230 lines
3.2 KiB
HTML
230 lines
3.2 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Quotes and Special Characters</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Bash Prompt HOWTO"
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TITLE="Bash Programming and Shell Scripts"
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TITLE="Variables"
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HREF="x191.html"><LINK
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REL="NEXT"
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TITLE="Command Substitution"
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HREF="x219.html"></HEAD
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>Bash Prompt HOWTO: </TH
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>Prev</A
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><TD
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>Chapter 3. Bash Programming and Shell Scripts</TD
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><DIV
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><H1
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CLASS="SECT1"
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><A
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NAME="AEN206"
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></A
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>3.2. Quotes and Special Characters</H1
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><P
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>If you wish to include a special character in a variable, you will have to
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quote it differently:</P
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><TABLE
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BORDER="1"
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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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>> newvar=$testvar
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> echo $newvar
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5
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> newvar="$testvar"
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> echo $newvar
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5
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> newvar='$testvar'
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> echo $newvar
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$testvar
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> newvar=\$testvar
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> echo $newvar
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$testvar
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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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>The dollar sign isn't the only character that's special to the Bash shell,
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but it's a simple example. An interesting step we can take to make use of
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assigning a variable name to another variable name is to use
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<TT
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CLASS="USERINPUT"
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><B
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>eval</B
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></TT
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> to dereference the stored variable name: </P
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><TABLE
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BORDER="1"
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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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>> echo $testvar
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5
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> echo $newvar
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$testvar
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> eval echo $newvar
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5
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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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>Normally, the shell does only one round of substitutions on the expression
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it is evaluating: if you say <TT
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CLASS="USERINPUT"
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><B
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>echo $newvar</B
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></TT
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> the shell
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will only go so far as to determine that <TT
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CLASS="VARNAME"
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>$newvar</TT
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> is
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equal to the text string <TT
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CLASS="VARNAME"
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>$testvar</TT
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>, it won't evaluate
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what <TT
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CLASS="VARNAME"
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>$testvar</TT
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> is equal to. <TT
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CLASS="USERINPUT"
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><B
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>eval</B
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></TT
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>
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forces that evaluation. </P
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></DIV
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><DIV
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>Prev</A
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>Home</A
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>Next</A
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>Variables</TD
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><TD
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>Up</A
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><TD
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>Command Substitution</TD
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></TR
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> |