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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 13] 13.13 The "Filename" - </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="1998-08-04T21:36:51Z"><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="ch13_01.htm"TITLE="13. Redirecting Input and Output"><LINKREL="prev"HREF="ch13_12.htm"TITLE="13.12 Writing to Multiple Terminals for Demonstrations "><LINKREL="next"HREF="ch13_14.htm"TITLE="13.14 What Can You Do with an Empty File? "></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="ch13_12.htm"TITLE="13.12 Writing to Multiple Terminals for Demonstrations "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 13.12 Writing to Multiple Terminals for Demonstrations "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 13<BR>Redirecting Input and Output</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch13_14.htm"TITLE="13.14 What Can You Do with an Empty File? "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 13.14 What Can You Do with an Empty File? "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-7362">13.13 The "Filename" - </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-14767"></A><ACLASS="indexterm"NAME="AUTOID-14770"></A><ACLASS="indexterm"NAME="AUTOID-14773"></A><ACLASS="indexterm"NAME="AUTOID-14775"></A>If you put filenames on the command line, a typical UNIX command willread those files.With no filenames, the command will read its standard input.How can you make the command read both files and standard input?Some UNIX systems, and utilities such as<SPANCLASS="link"><EMCLASS="emphasis">gawk</EM> (<ACLASS="linkend"HREF="ch33_12.htm"TITLE="Versions of awk ">33.12</A>)</SPAN>,support a special name like<SPANCLASS="link"><EMCLASS="emphasis">/dev/stdin</EM> (<ACLASS="linkend"HREF="ch13_01.htm#UPT-ART-1023"TITLE="Using Standard Input and Output">13.1</A>)</SPAN>.Some older UNIX commands, like<SPANCLASS="link"><EMCLASS="emphasis">cat</EM> (<ACLASS="linkend"HREF="ch25_02.htm"TITLE="Four Ways to Skin a cat ">25.2</A>)</SPAN>and<SPANCLASS="link"><EMCLASS="emphasis">diff</EM> (<ACLASS="linkend"HREF="ch28_01.htm#UPT-ART-2680"TITLE="Checking Differences with diff ">28.1</A>)</SPAN>,will accept a "filename" of <CODECLASS="literal">-</CODE> (dash).There's not actually a file named <CODECLASS="literal">-</CODE>; it's just a shorthandfor "read standard input."</P><BLOCKQUOTECLASS="note"><PCLASS="para"><STRONG>NOTE:</STRONG> This syntax might change in the future.</P></BLOCKQUOTE><PCLASS="para">For instance, here's how to compare two files on different computers.The <CODECLASS="literal">rsh</CODE> <CODECLASS="literal">snooze</CODE> <CODECLASS="literal">cat</CODE> <CODECLASS="literal">bin/aprog</CODE> commandsends a copy of the file <EMCLASS="emphasis">bin/aprog</EM> from the remote host <EMCLASS="emphasis">snooze</EM>down the pipe here on the local computer.<EMCLASS="emphasis">diff</EM> compares the local file <EMCLASS="emphasis">aprog.new</EM> to the standard inputfrom the pipe:</P><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"><ACLASS="co"HREF="ch01_33.htm"TITLE="1.33 UNIX Networking and Communications ">rsh</A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen">% <CODECLASS="userinput"><B>rsh snooze cat bin/aprog | diff - aprog.new</B></CODE></PRE></TD></TR></TABLE></P><PCLASS="para">For more examples, see articles<ACLASS="xref"HREF="ch09_11.htm"TITLE="Repeating a Command with a foreach Loop ">9.11</A>and<ACLASS="xref"HREF="ch13_07.htm"TITLE="The () Subshell Operators ">13.7</A>.</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="ch13_12.htm"TITLE="13.12 Writing to Multiple Terminals for Demonstrations "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 13.12 Writing to Multiple Terminals for Demonstrations "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="ch13_14.htm"TITLE="13.14 What Can You Do with an Empty File? 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