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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 13] 13.14 What Can You Do with an Empty File? </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="1998-08-04T21:36:52Z"><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_13.htm"TITLE='13.13 The "Filename" - '><LINKREL="next"HREF="ch13_15.htm"TITLE="13.15 What to Do with a Full Bit Bucket :-) "></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_13.htm"TITLE='13.13 The "Filename" - '><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT='Previous: 13.13 The "Filename" - '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_15.htm"TITLE="13.15 What to Do with a Full Bit Bucket :-) "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 13.15 What to Do with a Full Bit Bucket :-) "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-6910">13.14 What Can You Do with an Empty File? </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-14811"></A><ACLASS="indexterm"NAME="AUTOID-14813"></A><ACLASS="indexterm"NAME="AUTOID-14816"></A>It isn't a file, actually, though you can use it like one.<EMCLASS="emphasis">/dev/null</EM> is a UNIX device.[4]It's not a physical device.<EMCLASS="emphasis">/dev/null</EM> is a special device that "eats" any text written to it<ACLASS="indexterm"NAME="AUTOID-14824"></A>and returns "end-of-file" (a file of length 0) when you read from it.So what the heck can you use it for?</P><BLOCKQUOTECLASS="footnote"><PCLASS="para">[4] Well, okay.It's a <EMCLASS="emphasis">device file</EM>.</P></BLOCKQUOTE><ULCLASS="itemizedlist"><LICLASS="listitem"><PCLASS="para">Empty another file.<ACLASS="indexterm"NAME="AUTOID-14829"></A><SPANCLASS="link"> Just copy <EMCLASS="emphasis">/dev/null</EM> "on top of" the other file . (<ACLASS="linkend"HREF="ch24_01.htm#UPT-ART-1730"TITLE="Instead of Removing a File, Empty It ">24.1</A>)</SPAN></P></LI><LICLASS="listitem"><PCLASS="para">Make another program "quiet" by redirecting its output there.For instance, if you're putting a program into the backgroundand you don't want it to bother you, type:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="replaceable"><I>progname</I></CODE> <CODECLASS="userinput"><B>> /dev/null &</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para">That<SPANCLASS="link">redirects (<ACLASS="linkend"HREF="ch13_01.htm#UPT-ART-1023"TITLE="Using Standard Input and Output">13.1</A>)</SPAN>standard output but leaves standard error hooked to yourterminal, in case there is an error.</P></LI><LICLASS="listitem"><PCLASS="para">Answer a program that asks a lot of questions-you know you'll just press RETURNat each prompt.In a lot of cases, you can redirect the program's standard input from<EMCLASS="emphasis">/dev/null</EM>:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="replaceable"><I>progname</I></CODE> <CODECLASS="userinput"><B>< /dev/null</B></CODE>Want the default setup? If yes, press RETURN:Enter filename or press RETURN for default: ...</PRE></BLOCKQUOTE></P><PCLASS="para">You should test that with each program, though, before you assume this trickwill work.(If it doesn't work, try<SPANCLASS="link"><EMCLASS="emphasis">yes</EM> (<ACLASS="linkend"HREF="ch23_04.htm"TITLE='Answer "Yes" or "No" Forever with yes'>23.4</A>)</SPAN>.)</P></LI><LICLASS="listitem"><PCLASS="para">Where a program needs an extra filename but you don't want it to read orwrite an actual file.For instance, the<SPANCLASS="link"><EMCLASS="emphasis">grep</EM> (<ACLASS="linkend"HREF="ch27_01.htm#UPT-ART-7420"TITLE="Different Versions of grep ">27.1</A>)</SPAN>programs won't give the name of the file where they find a match unless thereare at least two filenames on the command line.When you use a wildcard in a directory where maybe only one file will match,<SPANCLASS="link"> use <EMCLASS="emphasis">/dev/null</EM> to be sure that <EMCLASS="emphasis">grep</EM> will always see more than one : (<ACLASS="linkend"HREF="ch17_20.htm"TITLE="grepping a Directory Tree (and a Gotcha) ">17.20</A>)</SPAN></P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>grep "</B></CODE><CODECLASS="replaceable"><I>whatever</I></CODE><CODECLASS="userinput"><B>" * /dev/null</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para">You're guaranteed that <EMCLASS="emphasis">grep</EM> won't match its regular expression in<EMCLASS="emphasis">/dev/null</EM>. <CODECLASS="literal">:-)</CODE></P></LI><LICLASS="listitem"><PCLASS="para">Article<ACLASS="xref"HREF="ch24_02.htm"TITLE="Save Space with Bit Bucket Log Files and Mailboxes">24.2</A>shows even more uses for <EMCLASS="emphasis">/dev/null</EM>.</P></LI></UL><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-14873"></A><ACLASS="indexterm"NAME="AUTOID-14876"></A>Another interesting device (mostly for programmers) is <EMCLASS="emphasis">/dev/zero</EM>.When you read it, you'll get<SPANCLASS="link">ASCII zeros (NUL characters) (<ACLASS="linkend"HREF="ch51_03.htm"TITLE="ASCII Characters: Listing and Getting Values ">51.3</A>)</SPAN>forever.There are no newlines either.For both of those reasons, many UNIX commands have trouble reading it.If you want to play, the command below will give you a start (and<SPANCLASS="link"><EMCLASS="emphasis">head</EM> (<ACLASS="linkend"HREF="ch25_20.htm"TITLE="Printing the Top of a File ">25.20</A>)</SPAN>will give you a stop!):[5]</P><BLOCKQUOTECLASS="footnote"><PCLASS="para">[5] On some UNIX versions, the <EMCLASS="emphasis">head</EM> program may not terminate afterit's printed the first ten lines.In that case, use <CODECLASS="literal">sed 10q</CODE> instead of <CODECLASS="literal">head</CODE>.</P></BLOCKQUOTE><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"><ACLASS="co"HREF="ch43_08.htm"TITLE="43.8 Fixing Margins with pr and fold ">fold</A> <ACLASS="co"HREF="ch25_07.htm"TITLE="25.7 Show Non-Printing Characters with cat -v or od -c ">od</A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen">% <CODECLASS="userinput"><B>fold -20 /dev/zero | od -c | head</B></CODE></PRE></TD></TR></TABLE></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_13.htm"TITLE='13.13 The "Filename" - 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