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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 1] 1.11 How the Shell Executes Other Commands </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="1998-08-04T21:31:09Z"><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="ch01_01.htm"TITLE="1. Introduction"><LINKREL="prev"HREF="ch01_10.htm"TITLE="1.10 Internal and External Commands "><LINKREL="next"HREF="ch01_12.htm"TITLE="1.12 What Makes a Shell Script? "></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="ch01_10.htm"TITLE="1.10 Internal and External Commands "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 1.10 Internal and External Commands "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 1<BR>Introduction</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch01_12.htm"TITLE="1.12 What Makes a Shell Script? "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 1.12 What Makes a Shell Script? "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-1041">1.11 How the Shell Executes Other Commands </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-1262"></A><ACLASS="indexterm"NAME="AUTOID-1265"></A>When the shell executes an<SPANCLASS="link">external command (<ACLASS="linkend"HREF="ch01_10.htm"TITLE="Internal and External Commands ">1.10</A>)</SPAN>,what happens?</P><PCLASS="para">UNIX programs are executed through a combination of two system calls(low-level requests to the operating system) called <EMCLASS="emphasis">fork</EM> and<EMCLASS="emphasis">exec</EM>.</P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-1273"></A><ACLASS="indexterm"NAME="AUTOID-1275"></A>The <EMCLASS="emphasis">exec</EM> system call tells the kernel to execute anotherprogram. However, the kernelreplaces the calling program with the new one being called. Thisdoesn't work too well if you want to return to the original programafter the second one has done its job.</P><PCLASS="para">To get around this problem, programs that want to stick around firstcopy themselves with the <EMCLASS="emphasis">fork</EM> system call. Then the copied program<EMCLASS="emphasis">exec</EM>sthe new program, terminating itself in the process.</P><PCLASS="para">You don't really need to know this little tidbit aboutwhat goes on behind the scenes, but it sure helps to know about <EMCLASS="emphasis">fork</EM>and <EMCLASS="emphasis">exec</EM> when reading some UNIX manuals.Article<ACLASS="xref"HREF="ch38_02.htm"TITLE="fork and exec ">38.2</A>has more information.</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="ch01_10.htm"TITLE="1.10 Internal and External Commands "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 1.10 Internal and External Commands "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="ch01_12.htm"TITLE="1.12 What Makes a Shell Script? 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