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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 45] 45.9 The Unappreciated Bourne Shell ":" Operator</TITLE><METANAME="DC.title"CONTENT="UNIX Power Tools"><METANAME="DC.creator"CONTENT="Jerry Peek, Tim O'Reilly & Mike Loukides"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1998-08-04T21:54:30Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-260-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch45_01.htm"TITLE="45. Shell Programming for the Initiated"><LINKREL="prev"HREF="ch45_08.htm"TITLE="45.8 Handling Signals to Child Processes "><LINKREL="next"HREF="ch45_10.htm"TITLE="45.10 Removing a File Once It's Opened - for Security and Easy Cleanup"></HEAD><BODYBGCOLOR="#FFFFFF"TEXT="#000000"><DIVCLASS="htmlnav"><H1><IMGSRC="gifs/smbanner.gif"ALT="UNIX Power Tools"USEMAP="#srchmap"BORDER="0"></H1><MAPNAME="srchmap"><AREASHAPE="RECT"COORDS="0,0,466,58"HREF="index.htm"ALT="UNIX Power Tools"><AREASHAPE="RECT"COORDS="467,0,514,18"HREF="jobjects/fsearch.htm"ALT="Search this book"></MAP><TABLEWIDTH="515"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch45_08.htm"TITLE="45.8 Handling Signals to Child Processes "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 45.8 Handling Signals to Child Processes "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 45<BR>Shell Programming for the Initiated</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch45_10.htm"TITLE="45.10 Removing a File Once It's Opened - for Security and Easy Cleanup"><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 45.10 Removing a File Once It's Opened - for Security and Easy Cleanup"BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-0200">45.9 The Unappreciated Bourne Shell ":" Operator</A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-52719"></A>Some people think that the Bourne shell's <CODECLASS="literal">:</CODE> is a comment character.It isn't, really.It evaluates its arguments and returns a zero<SPANCLASS="link">exit status (<ACLASS="linkend"HREF="ch44_07.htm"TITLE="Exit Status of UNIX Processes ">44.7</A>)</SPAN>.Here are a few places to use it:</P><ULCLASS="itemizedlist"><LICLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-52727"></A>Replace the UNIX <EMCLASS="emphasis">true</EM> command to make an endless<SPANCLASS="link"><EMCLASS="emphasis">while</EM> loop (<ACLASS="linkend"HREF="ch44_10.htm"TITLE="Loops That Test Exit Status ">44.10</A>)</SPAN>.This is more efficient because the shell doesn't have to start a newprocess each time around the loop(as it does when you use <CODECLASS="literal">while true</CODE>):</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">while :do <CODECLASS="replaceable"><I>commands</I></CODE>done</PRE></BLOCKQUOTE></P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-52738"></A><ACLASS="indexterm"NAME="AUTOID-52740"></A><ACLASS="indexterm"NAME="AUTOID-52743"></A>(Of course, one of the <CODECLASS="replaceable"><I>commands</I></CODE> will probably be <EMCLASS="emphasis">break</EM>,to end the loop eventually.)</P></LI><LICLASS="listitem"><PCLASS="para">When you want to use the <EMCLASS="emphasis">else</EM> in an<SPANCLASS="link"><EMCLASS="emphasis">if</EM> (<ACLASS="linkend"HREF="ch44_08.htm"TITLE="Test Exit Status with the if Statement ">44.8</A>)</SPAN>,but leave the<EMCLASS="emphasis">then</EM> empty,the <CODECLASS="literal">:</CODE> makes a nice "do-nothing" place filler:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">if <CODECLASS="replaceable"><I>something</I></CODE>then :else <CODECLASS="replaceable"><I>commands</I></CODE>fi</PRE></BLOCKQUOTE></P><PCLASS="para"></P></LI><LICLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-52762"></A><ACLASS="indexterm"NAME="AUTOID-52765"></A><ACLASS="indexterm"NAME="AUTOID-52768"></A><ACLASS="indexterm"NAME="AUTOID-52771"></A><ACLASS="indexterm"NAME="AUTOID-52774"></A><ACLASS="indexterm"NAME="AUTOID-52777"></A><ACLASS="indexterm"NAME="AUTOID-52780"></A>If your Bourne shell doesn't have a true <CODECLASS="literal">#</CODE> comment character,you can use <CODECLASS="literal">:</CODE> to "fake it."It's safest to use quotes so the shell won't try to interpretcharacters like <CODECLASS="literal">></CODE> or <CODECLASS="literal">|</CODE> in your "comment":</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">: 'read answer and branch if < 3 or > 6'</PRE></BLOCKQUOTE></P><PCLASS="para"></P></LI><LICLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-52792"></A><ACLASS="indexterm"NAME="AUTOID-52795"></A>Finally, it's useful with<SPANCLASS="link">parameter substitution (<ACLASS="linkend"HREF="ch45_12.htm"TITLE="Parameter Substitution ">45.12</A>)</SPAN>like<CODECLASS="literal">${</CODE><CODECLASS="replaceable"><I>var</I></CODE><CODECLASS="literal">?}</CODE>or <CODECLASS="literal">${</CODE><CODECLASS="replaceable"><I>var</I></CODE><CODECLASS="literal">=</CODE><CODECLASS="replaceable"><I>default</I></CODE><CODECLASS="literal">}</CODE>.For instance, using this line in your script will print an error and exitif either the <EMCLASS="emphasis">USER</EM> or <EMCLASS="emphasis">HOME</EM> variables aren't set:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">: ${USER?} ${HOME?}</PRE></BLOCKQUOTE></P></LI></UL><DIVCLASS="sect1info"><PCLASS="SECT1INFO">- <SPANCLASS="authorinitials">JP</SPAN></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="515"TITLE="footer"><TABLEWIDTH="515"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch45_08.htm"TITLE="45.8 Handling Signals to Child Processes "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 45.8 Handling Signals to Child Processes "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><ACLASS="book"HREF="index.htm"TITLE="UNIX Power Tools"><IMGSRC="gifs/txthome.gif"SRC="gifs/txthome.gif"ALT="UNIX Power Tools"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch45_10.htm"TITLE="45.10 Removing a File Once It's Opened - 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