PERL Beginners - Replace a string in a text file

Author

Replace a string in a text file

Mr Mojo

2004-03-18, 6:22 pm

Hello,

I have a text file (texst1.txt) that reads:

The quick brown fox jumped over the lazy dog.

I would like to replace the word fox with coyote. Is there an easier/simpler/

er way to do this?

#!/usr/bin/perl -w
use strict;
open (FILE, "test1.txt");
open (NEWFILE, ">>tmp");
while (<FILE> ) {
if ($_ =~ /fox/) {
s/fox/coyote/;
}
print NEWFILE $_;
}
close FILE;
close NEWFILE;
unlink "test1.txt";
rename "tmp", "test1.txt";

Thanks,
Greg
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James Edward Gray II

2004-03-18, 6:22 pm

On Mar 10, 2004, at 12:56 PM, Mr Mojo wrote:

> Hello,
>
> I have a text file (texst1.txt) that reads:
>
> The quick brown fox jumped over the lazy dog.
>
> I would like to replace the word fox with coyote. Is there an
> easier/simpler/

er way to do this?
>
> #!/usr/bin/perl -w
> use strict;
> open (FILE, "test1.txt");
> open (NEWFILE, ">>tmp");
> while (<FILE> ) {
> if ($_ =~ /fox/) {
> s/fox/coyote/;
> }
> print NEWFILE $_;
> }
> close FILE;
> close NEWFILE;
> unlink "test1.txt";
> rename "tmp", "test1.txt";

perl -pi -e 's/fox/coyote' text_file_name_here.txt

Is that

enough? ;)

James

Jayakumar Rajagopal

2004-03-18, 6:22 pm

Use this from unix prompt :

sed s/fox/coyote/g test.txt > tmp

--Jay

-----Original Message-----
From: Mr Mojo [mailto:mojohand@linuxmail.org]
Sent: Wednesday, March 10, 2004 1:57 PM
To: beginners@perl.org
Subject: Replace a string in a text file

Hello,

I have a text file (texst1.txt) that reads:

The quick brown fox jumped over the lazy dog.

I would like to replace the word fox with coyote. Is there an =
easier/simpler/

er way to do this?

#!/usr/bin/perl -w
use strict;
open (FILE, "test1.txt");
open (NEWFILE, ">>tmp");
while (<FILE> ) {
if ($_ =3D~ /fox/) {
s/fox/coyote/;
}
print NEWFILE $_;
}
close FILE;
close NEWFILE;
unlink "test1.txt";
rename "tmp", "test1.txt";

Thanks,
Greg
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John W. Krahn

2004-03-18, 6:22 pm

Mr Mojo wrote:
>
> Hello,

Hello,

> I have a text file (texst1.txt) that reads:
>
> The quick brown fox jumped over the lazy dog.
>
> I would like to replace the word fox with coyote. Is there an
> easier/simpler/

er way to do this?
>
> #!/usr/bin/perl -w
> use strict;
> open (FILE, "test1.txt");
> open (NEWFILE, ">>tmp");
> while (<FILE> ) {
> if ($_ =~ /fox/) {
> s/fox/coyote/;
> }
> print NEWFILE $_;
> }
> close FILE;
> close NEWFILE;
> unlink "test1.txt";
> rename "tmp", "test1.txt";

#!/usr/bin/perl -w
use strict;

{ local ( $^I, @ARGV ) = ( '.bak', 'test1.txt' );
while ( <> ) {
s/\bfox\b/coyote/;
print
}
}

John
--
use Perl;
program
fulfillment

John W. Krahn

2004-03-18, 6:22 pm

James Edward Gray II wrote:
>
> On Mar 10, 2004, at 12:56 PM, Mr Mojo wrote:
>
> perl -pi -e 's/fox/coyote' text_file_name_here.txt

Note that if you are running Windows the -i switch will not work
correctly without a value. Also the closing '/' is missing. :-)

John
--
use Perl;
program
fulfillment

Zsdc

2004-03-18, 6:22 pm

Mr Mojo wrote:

> I have a text file (texst1.txt) that reads:
>
> The quick brown fox jumped over the lazy dog.
>
> I would like to replace the word fox with coyote.
> Is there an easier/simpler/

er way to do this?

You can run this from the command line:

perl -pi -es/fox/coyote/g test1.txt

Or if you want to have a backup in test1.txt~:

perl -pi~ -es/fox/coyote/g test1.txt

or in test1.txt.bak:

perl -pi.bak -es/fox/coyote/g test1.txt

See perldoc perlrun for the command line switches.
If you want to save this script in a file, write:

#!/usr/bin/perl -pi~
s/fox/coyote/g;

and run it with your file as an argument:

./script test1.txt

As for your program, I might have few suggestions:

> #!/usr/bin/perl -w
> use strict;

You might prefer "use warnings" instead of -w for better control of
warnings. See: perldoc perllexwarn

> open (FILE, "test1.txt");
> open (NEWFILE, ">>tmp");

Always add at least 'or die $!' when you are opening files, because it
can fail for many reasons:

open FILE, "test1.txt" or die "Error opening test1.txt: $!\n";
open NEWFILE, ">>tmp" or die "Error opening tmp: $!\n";

You probably want '>' instead of '>>' here. If there already is a file
named 'tmp' opening it with '>>' will append new lines at the end, while
'>' will truncate it first.

See: perldoc perlopentut

If you don't want to add "or die" to every command which can fail, you
may write something like this instead:

use Fatal qw(open close unlink rename);

at the beginning of your program.

> while (<FILE> ) {
> if ($_ =~ /fox/) {
> s/fox/coyote/;
> }
> print NEWFILE $_;
> }

The "if ($_ =~ /fox/)" is redundant, because s/fox/coyote/ will change
only lines with "fox" anyway. s/fox/coyote/ will only change the first
"fox" in every line. Use s/fox/coyote/g to change every one if there are
more foxes in any single line. $_ is default for print. So the above is
equivalent to:

while (<FILE> ) {
s/fox/coyote/;
print NEWFILE;
}

> close FILE;
> close NEWFILE;
> unlink "test1.txt";
> rename "tmp", "test1.txt";

You might also check the return values of unlink and rename to "die" if
they fail:

unlink "test1.txt" or die "Unlinking test1.txt: $!\n";
rename "tmp", "test1.txt" or die "Renamimg tmp: $!\n";

or at least:

unlink "test1.txt" or die $!;
rename "tmp", "test1.txt" or die $!;

Or you can use the trick with "use Fatal" instead.

In the examples I showed above you always had to give file name as an
argument, while the file name is hardcoded into your program. To have
exactly the same functionality, you might write this program:

#!/usr/bin/perl -wpi
BEGIN {@ARGV = 'test1.txt'}
s/fox/coyote/g;

It does the same as yours with only few exceptions: it prints an error
when it can't open the file:

Can't open test1.txt: No such file or directory.

It changes every "fox" to "coyote" and not only the first one in a line
(thanks to the "g" at the end of substitution, see: perldoc perlre) and
it won't include any old "tmp" file if there is any.

The above program even if

(for some definition of

ness) may be
quite hard to understand at first, so if you have any questions I can
explain it in more details.

--
ZSDC Perl and Systems Security Consulting

Zsdc

2004-03-18, 6:22 pm

Jayakumar Rajagopal wrote:

> Use this from unix prompt :
>
> sed s/fox/coyote/g test.txt > tmp

Hey, this is not beginners@sed.org mailing list! ;)

--
ZSDC Perl and Systems Security Consulting

Mr Mojo

2004-03-18, 6:22 pm

Thank you to everyone for your help. I have learned a lot!

Any your answers were WAY

er than mine...

Greg

> perl -pi -e 's/fox/coyote' text_file_name_here.txt
>
> Is that

enough? ;)
>
> James
>

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