514 lines
8.0 KiB
HTML
514 lines
8.0 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>The for loop</TITLE
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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TITLE="The while loop"
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HREF="sect_09_02.html"></HEAD
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>Bash Guide for Beginners</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="chap_09.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 9. Repetitive tasks</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="sect_09_02.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="sect_09_01"
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></A
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>9.1. The for loop</H1
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_09_01_01"
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></A
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>9.1.1. How does it work?</H2
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><P
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>The <B
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CLASS="command"
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>for</B
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> loop is the first of the three shell looping constructs. This loop allows for specification of a list of values. A list of commands is executed for each value in the list.</P
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><P
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>The syntax for this loop is:</P
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><P
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><B
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CLASS="command"
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>for <TT
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CLASS="varname"
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>NAME</TT
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> [in LIST ]; do COMMANDS; done</B
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> </P
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><P
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>If <B
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CLASS="command"
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>[in LIST]</B
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> is not present, it is replaced with <B
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CLASS="command"
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>in <TT
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CLASS="varname"
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>$@</TT
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></B
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> and <B
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CLASS="command"
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>for</B
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> executes the <B
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CLASS="command"
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>COMMANDS</B
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> once for each positional parameter that is set (see <A
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HREF="sect_03_02.html#sect_03_02_05"
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>Section 3.2.5</A
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> and <A
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HREF="sect_07_02.html#sect_07_02_01_02"
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>Section 7.2.1.2</A
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>).</P
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><P
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>The return status is the exit status of the last command that executes. If no commands are executed because <TT
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CLASS="varname"
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>LIST</TT
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> does not expand to any items, the return status is zero.</P
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><P
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><TT
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CLASS="varname"
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>NAME</TT
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> can be any variable name, although <TT
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CLASS="varname"
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>i</TT
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> is used very often. <TT
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CLASS="varname"
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>LIST</TT
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> can be any list of words, strings or numbers, which can be literal or generated by any command. The <B
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CLASS="command"
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>COMMANDS</B
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> to execute can also be any operating system commands, script, program or shell statement. The first time through the loop, <TT
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CLASS="varname"
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>NAME</TT
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> is set to the first item in <TT
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CLASS="varname"
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>LIST</TT
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>. The second time, its value is set to the second item in the list, and so on. The loop terminates when <TT
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CLASS="varname"
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>NAME</TT
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> has taken on each of the values from <TT
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CLASS="varname"
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>LIST</TT
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> and no items are left in <TT
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CLASS="varname"
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>LIST</TT
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>.</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_09_01_02"
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></A
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>9.1.2. Examples</H2
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><DIV
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CLASS="sect3"
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><H3
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CLASS="sect3"
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><A
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NAME="sect_09_01_02_03"
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></A
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>9.1.2.1. Using command substitution for specifying LIST items</H3
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><P
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>The first is a command line example, demonstrating the use of a <B
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CLASS="command"
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>for</B
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> loop that makes a backup copy of each <TT
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CLASS="filename"
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>.xml</TT
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> file. After issuing the command, it is safe to start working on your sources:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> <TT
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CLASS="prompt"
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>[carol@octarine ~/articles]</TT
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> <B
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CLASS="command"
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>ls <TT
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CLASS="filename"
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>*.xml</TT
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></B
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>
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file1.xml file2.xml file3.xml
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<TT
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CLASS="prompt"
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>[carol@octarine ~/articles]</TT
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> <B
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CLASS="command"
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>ls <TT
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CLASS="filename"
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>*.xml</TT
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> > <TT
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CLASS="filename"
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>list</TT
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></B
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>
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<TT
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CLASS="prompt"
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>[carol@octarine ~/articles]</TT
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> <B
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CLASS="command"
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>for <TT
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CLASS="varname"
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>i</TT
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> in <TT
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CLASS="parameter"
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><I
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>`cat list`</I
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></TT
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>; do cp <TT
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CLASS="filename"
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>"$i" "$i".bak</TT
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> ; done</B
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>
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<TT
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CLASS="prompt"
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>[carol@octarine ~/articles]</TT
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> <B
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CLASS="command"
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>ls <TT
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CLASS="filename"
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>*.xml*</TT
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></B
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>
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file1.xml file1.xml.bak file2.xml file2.xml.bak file3.xml file3.xml.bak
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>This one lists the files in <TT
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CLASS="filename"
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>/sbin</TT
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> that are just plain text files, and possibly scripts:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> <B
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CLASS="command"
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>for <TT
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CLASS="varname"
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>i</TT
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> in <TT
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CLASS="parameter"
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><I
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>`ls /sbin`</I
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></TT
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>; do file <TT
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CLASS="filename"
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>/sbin/$i</TT
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> | grep <TT
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CLASS="parameter"
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><I
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>ASCII</I
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></TT
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>; done</B
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>
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="sect3"
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><H3
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CLASS="sect3"
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><A
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NAME="sect_09_01_02_02"
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></A
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>9.1.2.2. Using the content of a variable to specify LIST items</H3
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><P
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>The following is a specific application script for converting HTML files, compliant with a certain scheme, to PHP files. The conversion is done by taking out the first 25 and the last 21 lines, replacing these with two PHP tags that provide header and footer lines:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> <TT
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CLASS="prompt"
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>[carol@octarine ~/html]</TT
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> <B
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CLASS="command"
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>cat <TT
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CLASS="filename"
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>html2php.sh</TT
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></B
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>
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#!/bin/bash
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# specific conversion script for my html files to php
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LIST="$(ls *.html)"
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for i in "$LIST"; do
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NEWNAME=$(ls "$i" | sed -e 's/html/php/')
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cat beginfile > "$NEWNAME"
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cat "$i" | sed -e '1,25d' | tac | sed -e '1,21d'| tac >> "$NEWNAME"
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cat endfile >> "$NEWNAME"
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done
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>Since we don't do a line count here, there is no way of knowing the line number from which to start deleting lines until reaching the end. The problem is solved using <B
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CLASS="command"
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>tac</B
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>, which reverses the lines in a file.</P
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><DIV
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CLASS="tip"
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><P
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></P
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><TABLE
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CLASS="tip"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/tip.gif"
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HSPACE="5"
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ALT="Tip"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>The basename command</B
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></TH
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></TR
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><TR
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><TD
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> </TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>Instead of using <B
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CLASS="command"
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>sed</B
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> to replace the <TT
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CLASS="filename"
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>html</TT
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> suffix with <TT
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CLASS="filename"
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>php</TT
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>, it would be cleaner to use the <B
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CLASS="command"
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>basename</B
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> command. Read the man page for more info.</P
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="warning"
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><P
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></P
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><TABLE
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CLASS="warning"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/warning.gif"
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HSPACE="5"
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ALT="Warning"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>Odd characters</B
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></TH
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></TR
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><TR
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><TD
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> </TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>You will run into problems if the list expands to file names containing spaces and other irregular characters. A more ideal construct to obtain the list would be to use the shell's globbing feature, like this:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> for i in $PATHNAME/*; do
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commands
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done
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></TD
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></TR
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></TABLE
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></DIV
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></DIV
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></DIV
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="chap_09.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="sect_09_02.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Repetitive tasks</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="chap_09.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>The while loop</TD
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