ch07_08.htm

By Tom Christiansen and Nathan Torkington ISBN 1-56592-243-3 First Edition, published August 1998

>while</CODE
> <CODE
CLASS="literal"
>(&lt;&gt;)</CODE
> loop around your program text. It's normally used for filters like <EM
CLASS="emphasis"
>grep</EM
> or programs that summarize the data they read. The program is shown in <A
CLASS="xref"
HREF="ch07_08.htm#ch07-34632"
TITLE="findlogin1"
>Example 7.1</A
>.</P
><DIV
CLASS="example"
><H4
CLASS="example"
><A
CLASS="title"
NAME="ch07-34632"
>Example 7.1: findlogin1</A
></H4
><PRE
CLASS="programlisting"
>#!/usr/bin/perl   
# <A
CLASS="indexterm"
NAME="ch07-idx-1000009827-0"
></A
>findlogin1 - print all lines containing the string &quot;login&quot;   
while (&lt;&gt;) {# loop over files on command line 	
    print if /login/;     
}</PRE
></DIV
><P
CLASS="para"
>The program in <A
CLASS="xref"
HREF="ch07_08.htm#ch07-34632"
TITLE="findlogin1"
>Example 7.1</A
> could be written as shown in <A
CLASS="xref"
HREF="ch07_08.htm#ch07-36310"
TITLE="findlogin2"
>Example 7.2</A
>.</P
><DIV
CLASS="example"
><H4
CLASS="example"
><A
CLASS="title"
NAME="ch07-36310"
>Example 7.2: findlogin2</A
></H4
><PRE
CLASS="programlisting"
>#!/usr/bin/perl -n     
# findlogin2 - print all lines containing the string &quot;login&quot;     
print if /login/;</PRE
></DIV
><P
CLASS="para"
>You can combine the <B
CLASS="emphasis.bold"
>-n</B
> and <B
CLASS="emphasis.bold"
>-e</B
> options to run Perl code from the command line:</P
><PRE
CLASS="programlisting"
>% perl -ne 'print if /login/'</PRE
><P
CLASS="para"
>The <B
CLASS="emphasis.bold"
>-p</B
> option is like <B
CLASS="emphasis.bold"
>-n</B
> but it adds a <CODE
CLASS="literal"
>print</CODE
> at the end of the loop. It's normally used for programs that translate their input. This program is shown in <A
CLASS="xref"
HREF="ch07_08.htm#ch07-34498"
TITLE="lowercase1"
>Example 7.3</A
>.</P
><DIV
CLASS="example"
><H4
CLASS="example"
><A
CLASS="title"
NAME="ch07-34498"
>Example 7.3: lowercase1</A
></H4
><PRE
CLASS="programlisting"
>#!/usr/bin/perl    
# <A
CLASS="indexterm"
NAME="ch07-idx-1000009830-0"
></A
>lowercase - turn all lines into lowercase

use locale;
while (&lt;&gt;) {                 # loop over lines on command line
    s/([^\W0-9_])/\l$1/g;    # change all letters to lowercase
print;
}</PRE
></DIV
><P
CLASS="para"
>The program in <A
CLASS="xref"
HREF="ch07_08.htm#ch07-34498"
TITLE="lowercase1"
>Example 7.3</A
> could be written as shown in <A
CLASS="xref"
HREF="ch07_08.htm#ch07-32154"
TITLE="lowercase2"
>Example 7.4</A
>.</P
><DIV
CLASS="example"
><H4
CLASS="example"
><A
CLASS="title"
NAME="ch07-32154"
>Example 7.4: lowercase2</A
></H4
><PRE
CLASS="programlisting"
>#!/usr/bin/perl -p     
# lowercase - turn all lines into lowercase     
use locale;     
s/([^\W0-9_])/\l$1/g;# change all letters to lowercase</PRE
></DIV
><P
CLASS="para"
>Or written from the command line as:</P
><PRE
CLASS="programlisting"
>% perl -Mlocale -pe 's/([^\W0-9_])/\l$1/g'</PRE
><P
CLASS="para"
>While using<B
CLASS="emphasis.bold"
> -n</B
> or <B
CLASS="emphasis.bold"
>-p</B
> for implicit input looping, the special label <A
CLASS="indexterm"
NAME="ch07-idx-1000012067-0"
></A
><CODE
CLASS="literal"
>LINE:</CODE
> is silently created for the whole input loop. That means that from an inner loop, you can go on the following input record by using <CODE
CLASS="literal"
>next</CODE
> <CODE
CLASS="literal"
>LINE</CODE
> (this is like <EM
CLASS="emphasis"
>awk  </EM
>'s <CODE
CLASS="literal"
>next</CODE
>). Go on to the file by closing ARGV (this is like <EM
CLASS="emphasis"
>awk</EM
>  's <CODE
CLASS="literal"
>nextfile</CODE
>). This is shown in <A
CLASS="xref"
HREF="ch07_08.htm#ch07-34027"
TITLE="countchunks"
>Example 7.5</A
>.</P
><DIV
CLASS="example"
><H4
CLASS="example"
><A
CLASS="title"
NAME="ch07-34027"
>Example 7.5: countchunks</A
></H4
><PRE
CLASS="programlisting"
>#!/usr/bin/perl -n    
# <A
CLASS="indexterm"
NAME="ch07-idx-1000009833-0"
></A
>countchunks - count how many words are used.    
# skip comments, and bail on file if __END__   
# or __DATA__ seen.    
for (split /\W+/) { 
    next LINE if /^#/; 
    close ARGV if /__(DATA|END)__/; 
    $chunks++;     
}     
END { print &quot;Found $chunks chunks\n&quot; }</PRE
></DIV
><P
CLASS="para"
>The <EM
CLASS="emphasis"
>tcsh</EM
> keeps a <EM
CLASS="emphasis"
>.history</EM
> file in a format such that every other line contains a commented out timestamp in Epoch seconds:</P
><PRE
CLASS="programlisting"
>#+0894382237     
less /etc/motd     
#+0894382239     
vi ~/.exrc     
#+0894382242     
date     
#+0894382242     
who     
#+0894382288     
telnet home</PRE
><P
CLASS="para"
>A simple one-liner can render that legible:</P
><PRE
CLASS="programlisting"
>% perl -pe 's/^#\+(\d+)\n/localtime($1) . &quot; &quot;/e' 
<CODE
CLASS="userinput"
><B
><CODE
CLASS="replaceable"
><I
>Tue May  5 09:30:37 1998     less /etc/motd </I
></CODE
></B
></CODE
>
<CODE
CLASS="userinput"
><B
><CODE
CLASS="replaceable"
><I
>Tue May  5 09:30:39 1998     vi ~/.exrc </I
></CODE
></B
></CODE
>
<CODE
CLASS="userinput"
><B
><CODE
CLASS="replaceable"
><I
>Tue May  5 09:30:42 1998     date</I
></CODE
></B
></CODE
>
<CODE
CLASS="userinput"
><B
><CODE
CLASS="replaceable"
><I
>Tue May  5 09:30:42 1998     who </I
></CODE
></B
></CODE
>
<CODE
CLASS="userinput"
><B
><CODE
CLASS="replaceable"
><I
>Tue May  5 09:31:28 1998     telnet home</I
></CODE
></B
></CODE
></PRE
><P
CLASS="para"
>The <B
CLASS="emphasis.bold"
>-i</B
> option changes each file on the command line. It is described in <A
CLASS="xref"
HREF="ch07_10.htm"
TITLE="Modifying a File in Place with -i Switch"
>Recipe 7.9</A
>, and is normally used in conjunction with <B
CLASS="emphasis.bold"
>-p</B
>.</P
><P
CLASS="para"
>You have to say <CODE
CLASS="literal"
>use</CODE
> <CODE
CLASS="literal"
>locale</CODE
> to handle current character <A
CLASS="indexterm"
NAME="ch07-idx-1000009653-0"
></A
><A
CLASS="indexterm"
NAME="ch07-idx-1000009653-1"
></A
>set.<A
CLASS="indexterm"
NAME="ch07-idx-1000009629-0"
></A
><A
CLASS="indexterm"
NAME="ch07-idx-1000009629-1"
></A
><A
CLASS="indexterm"
NAME="ch07-idx-1000009629-2"
></A
></P
></DIV
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="ch07-pgfId-1000008971"
>See Also</A
></H3
><P
CLASS="para"
><I
CLASS="filename"
>perlrun </I
>(1), and the <A
CLASS="olink"
HREF="../prog/ch06_01.htm#PERL2-CH-6-SECT-1.2"
>"Switches"</A
> section of <A
CLASS="olink"
HREF="../prog/ch06_01.htm"
>Chapter 6</A
> of <A
CLASS="citetitle"
HREF="../prog/index.htm"
TITLE="Programming Perl"
><CITE
CLASS="citetitle"
>Programming Perl</CITE
></A
>; <A
CLASS="xref"
HREF="ch07_10.htm"
TITLE="Modifying a File in Place with -i Switch"
>Recipe 7.9</A
>; Recipe 16.6</P
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