" loop.
  39&nbsp;
  40&nbsp;abc=hi
  41&nbsp;echo -e "1\n2\n3" | while read l
  42&nbsp;     do abc="$l"
  43&nbsp;        echo $abc
  44&nbsp;     done
  45&nbsp;echo $abc
  46&nbsp;
  47&nbsp;#  Thanks, Bruno de Oliveira Schneider, for demonstrating this
  48&nbsp;#+ with the above snippet of code.
  49&nbsp;#  And, thanks, Brian Onn, for correcting an annotation error.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REDIR2A"
></A
><P
><B
>Example 20-6. Alternate form of redirected <I
CLASS="FIRSTTERM"
>while</I
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# This is an alternate form of the preceding script.
   4&nbsp;
   5&nbsp;#  Suggested by Heiner Steven
   6&nbsp;#+ as a workaround in those situations when a redirect loop
   7&nbsp;#+ runs as a subshell, and therefore variables inside the loop
   8&nbsp;# +do not keep their values upon loop termination.
   9&nbsp;
  10&nbsp;
  11&nbsp;if [ -z "$1" ]
  12&nbsp;then
  13&nbsp;  Filename=names.data     # Default, if no filename specified.
  14&nbsp;else
  15&nbsp;  Filename=$1
  16&nbsp;fi  
  17&nbsp;
  18&nbsp;
  19&nbsp;exec 3&#60;&#38;0                 # Save stdin to file descriptor 3.
  20&nbsp;exec 0&#60;"$Filename"        # Redirect standard input.
  21&nbsp;
  22&nbsp;count=0
  23&nbsp;echo
  24&nbsp;
  25&nbsp;
  26&nbsp;while [ "$name" != Smith ]
  27&nbsp;do
  28&nbsp;  read name               # Reads from redirected stdin ($Filename).
  29&nbsp;  echo $name
  30&nbsp;  let "count += 1"
  31&nbsp;done                      #  Loop reads from file $Filename
  32&nbsp;                          #+ because of line 20.
  33&nbsp;
  34&nbsp;#  The original version of this script terminated the "while" loop with
  35&nbsp;#+      done &#60;"$Filename" 
  36&nbsp;#  Exercise:
  37&nbsp;#  Why is this unnecessary?
  38&nbsp;
  39&nbsp;
  40&nbsp;exec 0&#60;&#38;3                 # Restore old stdin.
  41&nbsp;exec 3&#60;&#38;-                 # Close temporary fd 3.
  42&nbsp;
  43&nbsp;echo; echo "$count names read"; echo
  44&nbsp;
  45&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REDIR3"
></A
><P
><B
>Example 20-7. Redirected <I
CLASS="FIRSTTERM"
>until</I
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Same as previous example, but with "until" loop.
   3&nbsp;
   4&nbsp;if [ -z "$1" ]
   5&nbsp;then
   6&nbsp;  Filename=names.data         # Default, if no filename specified.
   7&nbsp;else
   8&nbsp;  Filename=$1
   9&nbsp;fi  
  10&nbsp;
  11&nbsp;# while [ "$name" != Smith ]
  12&nbsp;until [ "$name" = Smith ]     # Change  !=  to =.
  13&nbsp;do
  14&nbsp;  read name                   # Reads from $Filename, rather than stdin.
  15&nbsp;  echo $name
  16&nbsp;done &#60;"$Filename"             # Redirects stdin to file $Filename. 
  17&nbsp;#    ^^^^^^^^^^^^
  18&nbsp;
  19&nbsp;# Same results as with "while" loop in previous example.
  20&nbsp;
  21&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REDIR4"
></A
><P
><B
>Example 20-8. Redirected <I
CLASS="FIRSTTERM"
>for</I
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;if [ -z "$1" ]
   4&nbsp;then
   5&nbsp;  Filename=names.data          # Default, if no filename specified.
   6&nbsp;else
   7&nbsp;  Filename=$1
   8&nbsp;fi  
   9&nbsp;
  10&nbsp;line_count=`wc $Filename | awk '{ print $1 }'`
  11&nbsp;#           Number of lines in target file.
  12&nbsp;#
  13&nbsp;#  Very contrived and kludgy, nevertheless shows that
  14&nbsp;#+ it's possible to redirect stdin within a "for" loop...
  15&nbsp;#+ if you're clever enough.
  16&nbsp;#
  17&nbsp;# More concise is     line_count=$(wc -l &#60; "$Filename")
  18&nbsp;
  19&nbsp;
  20&nbsp;for name in `seq $line_count`  # Recall that "seq" prints sequence of numbers.
  21&nbsp;# while [ "$name" != Smith ]   --   more complicated than a "while" loop   --
  22&nbsp;do
  23&nbsp;  read name                    # Reads from $Filename, rather than stdin.
  24&nbsp;  echo $name
  25&nbsp;  if [ "$name" = Smith ]       # Need all this extra baggage here.
  26&nbsp;  then
  27&nbsp;    break
  28&nbsp;  fi  
  29&nbsp;done &#60;"$Filename"              # Redirects stdin to file $Filename. 
  30&nbsp;#    ^^^^^^^^^^^^
  31&nbsp;
  32&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>We can modify the previous example to also redirect the output of
        the loop.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REDIR4A"
></A
><P
><B
>Example 20-9. Redirected <I
CLASS="FIRSTTERM"
>for</I
> loop (both
	  <TT
CLASS="FILENAME"
>stdin</TT
> and <TT
CLASS="FILENAME"
>stdout</TT
>
	  redirected)</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;if [ -z "$1" ]
   4&nbsp;then
   5&nbsp;  Filename=names.data          # Default, if no filename specified.
   6&nbsp;else
   7&nbsp;  Filename=$1
   8&nbsp;fi  
   9&nbsp;
  10&nbsp;Savefile=$Filename.new         # Filename to save results in.
  11&nbsp;FinalName=Jonah                # Name to terminate "read" on.
  12&nbsp;
  13&nbsp;line_count=`wc $Filename | awk '{ print $1 }'`  # Number of lines in target file.
  14&nbsp;
  15&nbsp;
  16&nbsp;for name in `seq $line_count`
  17&nbsp;do
  18&nbsp;  read name
  19&nbsp;  echo "$name"
  20&nbsp;  if [ "$name" = "$FinalName" ]
  21&nbsp;  then
  22&nbsp;    break
  23&nbsp;  fi  
  24&nbsp;done &#60; "$Filename" &#62; "$Savefile"     # Redirects stdin to file $Filename,
  25&nbsp;#    ^^^^^^^^^^^^^^^^^^^^^^^^^^^       and saves it to backup file.
  26&nbsp;
  27&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REDIR5"
></A
><P
><B
>Example 20-10. Redirected <I
CLASS="FIRSTTERM"
>if/then</I
> test</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;if [ -z "$1" ]
   4&nbsp;then
   5&nbsp;  Filename=names.data   # Default, if no filename specified.
   6&nbsp;else
   7&nbsp;  Filename=$1
   8&nbsp;fi  
   9&nbsp;
  10&nbsp;TRUE=1
  11&nbsp;
  12&nbsp;if [ "$TRUE" ]          # if true    and   if :   also work.
  13&nbsp;then
  14&nbsp; read name
  15&nbsp; echo $name
  16&nbsp;fi &#60;"$Filename"
  17&nbsp;#  ^^^^^^^^^^^^
  18&nbsp;
  19&nbsp;# Reads only first line of file.
  20&nbsp;# An "if/then" test has no way of iterating unless embedded in a loop.
  21&nbsp;
  22&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="NAMESDATA"
></A
><P
><B
>Example 20-11. Data file <I
CLASS="FIRSTTERM"
>names.data</I
> for above
	examples</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;Aristotle
   2&nbsp;Arrhenius
   3&nbsp;Belisarius
   4&nbsp;Capablanca
   5&nbsp;Dickens
   6&nbsp;Euler
   7&nbsp;Goethe
   8&nbsp;Hegel
   9&nbsp;Jonah
  10&nbsp;Laplace
  11&nbsp;Maroczy
  12&nbsp;Purcell
  13&nbsp;Schmidt
  14&nbsp;Schopenhauer
  15&nbsp;Semmelweiss
  16&nbsp;Smith
  17&nbsp;Steinmetz
  18&nbsp;Tukhashevsky
  19&nbsp;Turing
  20&nbsp;Venn
  21&nbsp;Warshawski
  22&nbsp;Znosko-Borowski
  23&nbsp;
  24&nbsp;#  This is a data file for
  25&nbsp;#+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh".</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Redirecting the <TT
CLASS="FILENAME"
>stdout</TT
> of a code
	block has the effect of saving its output to a file. See <A
HREF="special-chars.html#RPMCHECK"
>Example 3-2</A
>.</P
><P
><A
HREF="here-docs.html#HEREDOCREF"
>Here documents</A
>
        are a special case of redirected code blocks. That being the case,
	it should be possible to feed the output of a <I
CLASS="FIRSTTERM"
>here
	document</I
> into the <TT
CLASS="FILENAME"
>stdin</TT
> for a
	<I
CLASS="FIRSTTERM"
>while loop</I
>.</P
><P
>	    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# This example by Albert Siersema
   2&nbsp;# Used with permission (thanks!).
   3&nbsp;
   4&nbsp;function doesOutput()
   5&nbsp; # Could be an external command too, of course.
   6&nbsp; # Here we show you can use a function as well.
   7&nbsp;{
   8&nbsp;  ls -al *.jpg | awk '{print $5,$9}'
   9&nbsp;}
  10&nbsp;
  11&nbsp;
  12&nbsp;nr=0          #  We want the while loop to be able to manipulate these and
  13&nbsp;totalSize=0   #+ to be able to see the changes after the 'while' finished.
  14&nbsp;
  15&nbsp;while read fileSize fileName ; do
  16&nbsp;  echo "$fileName is $fileSize bytes"
  17&nbsp;  let nr++
  18&nbsp;  totalSize=$((totalSize+fileSize))   # Or: "let totalSize+=fileSize"
  19&nbsp;done&#60;&#60;EOF
  20&nbsp;$(doesOutput)
  21&nbsp;EOF
  22&nbsp;
  23&nbsp;echo "$nr files totaling $totalSize bytes"</PRE
></TD
></TR
></TABLE
>
            </P
></DIV
><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="io-redirection.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="redirapps.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>I/O Redirection</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="io-redirection.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Applications</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>