There are a
couple of switches that make Perl a very useful command-line editing
tool. Learn these switches, and you too can learn how to mumble magic
Perl one-liners to confound your friends (and frighten your project
manager).
The first is -e. Give
Perl a -e followed
by a line of code, and it will run it as if it were an ordinary Perl
script:
rob@catlin:~$ perl -e 'print "Hi, Ma!\n"'
Hi, Ma!
Note that a trailing ; isn't needed on
one-liners. It's generally a good idea to wrap your
line in single quotes to prevent the shell from attempting to
interpret special characters (like the ! and
\ above).
The next switch is a little more
complicated, it but becomes second nature once you start using
it: -p. From perldoc perlrun:
-p causes Perl to assume the following loop around your
program, which makes it iterate over filename arguments
somewhat like sed:
LINE:
while (<>) {
... # your program goes here
} continue {
print or die "-p destination: $!\n";
}
The line in question ($_) is automatically printed
on every iteration of the loop. If you combine
-p and -e, you get a
one-liner that iterates over every file on the command line or on the
text fed to it on STDIN. For example, here's a
complicated cat command:
rob@catlin:~$ perl -pe 1 /etc/hosts
#
# hosts This file describes a number of hostname-to-address
# mappings for the TCP/IP subsystem.
#
# For loopbacking.
127.0.0.1 localhost
The 1 is just a return code (as if you entered
1; in a Perl script, which is equivalent to
return 1;). Since the lines are printed
automatically, we don't really need the program we
specify with -e to do anything.
Where it gets interesting is in providing a bit of code to manipulate
the current line before it gets printed. For example, suppose you
wanted to append the local machine name to the localhost line:
rob@catlin:~$ perl -pe 's/localhost/localhost $ENV{HOSTNAME}/' /etc/hosts
#
# hosts This file describes a number of hostname-to-address
# mappings for the TCP/IP subsystem.
#
# For loopbacking.
127.0.0.1 localhost catlin.nocat.net
or maybe you'd like to manipulate your
inetd settings:
rob@caligula:~$ perl -pe 's/^(\s+)?(telnet|shell|login|exec)/# $2/' \ /etc/inetd.conf
That will print the contents of /etc/inetd.conf
to STDOUT, commenting out any uncommented
telnet, shell,
login, or exec lines along
the way. Naturally, we could redirect that back out to a file, but if
we just want to edit a file in place, there's a
better way: the
-i switch.
-i lets you edit files in place. So, to comment
out all of the above lines in /etc/inetd.conf,
you might try:
root@catlin:~# perl -pi -e 's/^(\s+)?(telnet|shell|login|exec)/# $2/' /etc/inetd.conf
or better yet:
root@catlin:~# perl -pi.orig -e 's/^(\s+)?(telnet|shell|login|exec)/# $2/' /etc/inetd.conf
The second example will backup /etc/inetd.conf
to /etc/inetd.conf.orig before changing the
original. Don't forget to HUP
inetd to make your changes take.
It's just as easy to edit multiple files in place at
the same time. You can specify any number of files (or wildcards) on
the command line:
rob@catlin:~$ perl -pi.bak -e 's/bgcolor=#ffffff/bgcolor=#000000/i' *.html
This will change the background color of all html pages in the
current directory from white to black. Don't forget
that trailing i to make the match case insensitive
(to match bgcolor=#FFFFFF or even BGColor=#FfFffF).
What if you're in the middle of working on a
CVS project, and need to change the
CVS server that you'd like to commit to?
It's easy, if you pipe the output of a
find through an xargs
running a perl -pi -e:
schuyler@ganesh:~/nocat$ find -name Root | xargs perl -pi -e
's/cvs.oldserver.com/cvs.newserver.org/g'
Then reset your $CVSROOT and do your CVS check in as normal, and your
project will automagically end up checked into
cvs.newserver.org.
Using Perl from the command line can help you do some powerful
transformations on the fly. Study your regular expressions, and use
it wisely, and it can save piles of hand edits.
