ch45_28.htm

the unix power tools

><A
CLASS="indexterm"
NAME="AUTOID-54439"
></A
></P
></DD
></DL
></DD
><DT
CLASS="term"
>Logical operators</DT
><DD
CLASS="listitem"
><P
CLASS="para"
>Use these to compare two arguments. Depending on the values,
the result written to standard output
can be <CODE
CLASS="replaceable"
><I
>arg1</I
></CODE
> (or some portion of it), <CODE
CLASS="replaceable"
><I
>arg2</I
></CODE
>, or 0.
The symbols <CODE
CLASS="literal"
>|</CODE
> and <CODE
CLASS="literal"
>&amp;</CODE
> must be escaped.</P
><DL
CLASS="variablelist"
><DT
CLASS="term"
><CODE
CLASS="literal"
>|</CODE
></DT
><DD
CLASS="listitem"
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-54456"
></A
>Logical OR; if <CODE
CLASS="replaceable"
><I
>arg1</I
></CODE
> has a non-zero (and non-null) value,
the output is <CODE
CLASS="replaceable"
><I
>arg1</I
></CODE
>; otherwise, the output is <CODE
CLASS="replaceable"
><I
>arg2</I
></CODE
>.</P
></DD
><DT
CLASS="term"
><CODE
CLASS="literal"
>&amp;</CODE
></DT
><DD
CLASS="listitem"
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-54467"
></A
>Logical AND; if both <CODE
CLASS="replaceable"
><I
>arg1</I
></CODE
> and <CODE
CLASS="replaceable"
><I
>arg2</I
></CODE
> have a non-zero 
(and non-null) value, the output is <CODE
CLASS="replaceable"
><I
>arg1</I
></CODE
>; otherwise, 
the output is 0.</P
></DD
><DT
CLASS="term"
><CODE
CLASS="literal"
>:</CODE
></DT
><DD
CLASS="listitem"
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-54478"
></A
>Sort of like
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>grep</EM
> (<A
CLASS="linkend"
HREF="ch27_01.htm#UPT-ART-7420"
TITLE="Different Versions of grep "
>27.1</A
>)</SPAN
>;
<CODE
CLASS="replaceable"
><I
>arg2</I
></CODE
> is a pattern to search for in <CODE
CLASS="replaceable"
><I
>arg1</I
></CODE
>.
<CODE
CLASS="replaceable"
><I
>arg2</I
></CODE
> must be a regular expression in this case. If the <CODE
CLASS="replaceable"
><I
>arg2</I
></CODE
> pattern
is enclosed in <CODE
CLASS="literal"
>\(&nbsp;\)</CODE
>, the output is the portion of <CODE
CLASS="replaceable"
><I
>arg1</I
></CODE
> that matches;
otherwise, the output is simply the number of characters that match. A 
pattern match always applies to the beginning of the 
argument (the <CODE
CLASS="literal"
>^</CODE
> 
symbol is assumed by default).</P
></DD
></DL
></DD
></DL
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-4030-SECT-1.0.1"
>45.28.1 Examples </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-54493"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54496"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54499"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54502"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54505"
></A
>Division happens first; output is 10:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>expr 5 + 10 / 2</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Addition happens first; output is 7 (truncated from 7.5):</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>expr \( 5 + 10 \) / 2</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Add 1 to variable <EM
CLASS="emphasis"
>i</EM
>; this is how variables are incremented in
Bourne shell scripts:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
><CODE
CLASS="userinput"
><B
>i=`expr &quot;$i&quot; + 1`</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-54521"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54524"
></A
>
Output 1 (true) if variable <EM
CLASS="emphasis"
>a</EM
> is the string &quot;hello&quot;:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>expr &quot;$a&quot; = hello</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Output 1 (true) if variable <EM
CLASS="emphasis"
>b</EM
> plus 5 equals 10 or more:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>expr &quot;$b&quot; + 5 \&gt;= 10</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>&#13;In the examples below, variable <EM
CLASS="emphasis"
>p</EM
> is the string 
&quot;version.100&quot;. This command returns the number of characters in <EM
CLASS="emphasis"
>p</EM
>:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
><A
CLASS="indexterm"
NAME="AUTOID-54541"
></A
>$ <CODE
CLASS="userinput"
><B
>expr &quot;$p&quot; : '.*'</B
></CODE
>   <I
CLASS="lineannotation"
>Output is 11</I
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Match all characters and print them:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>expr &quot;$p&quot; : '\(.*\)'</B
></CODE
>   <I
CLASS="lineannotation"
>Output is &quot;version.100&quot;</I
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Output the number of lowercase letters matched:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>expr &quot;$p&quot; : '[a-z]*'</B
></CODE
>   <I
CLASS="lineannotation"
>Output is 7</I
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Match a string of lowercase letters:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>expr &quot;$p&quot; : '\([a-z]*\)'</B
></CODE
>   <I
CLASS="lineannotation"
>Output is \&quot;version&quot;</I
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Truncate <CODE
CLASS="literal"
>$x</CODE
> if it contains five or more characters; 
if not, just output <CODE
CLASS="literal"
>$x</CODE
>. (Logical OR uses the second argument when
the first one is 0 or null; i.e., when the match fails.)</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>expr &quot;$x&quot; : '\(.....\)'  &quot;$x&quot;</B
></CODE
></PRE
></BLOCKQUOTE
></P
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>DG</SPAN
> <SPAN
CLASS="bibliomisc"
>from O'Reilly &amp; Associates' <CITE
CLASS="citetitle"
>UNIX in a Nutshell (SVR4/Solaris)</CITE
></SPAN
></P
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