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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 6] 6.8 Shell Variables </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="1998-10-23T15:38:06Z"><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="ch06_01.htm"TITLE="6. Shell and Environment Variables"><LINKREL="prev"HREF="ch06_07.htm"TITLE="6.7 What Time Is It in Japan? "><LINKREL="next"HREF="ch06_09.htm"TITLE="6.9 Special C Shell Variables "></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="ch06_07.htm"TITLE="6.7 What Time Is It in Japan? "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 6.7 What Time Is It in Japan? "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 6<BR>Shell and Environment Variables</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch06_09.htm"TITLE="6.9 Special C Shell Variables "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 6.9 Special C Shell Variables "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-1180">6.8 Shell Variables </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-6207"></A><ACLASS="indexterm"NAME="AUTOID-6210"></A>Shell variables are really just the "general case" of<SPANCLASS="link">environment variables (<ACLASS="linkend"HREF="ch06_01.htm#UPT-ART-1170"TITLE="What Environment Variables Are Good For ">6.1</A>)</SPAN>.If you're a programmer, remember that a UNIX shell really runsan interpretedprogramming language. Shell variables belong to the shell; you can set them,print them, and work with them much as you can in a C program (or aFORTRAN program or a BASIC program). If you're not a programmer,just remember that shell variables are pigeonholes that storeinformation for you or your shell to use.</P><PCLASS="para">If you've read the section on environment variables, you realize thatwe defined them in exactly the same way. How are shell variablesdifferent from environment variables? Whenever you start a new shellor a UNIX program, it inherits all of its parent's environmentvariables. However, it does <EMCLASS="emphasis">not</EM> inherit any shell variables;it starts with a clean slate. If you're a programmer, you can thinkof environment variables as "global" variables, while shell variablesare "local" variables. By convention, shell variables have lowercasenames.</P><PCLASS="para">Just as some programs use certain environment variables, the shell expectsto use certain shell variables.<ACLASS="indexterm"NAME="AUTOID-6218"></A>For example, the C shell usesthe<ACLASS="indexterm"NAME="AUTOID-6221"></A><SPANCLASS="link"><EMCLASS="emphasis">history</EM> (<ACLASS="linkend"HREF="ch11_01.htm#UPT-ART-4920"TITLE="The Lessons of History ">11.1</A>)</SPAN>variable to determine how many of your previous commandsto remember; if the<ACLASS="indexterm"NAME="AUTOID-6225"></A><SPANCLASS="link"><EMCLASS="emphasis">noclobber</EM> (<ACLASS="linkend"HREF="ch13_06.htm"TITLE="Safe I/O Redirection with noclobber ">13.6</A>)</SPAN>variable is defined, the C shellprevents you from damaging files by making mistakes with standardoutput. Most users insert code into their<SPANCLASS="link"><EMCLASS="emphasis">.cshrc</EM> files (<ACLASS="linkend"HREF="ch02_02.htm"TITLE="Shell Setup Files-Which, Where, and Why ">2.2</A>)</SPAN>to define theseimportant variables appropriately.</P><PCLASS="para">To set a shell variable, use one of these commands:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>set </B></CODE><CODECLASS="replaceable"><I>name</I></CODE><CODECLASS="userinput"><B>=</B></CODE><CODECLASS="replaceable"><I>value </I></CODE><EMCLASS="emphasis">C shell</EM>$ <CODECLASS="replaceable"><I>name</I></CODE><CODECLASS="userinput"><B>=</B></CODE><CODECLASS="replaceable"><I>value </I></CODE><EMCLASS="emphasis">Bourne shell</EM> </PRE></BLOCKQUOTE></P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-6244"></A>As a special case, if you omit <EMCLASS="emphasis">value</EM>, the shell variable is setto a "null" value. For example, the following commandsare valid:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>set </B></CODE><CODECLASS="replaceable"><I>name </I></CODE><EMCLASS="emphasis">C shell</EM>$ <CODECLASS="replaceable"><I>name</I></CODE><CODECLASS="userinput"><B>= </B></CODE><EMCLASS="emphasis">Bourne shell</EM> </PRE></BLOCKQUOTE></P><PCLASS="para">This is important:giving a variable a null value is not the same as deletingthe value.Some programs look at variables to see whether or not they exist; theydon't care what the actual value is, and an empty value is as good as anythingelse. If you want to make the shell forget that a variable everexisted, use the <EMCLASS="emphasis">unset</EM> command.Unfortunately, older Bourne shells don't have a command like <EMCLASS="emphasis">unset</EM>:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>unset </B></CODE><CODECLASS="replaceable"><I>name </I></CODE><EMCLASS="emphasis">C shell</EM>$ <CODECLASS="userinput"><B>unset </B></CODE><CODECLASS="replaceable"><I>name </I></CODE><EMCLASS="emphasis">Bourne shell</EM></PRE></BLOCKQUOTE></P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-6268"></A><ACLASS="indexterm"NAME="AUTOID-6270"></A><ACLASS="indexterm"NAME="AUTOID-6272"></A>If you want to list all of your environment variables, use thecommand <EMCLASS="emphasis">printenv</EM> (Berkeley UNIX) or <EMCLASS="emphasis">env</EM> (System V).[1]If you want to list all of your Bourne or C shell variables, just type<CODECLASS="literal">set</CODE>.Here's a typical report in the C shell:</P><BLOCKQUOTECLASS="footnote"><PCLASS="para">[1] <EMCLASS="emphasis">printenv</EM> and <EMCLASS="emphasis">env</EM> are<SPANCLASS="link">external (<ACLASS="linkend"HREF="ch01_10.htm"TITLE="Internal and External Commands ">1.10</A>)</SPAN>commands; they work with any shell.</P></BLOCKQUOTE><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>set</B></CODE>argv ()cwd /home/los/mikel/power/articleshistory 40home /home/los/mikelnoclobberpath (/home/los/mikel/bin /usr/local/bin /usr/ucb /bin /usr/bin .)prompt los%shell /bin/cshstatus 0term sunuser mikel</PRE></BLOCKQUOTE></P><PCLASS="para">If you want to print the value of an individual variable, give the command:<ACLASS="indexterm"NAME="AUTOID-6286"></A></P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>echo "$</B></CODE><CODECLASS="replaceable"><I>variable-name</I></CODE><CODECLASS="userinput"><B>"</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para">(While the example above gives a C shell prompt, this commandworks in all UNIX shells.)</P><PCLASS="para">Whenever you need the value of a shell variable - not just with<SPANCLASS="link"><EMCLASS="emphasis">echo</EM> (<ACLASS="linkend"HREF="ch08_06.htm"TITLE="Output Command-Line Arguments ">8.6</A>)</SPAN>-<ACLASS="indexterm"NAME="AUTOID-6297"></A>you need to put a dollar sign (<CODECLASS="literal">$</CODE>) in front of the name.You don't need the dollar sign when you're assigning a new value to a<ACLASS="indexterm"NAME="AUTOID-6301"></A>shell variable. You can also stick curly braces (<CODECLASS="literal">{}</CODE>) aroundthe name, if you want (e.g., <CODECLASS="literal">${</CODE><CODECLASS="replaceable"><I>name</I></CODE><CODECLASS="literal">}</CODE>); when you're writing shellprograms, this can often make your code much clearer.Curly braces aremostly used when you need to separate the variable name from what comes after it.</P><PCLASS="para">But that's getting us out of the range of interactive variable useand into<SPANCLASS="link">shell programming (<ACLASS="linkend"HREF="ch44_01.htm#UPT-ART-0065"TITLE="Everyone Should Learn Some Shell Programming ">44.1</A>)</SPAN>.</P><DIVCLASS="sect1info"><PCLASS="SECT1INFO">- <SPANCLASS="authorinitials">ML</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="ch06_07.htm"TITLE="6.7 What Time Is It in Japan? 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