ch16_04.htm

by Randal L. Schwartz and Tom Phoenix ISBN 0-596-00132-0 Third Edition, published July 2001. (See

<p>We already know about much of the diamond's magic -- it will
automatically open and close a series of files for you, or read from
the standard-input stream if there aren't any filenames given.
But when there's a string in <tt class="literal">$^I</tt>, that
string is used as a backup filename's extension. Let's
see that in action.
</p>

<p>Let's say it's time for the diamond to open our file
<em class="filename">fred03.dat</em>. It opens it like before, but now it
renames it, calling it <em class="filename">fred03.dat.bak</em>.<a href="#FOOTNOTE-357">[357]</a> We've
still got the same file open, but now it has a different name on the
disk. Next, the diamond creates a new file and gives it the name
<em class="filename">fred03.dat</em>. That's okay; we weren't
using that name any more. And now the diamond selects the new file as
the default for output, so that anything that we print will go into
that file.<a href="#FOOTNOTE-358">[358]</a>
</p><blockquote class="footnote">
<a name="FOOTNOTE-357" /><p>[357]Some of the details of this procedure will vary on non-Unix
systems, but the end result should be nearly the same. See the
release notes for your port of Perl.</p> </blockquote><blockquote class="footnote"> <a name="FOOTNOTE-358" /><p>[358]The diamond also tries to duplicate the
original file's permission and ownership settings as much as
possible; for example, if the old one was world-readable, the new one
should be, as well.</p> </blockquote>

<p>So now the <tt class="literal">while</tt> loop will read a line from the
old file, update that, and print it out to the new file. This program
can update hundreds of files in a few seconds on a typical machine.
Pretty powerful, huh?
</p>

<p>Once the program has finished, what does the user see? The user says,
"Ah, I see what happened! Perl edited my file
<em class="filename">fred03.dat</em>, making the changes I needed, and
saved me a copy of the original in the <a name="INDEX-1075" />backup file
<em class="filename">fred03.dat.bak</em> just to be helpful!" But we
now know the truth: Perl didn't really edit any file. It made a
modified copy, said "Abracadabra!", and switched the
files around while we were watching sparks come out of the magic
wand. Tricky.
</p>

<p>Some folks use a <a name="INDEX-1076" />
<a name="INDEX-1077" />tilde
("<tt class="literal">~</tt>") as the value for
<tt class="literal">$^I</tt>, since that resembles what
<i class="command">emacs</i> does for backup files. Another possible
value for <tt class="literal">$^I</tt> is the empty string. This enables
in-place editing, but doesn't save the original data in a
backup file. But since a small typo in your pattern could wipe out
all of the old data, using the empty string is recommended only if
you want to find out how good your backup tapes are. It's easy
enough to delete the backup files when you're done. And when
something goes wrong and you need to rename the backup files to their
original names, you'll be glad that you know how to use Perl to
do that (see the multiple-file rename example in <a href="ch13_01.htm">Chapter 13, "Manipulating Files and Directories"</a>).
</p>

<a name="lperl3-CHP-16-SECT-4.1" /><div class="sect2">
<h3 class="sect2">16.4.1. In-place Editing from the Command Line</h3>

<p><a name="INDEX-1078" /> <a name="INDEX-1079" />A program like the example
from the previous section is fairly easy to write. But Larry decided
it wasn't easy enough.
</p>

<p>Imagine that you need to update hundreds of files that have the
misspelling <tt class="literal">Randal</tt> instead of the
one-<tt class="literal">l</tt> name <tt class="literal">Randal</tt>. You could
write a program like the one in the previous section. Or you could do
it all with a one-line program, right on the command line:
</p>

<blockquote><pre class="code">$ <tt class="userinput"><b>perl -p -i.bak -w -e 's/Randal/Randal/g' fred*.dat</b></tt></pre></blockquote>

<p>Perl has a whole slew of command-line options that can be used to
build a complete program in a few keystrokes.<a href="#FOOTNOTE-359">[359]</a>
Let's see what these few do.
</p><blockquote class="footnote"> <a name="FOOTNOTE-359" /><p>[359]See
the<a name="INDEX-1080" /> <em class="emphasis">perlrun</em>manpage for the complete list.</p> </blockquote>

<p>Starting the command with <tt class="literal">perl</tt> does something like
putting <tt class="literal">#!/usr/bin/perl</tt> at the top of a file does:
it says to use the program <em class="filename">perl</em> to process what
follows.
</p>

<p>The <tt class="literal">-p</tt><a name="INDEX-1081" /> option tells Perl to write a program for
you. It's not much of a program, though; it looks something
like this:<a href="#FOOTNOTE-360">[360]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-360" /><p>[360]Actually, the <tt class="literal">print</tt>
occurs in a <tt class="literal">continue</tt> block. See the
<em class="emphasis">perlsyn</em>and
<em class="emphasis">perlrun</em>manpages for more
information.</p> </blockquote>

<blockquote><pre class="code">while (&lt;&gt;) { print; }. </pre></blockquote>

<p>If you want even less, you could use
<tt class="literal">-n</tt><a name="INDEX-1082" /> instead; that leaves out the
<tt class="literal">print</tt> statement. (Fans of <i class="command">awk</i>
will recognize <tt class="literal">-p</tt> and <tt class="literal">-n</tt>.)
Again, it's not much of a program, but it's pretty good
for the price of a few keystrokes.
</p>

<p>The next option is
<tt class="literal">-i.bak</tt><a name="INDEX-1083" />, which you might have guessed sets
<tt class="literal">$^I</tt> to <tt class="literal">".bak"</tt> before the
program starts. If you don't want a backup file, you can use
<tt class="literal">-i</tt> alone, with no extension.
</p>

<p>We've seen <tt class="literal">-w</tt><a name="INDEX-1084" /> before -- it turns on warnings.
</p>

<p>The <tt class="literal">-e</tt><a name="INDEX-1085" /> option says "executable code
follows." That means that the
<tt class="literal">s/Randal/Randal/g</tt> string is treated as Perl code.
Since we've already got a <tt class="literal">while</tt> loop (from
the <tt class="literal">-p</tt> option), this code is put inside the loop,
before the <tt class="literal">print</tt>. For technical reasons, the last
semicolon in the <tt class="literal">-e</tt> code is optional. But if you
have more than one <tt class="literal">-e</tt>, and thus more than one
chunk of code, only the semicolon at the end of the last one may
safely be omitted.
</p>

<p>The last command-line parameter is <tt class="literal">fred*.dat</tt>,
which says that <tt class="literal">@ARGV</tt> should hold the list of
filenames that match that glob. Put the pieces all together, and
it's as if we had written a program like this:
</p>

<blockquote><pre class="code">#!/usr/bin/perl -w

@ARGV = glob "fred*.dat";
$^I = ".bak";

while (&lt;&gt;) {
  s/Randal/Randal/g;
  print;
}</pre></blockquote>

<p>Compare this program to the one we used in the previous section.
It's pretty similar. These command-line options are pretty
handy, aren't they?<a name="INDEX-1086" /> <a name="INDEX-1087" /> <a name="INDEX-1088" />
</p>

</div>


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