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<article>
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<title>BASH Programming - Introduction HOW-TO</title>
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<author>by Mike G <tt/mikkey@dynamo.com.ar/</author>
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<date>v0.04, 31 January 2000</date>
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<date>v0.05, 20 June 2000</date>
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<abstract>
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This article intends to help you to start programming
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basic-intermediate shell scripts. It does not intend to be an
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advanced document (see the title). I am NOT an expert nor guru
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shell programmer. I decided to write this because I'll learn a
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lot and it might be useful to other people.
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lot and it might be useful to other people. Any feedback will be apreciated,
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specially in the patch form :)
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</abstract>
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<!-- Table of contents -->
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@ -24,23 +25,18 @@
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<!-- Requisites -->
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<sect>
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Introduction
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<!--
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<sect1>
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Getting the latest version
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<P> Download latest version
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<htmlurl url="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/BASH-PROG-INTRO-HOWTO.tgz"
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name="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/BASH-PROG-INTRO-HOWTO.tgz">
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<P> Get sgml source
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<htmlurl url="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/BASH-PROG-INTRO-HOWTO.sgml"
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name="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/BASH-PROG-INTRO-HOWTO.sgml">
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<P> View it online
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<htmlurl url="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/html/BASH-PROG-INTRO-HOWTO.html"
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name="http://www.digimedia.com.ar/~mike/contrib/BASH-INTRO-PROG-HOW-TO/html/BASH-PROG-INTRO-HOWTO.html">
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<htmlurl url=""
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name="">
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</sect1>
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-->
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<sect1>
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Requisites
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<P> Familiarity with GNU/Linux command lines, and familiarity
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@ -109,6 +105,156 @@ Very simple Scripts
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</sect1>
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</sect>
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<!-- Redirection -->
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<sect>
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All about redirection
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<sect1>
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Theory and quick reference
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<p> There are 3 file descriptors, stdin, stdout and stderr (std=standar)
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<p>Basically you can:
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<enum>
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<item> redirect stdout to a file
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<item> redirect stderr to a file
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<item> redirect stdout to a stderr
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<item> redirect stderr to a stdout
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<item> redirect stderr and stdout to a file
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<item> redirect stderr and stdout to stdout
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<item> redirect stderr and stdout to stderr
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</enum>
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1 'represents' stdout and 2 stderr
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<p> A little note for seeing this things: with the less command you can view both stdout
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(which will remain on the buffer) and the stderr that will be printed on the screen, but erased as
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you try to 'browse' the buffer.
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</sect1>
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<sect1>
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Sample: stdout 2 file
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<p> This will cause the ouput of a program to be writen to a file
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<tscreen><verb>
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ls -l > ls-l.txt
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</verb></tscreen>
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Here, a file called 'ls-l.txt' will be created and it will contain what you would see on the
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screen if you type the command 'ls -l' and execute it.
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</sect1>
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<sect1>
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Sample: stderr 2 file
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<p> This will cause the stderr ouput of a program to be writen to a file
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<tscreen><verb>
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grep da * 2> grep-errors.txt
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</verb></tscreen>
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Here, a file called 'grep-errors.txt' will be created and it will contain what you would see
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the stderr portion of the output of the 'grep da *' command.
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</sect1>
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<sect1>
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Sample: stdout 2 stderr
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<p> This will cause the stderr ouput of a program to be writen to the same filedescriptor
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than stdout.
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<tscreen><verb>
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grep da * 1>&2
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</verb></tscreen>
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Here, the stdout portion of the command is sent to stderr, you may notice that in differen ways.
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</sect1>
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<sect1>
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Sample: stderr 2 stdout
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<p> This will cause the stderr ouput of a program to be writen to the same filedescriptor
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than stdout.
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<tscreen><verb>
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grep * 2>&1
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</verb></tscreen>
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Here, the stderr portion of the command is sent to stdout, if you pipe to less, you'll see that
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lines that normally 'dissapear' (as they are written to stderr) are being kept now (because
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they're on stdout).
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</sect1>
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<sect1>
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Sample: stderr and stdout 2 file
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<p> This will place every output of a program to a file. This is suitable sometimes
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for cron entries, if you want a command to pass in absolute silence.
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<tscreen><verb>
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rm -f $(find / -name core) &> /dev/null
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</verb></tscreen>
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This (thinking on the cron entry) will delete every file called 'core' in any directory. Notice
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that you should be pretty sure of what a command is doing if you are going to wipe it's output.
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</sect1>
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<!--
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redirect stderr and stdout to stdout
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redirect stderr and stdout to stderr
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-->
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<!--
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<sect1>
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Sample:
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<p> This will
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<tscreen><verb>
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command
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</verb></tscreen>
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Explanation
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</sect1>
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-->
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</sect>
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<!-- pipes -->
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<sect>
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Pipes
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<p> This section explains in a very simple and practical way how to use pipes,
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nd why you may want it.
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<sect1>
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What they are and why you'll want to use them
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<p> Pipes let you use (very simple, I insist) the output of a program as the
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input of another one
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</sect1>
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<sect1>
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Sample: simple pipe with sed
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<p> This is very simple way to use pipes.
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<tscreen><verb>
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ls -l | sed -e "s/[aeio]/u/g"
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</verb></tscreen>
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Here, the following happens: first the command ls -l is executed, and it's output,
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instead of being printed, is sent (piped) to the sed program, which in turn, prints
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what it has to.
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</sect1>
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<sect1>
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Sample: an alternative to ls -l *.txt
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<p> Probably, this is a more difficult way to do ls -l *.txt, but it is here for illustrating pipes,
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not for solving such listing dilema.
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<tscreen><verb>
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ls -l | grep "\.txt$"
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</verb></tscreen>
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Here, the output of the program ls -l is sent to the grep program, which, in turn, will print
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lines which match the regex "\.txt$".
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</sect1>
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<!--
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<sect1>
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Sample:
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<p> This will
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<tscreen><verb>
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command
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</verb></tscreen>
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Explanation
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</sect1>
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-->
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</sect>
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<!-- Variables -->
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<sect>
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Variables
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@ -683,11 +829,20 @@ About the document
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<item> Felix Hudson for writing the <it/renna/ script
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</itemize>
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</sect1>
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<sect1>
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History
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<p> v5 Added the redirection section
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<p> v4 disapperd from its location due to my ex-boss and thid doc found it's new place
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at the proper url: www.linuxdoc.org
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<p> prior: I don't rememeber and I didn't use rcs nor cvs :(
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</sect1>
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<sect1>
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More resources
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<P>
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<P> Introduction to bash (under BE)
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<htmlurl http://org.laol.net/lamug/beforever/bashtut.htm
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<htmlurl url="http://org.laol.net/lamug/beforever/bashtut.htm"
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name="http://org.laol.net/lamug/beforever/bashtut.htm">
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<P> Bourne Shell Programming
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http://207.213.123.70/book/
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</sect1>
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