📄 ch45_28.htm
字号:
<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 45] 45.28 Quick Reference: expr </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="1998-08-04T21:55:00Z"><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="ch45_01.htm"TITLE="45. Shell Programming for the Initiated"><LINKREL="prev"HREF="ch45_27.htm"TITLE='45.27 Turn Off echo for "Secret" Answers'><LINKREL="next"HREF="ch45_29.htm"TITLE="45.29 Testing Characters in a String with expr "></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="ch45_27.htm"TITLE='45.27 Turn Off echo for "Secret" Answers'><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT='Previous: 45.27 Turn Off echo for "Secret" Answers'BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 45<BR>Shell Programming for the Initiated</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch45_29.htm"TITLE="45.29 Testing Characters in a String with expr "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 45.29 Testing Characters in a String with expr "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-4030">45.28 Quick Reference: expr </A></H2><TABLECLASS="para.programreference"BORDER="1"><TR><THVALIGN="TOP"><ACLASS="programreference"HREF="examples/index.htm"TITLE="expr">expr</A><BR></TH><TDVALIGN="TOP"><ACLASS="indexterm"NAME="AUTOID-54267"></A><ACLASS="indexterm"NAME="AUTOID-54269"></A><ACLASS="indexterm"NAME="AUTOID-54272"></A><EMCLASS="emphasis">expr</EM> is a very handy tool in shell programming, since itprovides the ability to evaluate a wide range of arithmetic, logical,and relational expressions.It evaluates its arguments as expressions and prints the result.<EMCLASS="emphasis">expr</EM> is a standard UNIX utility; the GNU versionis on the CD-ROM.</TD></TR></TABLE><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54279"></A>Here's the syntax.The [brackets] mean "optional"; don't type the brackets:</P><BLOCKQUOTECLASS="blockquote"><PCLASS="para"><CODECLASS="literal">expr</CODE> <CODECLASS="replaceable"><I>arg1 operator arg2 </I></CODE><CODECLASS="literal">[</CODE> <CODECLASS="replaceable"><I>operator arg3 </I></CODE><CODECLASS="literal">... ]</CODE> </P></BLOCKQUOTE><PCLASS="para">Arguments and operators must be separated by spaces.In many cases, an argument is an integer, typed literally orrepresented by a shell variable.There are three types of operators: arithmetic, relational, and logical.</P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54291"></A><ACLASS="indexterm"NAME="AUTOID-54294"></A><SPANCLASS="link">Exit status (<ACLASS="linkend"HREF="ch44_07.htm"TITLE="Exit Status of UNIX Processes ">44.7</A>)</SPAN>values for <EMCLASS="emphasis">expr</EM> are 0 if the expression evaluates non-zero and non-null,1 if the expression evaluates to 0 or null, or 2 if the expression is invalid.<ACLASS="indexterm"NAME="AUTOID-54299"></A><ACLASS="indexterm"NAME="AUTOID-54302"></A></P><DLCLASS="variablelist"><DTCLASS="term">Arithmetic operators</DT><DDCLASS="listitem"><PCLASS="para">Use these to produce mathematical expressions whose results are printed.</P><DLCLASS="variablelist"><DTCLASS="term"><CODECLASS="literal">+</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54316"></A>Add <CODECLASS="replaceable"><I>arg2</I></CODE> to <CODECLASS="replaceable"><I>arg1</I></CODE>.</P></DD><DTCLASS="term"><CODECLASS="literal">-</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54326"></A>Subtract <CODECLASS="replaceable"><I>arg2</I></CODE> from <CODECLASS="replaceable"><I>arg1</I></CODE>.</P></DD><DTCLASS="term"><CODECLASS="literal">*</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54336"></A>Multiply the arguments.</P></DD><DTCLASS="term"><CODECLASS="literal">/</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54344"></A>Divide <CODECLASS="replaceable"><I>arg1</I></CODE> by <CODECLASS="replaceable"><I>arg2</I></CODE>.</P></DD><DTCLASS="term"><CODECLASS="literal">%</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54354"></A>Take the remainder when <CODECLASS="replaceable"><I>arg1</I></CODE> is divided by <CODECLASS="replaceable"><I>arg2</I></CODE>.</P></DD></DL><PCLASS="para">Addition and subtraction are evaluated last, unless they are grouped insideparentheses. The symbols <CODECLASS="literal">*</CODE>, <CODECLASS="literal">(</CODE>, and <CODECLASS="literal">)</CODE> have meaning to the shell,so they must be escaped (preceded by a backslash or enclosed inquotes).<ACLASS="indexterm"NAME="AUTOID-54363"></A><ACLASS="indexterm"NAME="AUTOID-54366"></A></P></DD><DTCLASS="term">Relational operators</DT><DDCLASS="listitem"><PCLASS="para">Use these to compare two arguments. Arguments can also be words, in whichcase comparisons assume a<CODECLASS="literal"><</CODE>z and A<CODECLASS="literal"><</CODE>Z.If the comparison statement is true, <EMCLASS="emphasis">expr</EM> writes 1 to<SPANCLASS="link">standard output (<ACLASS="linkend"HREF="ch13_01.htm#UPT-ART-1023"TITLE="Using Standard Input and Output">13.1</A>)</SPAN>;if false, it writes 0.The symbols <CODECLASS="literal">></CODE> and <CODECLASS="literal"><</CODE> must be escaped.</P><DLCLASS="variablelist"><DTCLASS="term"><CODECLASS="literal">=</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54385"></A>Are the arguments equal?</P></DD><DTCLASS="term"><CODECLASS="literal">!=</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54393"></A>Are the arguments different?</P></DD><DTCLASS="term"><CODECLASS="literal">></CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54401"></A>Is <CODECLASS="replaceable"><I>arg1</I></CODE> greater than <CODECLASS="replaceable"><I>arg2</I></CODE>?</P></DD><DTCLASS="term"><CODECLASS="literal">>=</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54411"></A>Is <CODECLASS="replaceable"><I>arg1</I></CODE> greater than or equal to <CODECLASS="replaceable"><I>arg2</I></CODE>?</P></DD><DTCLASS="term"><CODECLASS="literal"><</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54421"></A>Is <CODECLASS="replaceable"><I>arg1</I></CODE> less than <CODECLASS="replaceable"><I>arg2</I></CODE>?</P></DD><DTCLASS="term"><CODECLASS="literal"><=</CODE></DT><DDCLASS="listitem"><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-54431"></A>Is <CODECLASS="replaceable"><I>arg1</I></CODE> less than or equal to <CODECLASS="replaceable"><I>arg2</I></CODE>?<ACLASS="indexterm"NAME="AUTOID-54436"></A⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -