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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 13] 13.7 The () Subshell Operators </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="1998-08-04T21:36:45Z"><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_06.htm"TITLE="13.6 Safe I/O Redirection with noclobber "><LINKREL="next"HREF="ch13_08.htm"TITLE="13.8 Using {list} to Group Bourne Shell Commands "></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_06.htm"TITLE="13.6 Safe I/O Redirection with noclobber "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 13.6 Safe I/O Redirection with noclobber "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_08.htm"TITLE="13.8 Using {list} to Group Bourne Shell Commands "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 13.8 Using {list} to Group Bourne Shell Commands "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-6800">13.7 The () Subshell Operators </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-14430"></A><ACLASS="indexterm"NAME="AUTOID-14433"></A>A useful shell trick is to use parentheses, <CODECLASS="literal">()</CODE>, togroup commands.</P><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="UPT-ART-6800-SECT-1.1">13.7.1 Combining Several Commands </A></H3><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-14440"></A>The output of the entire group can be passed together into a single pipeline.For example:</P><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"><ACLASS="co"HREF="ch08_06.htm"TITLE="8.6 Output Command-Line Arguments ">echo</A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen">$ <CODECLASS="userinput"><B>(cat file1; echo .bp; cat file2) | nroff</B></CODE></PRE></TD></TR></TABLE></P><PCLASS="para">This will interpose the<SPANCLASS="link"><EMCLASS="emphasis">nroff</EM> (<ACLASS="linkend"HREF="ch43_13.htm"TITLE="The Text Formatters nroff, troff, ditroff, ... ">43.13</A>)</SPAN><CODECLASS="literal">.bp</CODE> (break page)request between two files to be formatted.[3]</P><BLOCKQUOTECLASS="footnote"><PCLASS="para">[3] If you're using only <EMCLASS="emphasis">cat</EM> and a single <EMCLASS="emphasis">echo</EM>,you can use thiscommand instead:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">$ <CODECLASS="userinput"><B>echo .bp | cat file1 - file2 | nroff</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para">The <EMCLASS="emphasis">cat</EM><SPANCLASS="link">\- option (<ACLASS="linkend"HREF="ch13_13.htm"TITLE='The "Filename" - '>13.13</A>)</SPAN>tells <EMCLASS="emphasis">cat</EM> to read its standard input (inthis case, from the pipe and the <EMCLASS="emphasis">echo</EM>) at that point.<EMCLASS="emphasis">nroff</EM> gets exactly the same input.</P></BLOCKQUOTE><PCLASS="para">Parentheses are also very useful in the Bourneshell if you want to put an entire sequence of commands separated bysemicolons into the background.In the C shell, the command line below willgo immediately into the background.</P><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"><ACLASS="co"HREF="ch01_26.htm"TITLE="1.26 What a Multiuser System Can Do for You ">&</A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen">% <CODECLASS="userinput"><B>nroff -ms file1; nroff -ms file2 &</B></CODE></PRE></TD></TR></TABLE></P><PCLASS="para">But in the Bourne shell, the background request (<CODECLASS="literal">&</CODE>)will only apply to the second command, forcing youto wait for completion of the first job before youget back the system prompt.To get right back towork, you can type:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">$ <CODECLASS="userinput"><B>(nroff -ms file1; nroff -ms file2) &</B></CODE></PRE></BLOCKQUOTE></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="UPT-ART-6800-SECT-1.2">13.7.2 Temporary Change of Directory and Environment </A></H3><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-14477"></A><ACLASS="indexterm"NAME="AUTOID-14480"></A>The parentheses start a<SPANCLASS="link">subshell (<ACLASS="linkend"HREF="ch38_04.htm"TITLE="Subshells ">38.4</A>)</SPAN>.Commands that run between the parentheses won't affect the parent shell'senvironment.For instance,to run a command in another directory<SPANCLASS="link"> without changingyour active shell's current directory : (<ACLASS="linkend"HREF="ch38_03.htm"TITLE="Managing Processes: Overall Concepts ">38.3</A>)</SPAN></P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>pwd</B></CODE>/home/trent% <CODECLASS="userinput"><B>(cd</B></CODE> <CODECLASS="replaceable"><I>somewhere-else</I></CODE><CODECLASS="userinput"><B>; nroff -ms file1 > file.out) &</B></CODE>[1] 22670% <CODECLASS="userinput"><B>pwd</B></CODE>/home/trent</PRE></BLOCKQUOTE></P><PCLASS="para">The file <EMCLASS="emphasis">file.out</EM> will be createdin the <EMCLASS="emphasis">somewhere-else</EM> directory.</P><PCLASS="para">Article<ACLASS="xref"HREF="ch13_08.htm"TITLE="Using {list} to Group Bourne Shell Commands ">13.8</A>shows another method for the Bourne shell.It's more limited but can also be more efficient.</P></DIV><DIVCLASS="sect1info"><PCLASS="SECT1INFO">- <SPANCLASS="authorinitials">TOR</SPAN>, <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_06.htm"TITLE="13.6 Safe I/O Redirection with noclobber "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 13.6 Safe I/O Redirection with noclobber "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_08.htm"TITLE="13.8 Using {list} to Group Bourne Shell Commands "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 13.8 Using {list} to Group Bourne Shell Commands "BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172">13.6 Safe I/O Redirection with noclobber </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">13.8 Using {list} to Group Bourne Shell Commands </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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