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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 35] 35.6 Low-Level File Butchery with dd </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="1998-08-04T21:48:03Z"><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="ch35_01.htm"TITLE="35. You Can't Quite Call This Editing"><LINKREL="prev"HREF="ch35_05.htm"TITLE="35.5 Remove Mail/News Headers with behead "><LINKREL="next"HREF="ch35_07.htm"TITLE="35.7 offset: Indent Text "></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="ch35_05.htm"TITLE="35.5 Remove Mail/News Headers with behead "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 35.5 Remove Mail/News Headers with behead "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 35<BR>You Can't Quite Call This Editing</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch35_07.htm"TITLE="35.7 offset: Indent Text "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 35.7 offset: Indent Text "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-7845">35.6 Low-Level File Butchery with dd </A></H2><PCLASS="para">Want to strip off some arbitrary number of characters from the frontof a file?</P><PCLASS="para"><EMCLASS="emphasis">dd</EM>provides an unexpectedly easy answer. Let's sayyou wanted to delete the first 100 characters in a file. Here's thecommand that will do the trick (assuming of course that you give <EMCLASS="emphasis">dd</EM> a filename withthe <EMCLASS="emphasis">if=</EM> option or data from a pipe):<ACLASS="indexterm"NAME="AUTOID-40345"></A></P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>dd bs=100 skip=1</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para">Or you could try:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>dd bs=1 skip=100</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para"><EMCLASS="emphasis">dd</EM> normally reads and writes data in 512-byte blocks; the input block sizecan be changed with the <EMCLASS="emphasis">ibs=</EM> option, and the output block size with <EMCLASS="emphasis">obs=</EM>. Use <EMCLASS="emphasis">bs=</EM> to set both. <EMCLASS="emphasis">skip=</EM> sets thenumber of blocks to skip at the start of the file.</P><PCLASS="para">Why would you want to do this? Article <ACLASS="xref"HREF="ch22_17.htm"TITLE="Ways of Improving the Security of crypt ">22.17</A>gives an interesting example when encrypting files.Article<ACLASS="xref"HREF="ch20_06.htm"TITLE="Writing a Tape Drive on a Remote Machine ">20.6</A>explains using <EMCLASS="emphasis">dd</EM> over a network with a tape drive.To convert files between ASCII and EBCDIC,see article<ACLASS="xref"HREF="ch35_12.htm"TITLE="Converting Between ASCII and EBCDIC ">35.12</A>.Article<ACLASS="xref"HREF="ch35_13.htm"TITLE="Other Conversions with dd ">35.13</A>shows even more uses for <EMCLASS="emphasis">dd</EM>.</P><DIVCLASS="sect1info"><PCLASS="SECT1INFO">- <SPANCLASS="authorinitials">TOR</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="ch35_05.htm"TITLE="35.5 Remove Mail/News Headers with behead "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 35.5 Remove Mail/News Headers with behead "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="ch35_07.htm"TITLE="35.7 offset: Indent Text "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 35.7 offset: Indent Text "BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172">35.5 Remove Mail/News Headers with behead </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">35.7 offset: Indent Text </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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