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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 14] 14.13 Which Directory Am I in, Really? </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="1998-08-04T21:37:12Z"><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="ch14_01.htm"TITLE="14. Moving Around in a Hurry"><LINKREL="prev"HREF="ch14_12.htm"TITLE="14.12 Marking Your Place with a Shell Variable "><LINKREL="next"HREF="ch14_14.htm"TITLE="14.14 Automatic Setup When You Enter/Exit a Directory "></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="ch14_12.htm"TITLE="14.12 Marking Your Place with a Shell Variable "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 14.12 Marking Your Place with a Shell Variable "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 14<BR>Moving Around in a Hurry</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch14_14.htm"TITLE="14.14 Automatic Setup When You Enter/Exit a Directory "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 14.14 Automatic Setup When You Enter/Exit a Directory "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-1430">14.13 Which Directory Am I in, Really? </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-15772"></A><ACLASS="indexterm"NAME="AUTOID-15774"></A>The C shell, and some other shells too, keep their own idea of what yourcurrent directory is.The <EMCLASS="emphasis">csh</EM> will give you the current directory's absolute pathname in <CODECLASS="literal">$cwd</CODE>;<EMCLASS="emphasis">bash</EM> uses <CODECLASS="literal">$PWD</CODE>.But sometimes this can give you the wrong pathname.</P><PCLASS="para">Why?<ACLASS="indexterm"NAME="AUTOID-15782"></A><ACLASS="indexterm"NAME="AUTOID-15784"></A><ACLASS="indexterm"NAME="AUTOID-15786"></A>Because the <EMCLASS="emphasis">cwd</EM> variable was added before many versions of UNIXhad <SPANCLASS="link">symlinks (<ACLASS="linkend"HREF="ch18_04.htm"TITLE="More About Links ">18.4</A>)</SPAN>(symbolic links).As article<ACLASS="xref"HREF="ch18_07.htm"TITLE="Linking Directories ">18.7</A>explains,symlinks can point to directories any place else on the filesystem oreven (for some UNIXes) directories on another computer.Poor <EMCLASS="emphasis">cwd</EM> couldn't cope: it assumed that the current directory was thename of the symlink itself (instead of the directory that the link points to).That led to problems like the one below: <EMCLASS="emphasis">cd</EM>ing to a "directory" named<EMCLASS="emphasis">wpa</EM> that's actually a symlink to <EMCLASS="emphasis">/work/pwrtools/articles</EM>.The value of <CODECLASS="literal">$cwd</CODE>, shown in the prompt, is wrong.The <EMCLASS="emphasis">/bin/pwd</EM> command<SPANCLASS="link">shows the real current directory (<ACLASS="linkend"HREF="ch14_04.htm"TITLE="How Does UNIX Find Your Current Directory? ">14.4</A>)</SPAN>(you shouldtype all of <CODECLASS="literal">/bin/pwd</CODE> because some shells and users have plain<CODECLASS="literal">pwd</CODE> aliased to do <CODECLASS="literal">echo</CODE> <CODECLASS="literal">$cwd</CODE>):</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">/home/jerry% <CODECLASS="userinput"><B>pwd</B></CODE>/home/jerry% <CODECLASS="userinput"><B>ls -l wpa</B></CODE>lrwxrwxrwx 1 jerry 23 Sep 8 13:55 wpa -> /work/pwrtools/articles/home/jerry% <CODECLASS="userinput"><B>cd wpa</B></CODE>/home/jerry/wpa% <CODECLASS="userinput"><B>/bin/pwd</B></CODE>/work/pwrtools/articles/home/jerry/wpa%</PRE></BLOCKQUOTE></P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-15810"></A><ACLASS="indexterm"NAME="AUTOID-15812"></A>By now, a lot of C shells have a variable named <EMCLASS="emphasis">hardpaths</EM>;the <EMCLASS="emphasis">bash</EM> variable is <EMCLASS="emphasis">nolinks</EM>.If you set the shell variable (usually in your<SPANCLASS="link">shell setup file (<ACLASS="linkend"HREF="ch02_02.htm"TITLE="Shell Setup Files-Which, Where, and Why ">2.2</A>)</SPAN>),the shell won't befooled by symlinks.Watch:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">/home/jerry/wpa% <CODECLASS="userinput"><B>cd</B></CODE>/home/jerry% <CODECLASS="userinput"><B>set hardpaths</B></CODE> <EMCLASS="emphasis">(on bash, </EM><CODECLASS="literal">nolinks=</CODE>1<EMCLASS="emphasis">)</EM>/home/jerry% <CODECLASS="userinput"><B>cd wpa</B></CODE>/work/pwrtools/articles%</PRE></BLOCKQUOTE></P><PCLASS="para">Setting <EMCLASS="emphasis">hardpaths</EM> or <EMCLASS="emphasis">nolinks</EM> makes the shell do extra work,so don't bother with it unless you use <CODECLASS="literal">$cwd</CODE>.</P><PCLASS="para">The<ACLASS="indexterm"NAME="AUTOID-15831"></A><SPANCLASS="link"><EMCLASS="emphasis">dirs</EM> (<ACLASS="linkend"HREF="ch14_06.htm"TITLE="The Shells' pushd and popd Commands ">14.6</A>)</SPAN>command has the same problem.Setting <EMCLASS="emphasis">hardpaths</EM> or <EMCLASS="emphasis">nolinks</EM> helps there, too.</P><PCLASS="para">If your system has symlinks but your shell doesn't recognize avariable like <EMCLASS="emphasis">hardpaths</EM>, here are workarounds for the <EMCLASS="emphasis">.cshrc</EM> file:<ACLASS="indexterm"NAME="AUTOID-15840"></A></P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">alias setprompt 'set prompt="${cwd}% "'alias cd 'chdir \!* && set cwd=`/bin/pwd` && setprompt'alias pushd 'pushd \!* && cd .'alias popd 'popd \!* && cd .'</PRE></BLOCKQUOTE></P><PCLASS="para">When you <EMCLASS="emphasis">cd</EM>, that alias resets the <EMCLASS="emphasis">cwd</EM> variable to theoutput of <EMCLASS="emphasis">/bin/pwd</EM>, then resets the prompt to the new <EMCLASS="emphasis">cwd</EM>.Using<SPANCLASS="link"><EMCLASS="emphasis">pushd</EM> or <EMCLASS="emphasis">popd</EM> (<ACLASS="linkend"HREF="ch14_06.htm"TITLE="The Shells' pushd and popd Commands ">14.6</A>)</SPAN>runs the <EMCLASS="emphasis">cd</EM> alias, too - this changesto the current directory (<CODECLASS="literal">.</CODE>), which fixes<EMCLASS="emphasis">cwd</EM> (as well as the <EMCLASS="emphasis">dirs</EM> command) and resets the prompt.</P><PCLASS="para">Whew.Are symlinks worth the work?(I think they are.)</P><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="ch14_12.htm"TITLE="14.12 Marking Your Place with a Shell Variable "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 14.12 Marking Your Place with a Shell Variable "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="ch14_14.htm"TITLE="14.14 Automatic Setup When You Enter/Exit a Directory "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 14.14 Automatic Setup When You Enter/Exit a Directory "BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172">14.12 Marking Your Place with a Shell Variable </TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><ACLASS="index"HREF="index/idx_0.htm"TITLE="Book Index"><IMGSRC="gifs/index.gif"SRC="gifs/index.gif"ALT="Book Index"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172">14.14 Automatic Setup When You Enter/Exit a Directory </TD></TR></TABLE><HRALIGN="LEFT"WIDTH="515"TITLE="footer"><IMGSRC="gifs/smnavbar.gif"SRC="gifs/smnavbar.gif"USEMAP="#map"BORDER="0"ALT="The UNIX CD Bookshelf Navigation"><MAPNAME="map"><AREASHAPE="RECT"COORDS="0,0,73,21"HREF="../index.htm"ALT="The UNIX CD Bookshelf"><AREASHAPE="RECT"COORDS="74,0,163,21"HREF="index.htm"ALT="UNIX Power Tools"><AREASHAPE="RECT"COORDS="164,0,257,21"HREF="../unixnut/index.htm"ALT="UNIX in a Nutshell"><AREASHAPE="RECT"COORDS="258,0,321,21"HREF="../vi/index.htm"ALT="Learning the vi Editor"><AREASHAPE="RECT"COORDS="322,0,378,21"HREF="../sedawk/index.htm"ALT="sed & awk"><AREASHAPE="RECT"COORDS="379,0,438,21"HREF="../ksh/index.htm"ALT="Learning the Korn Shell"><AREASHAPE="RECT"COORDS="439,0,514,21"HREF="../lrnunix/index.htm"ALT="Learning the UNIX Operating System"></MAP></DIV></BODY></HTML>⌨️ 快捷键说明
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