Question

I am trying to do a dynamic search and replace with Perl on the command line with part of the replacement text being the output of a grep command within backticks. Is this possible to do on the command line, or will I need to write a script to do this?

Here is the command that I thought would do the trick. I thought that Perl would treat the backticks as a command substitution, but instead it just treats the backticks and the content within them as a string:

perl -p -i -e 's/example.xml/http:\/\/exampleURL.net\/`grep -ril "example_needle" *`\/example\/path/g' `grep -ril "example_needle" *`

UPDATE:

Thanks for the helpful answers. Yes, there was a typo in my original one-liner: the target file of grep is supposed to be *.

I wrote a small script based on Schewrn’s example, but am having confusing results. Here is the script I wrote:

 #!/usr/bin/env perl -p -i

my $URL_First = "http://examplesite.net/some/path/";
my $URL_Last = "/example/example.xml";

my @files = `grep -ril $URL_Last .`;
chomp @files;

foreach my $val (@files) {
        @dir_names = split('/',$val);

        if(@dir_names[1] ne $0) {

            my $url = $URL_First .  @dir_names[1] . $URL_Last;

            open INPUT, "+<$val" or die $!;

            seek INPUT,0,0;

            while(<INPUT>) {
                    $_ =~ s{\Q$URL_Last}{$url}g;
                    print INPUT $_;
                    }
            close INPUT;
            }
    }

Basically what I am trying to do is:

1. Find files that contain $URL_Last.
2. Replace $URL_Last with $URL_First plus the name of the directory that the matched file is in, plus $URL_Last.
3. Write the above change to the input file without modifying anything else in the input file.

After running my script, it completely garbled the HTML code in the input file and it cut off the first few characters of each line in the file. This is strange, because I know for sure that $URL_Last only occurs once in each file, so it should only be matched once and replaced once. Is this being caused by a misuse of the seek function?

Answer 1

Everyone’s answers were very helpful to my writing a script that wound up working for me. I actually found a bash script solution yesterday, but wanted to post a Perl answer in case anyone else finds this question through Google.

The script that @TLP posted at http://codepad.org/BFpIwVtz is an alternative way of doing this.

Here is what I ended up writing:

#!/usr/bin/perl

use Tie::File;

my $URL_First = 'http://example.com/foo/bar/';
my $Search = 'path/example.xml';
my $URL_Last = '/path/example.xml';

# This grep returns a list of files containing "path/example.xml"
my @files = `grep -ril $Search .`;
chomp @files;

foreach my $File_To_Edit (@files) {

# The output of $File_To_Edit looks like this: "./some_path/index.html"
# I only need the "some_path" part, so I'm going to split up the output and only use @output[1] ("some_path")
    @output = split('/',$File_To_Edit);

# "some_path" is the parent directory of "index.html", so I'll call this "$Parent_Dir"
    my $Parent_Dir = @output[1];

# Make sure that we don't edit the contents of this script by checking that $Parent_Dir doesn't equal our script's file name.
    if($Parent_Dir ne $0) {

            # The $File_To_Edit is "./some_path/index.html"
            tie @lines, 'Tie::File', $File_To_Edit or die "Can't read file: $!\n";
            foreach(@lines) {
                    # Finally replace "path/example.xml" with "http://example.com/foo/bar/some_path/path/example.xml" in the $File_To_Edit
                    s{$Search}{$URL_First$Parent_Dir$URL_Last}g;
                    }
            untie @lines;
            }
    }