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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 13] 13.8 Using {list} to Group Bourne Shell 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="1998-08-04T21:36:47Z"><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_07.htm"TITLE="13.7 The () Subshell Operators "><LINKREL="next"HREF="ch13_09.htm"TITLE="13.9 Send Output Two or More Places with tee "></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_07.htm"TITLE="13.7 The () Subshell Operators "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 13.7 The () Subshell Operators "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_09.htm"TITLE="13.9 Send Output Two or More Places with tee "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 13.9 Send Output Two or More Places with tee "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-0220">13.8 Using {list} to Group Bourne Shell Commands </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-14502"></A><ACLASS="indexterm"NAME="AUTOID-14504"></A>A lot of people know that you can group the output of a series ofcommands by using a<SPANCLASS="link">subshell (<ACLASS="linkend"HREF="ch13_07.htm"TITLE="The () Subshell Operators ">13.7</A>)</SPAN>.That is, instead of this:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">$ <CODECLASS="userinput"><B>date > log</B></CODE>$ <CODECLASS="userinput"><B>who >> log</B></CODE>$ <CODECLASS="userinput"><B>ls >> log</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para">they start a subshell with parentheses:</P><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"> <ACLASS="co"HREF="ch09_13.htm"TITLE="9.13 Multiline Commands, Secondary Prompts ">></A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen">$ <CODECLASS="userinput"><B>(date</B></CODE>> <CODECLASS="userinput"><B>who</B></CODE>> <CODECLASS="userinput"><B>ls) > log</B></CODE></PRE></TD></TR></TABLE></P><PCLASS="para">and only redirect once to <EMCLASS="emphasis">log</EM>.But a subshell takes an extra process and takes time to start on a busysystem.If all you need to do is redirect output (or input) of a set ofcommands, use the Bourne shell's list operators <CODECLASS="literal">{}</CODE> (curly braces):<ACLASS="indexterm"NAME="AUTOID-14523"></A></P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">$ <CODECLASS="userinput"><B>{ date</B></CODE>> <CODECLASS="userinput"><B>who</B></CODE>> <CODECLASS="userinput"><B>ls</B></CODE>> <CODECLASS="userinput"><B>} > log</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para">Notice the spaces and the extra RETURN at the end.Each command must be separated from others.You can also write (note the semicolon after the last command):</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">$ <CODECLASS="userinput"><B>{ date; who; ls; } > log</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para">Here are two other differences between the subshell (parentheses) and list (curly braces)operators.<ACLASS="indexterm"NAME="AUTOID-14537"></A>A <EMCLASS="emphasis">cd</EM> command in the subshell doesn't change the parent shell's currentdirectory; it does in a list.<ACLASS="indexterm"NAME="AUTOID-14541"></A>Also, a variable set in a subshell isn't passed to the parent shell;from a list, the variable is passed out.</P><BLOCKQUOTECLASS="note"><PCLASS="para"><STRONG>NOTE:</STRONG> <ACLASS="indexterm"NAME="AUTOID-14546"></A>Jonathan I. Kamens points out that some Bourne shells may run a list ina subshell anyway, especially if there's a pipe involved.If your Bourne shell works like the example shown here, it's using a subshell, too: </P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">$ <CODECLASS="userinput"><B>{ echo frep; foo=bar; } | cat</B></CODE>frep$ <CODECLASS="userinput"><B>echo $foo</B></CODE>$ <CODECLASS="userinput"><B>{ echo frep; foo=bar; }</B></CODE>frep$ <CODECLASS="userinput"><B>echo $foo</B></CODE>bar</PRE></BLOCKQUOTE></P></BLOCKQUOTE><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_07.htm"TITLE="13.7 The () Subshell Operators "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 13.7 The () Subshell Operators "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_09.htm"TITLE="13.9 Send Output Two or More Places with tee "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 13.9 Send Output Two or More Places with tee "BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172">13.7 The () Subshell Operators </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">13.9 Send Output Two or More Places with tee </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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