FAQ 5.2 How do I change, delete, or insert a line in a file, or append to the beginning of...

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5.2: How do I change, delete, or insert a line in a file, or append to the
beginning of a file?

    (contributed by brian d foy)

    The basic idea of inserting, changing, or deleting a line from a text
    file involves reading and printing the file to the point you want to
    make the change, making the change, then reading and printing the rest
    of the file. Perl doesn't provide random access to lines (especially
    since the record input separator, $/, is mutable), although modules such
    as "Tie::File" can fake it.

    A Perl program to do these tasks takes the basic form of opening a file,
    printing its lines, then closing the file:

            open my $in,  '<',  $file      or die "Can't read old file: $!";
            open my $out, '>', "$file.new" or die "Can't write new file: $!";

            while( <$in> )
                    print $out $_;

       close $out;

    Within that basic form, add the parts that you need to insert, change,
    or delete lines.

    To prepend lines to the beginning, print those lines before you enter
    the loop that prints the existing lines.

            open my $in,  '<',  $file      or die "Can't read old file: $!";
            open my $out, '>', "$file.new" or die "Can't write new file: $!";

            print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC

            while( <$in> )
                    print $out $_;

       close $out;

    To change existing lines, insert the code to modify the lines inside the
    "while" loop. In this case, the code finds all lowercased versions of
    "perl" and uppercases them. The happens for every line, so be sure that
    you're supposed to do that on every line!

            open my $in,  '<',  $file      or die "Can't read old file: $!";
            open my $out, '>', "$file.new" or die "Can't write new file: $!";

            print $out "# Add this line to the top\n";

            while( <$in> )
                    print $out $_;

       close $out;

    To change only a particular line, the input line number, $., is useful.
    First read and print the lines up to the one you want to change. Next,
    read the single line you want to change, change it, and print it. After
    that, read the rest of the lines and print those:

            while( <$in> )   # print the lines before the change
                    print $out $_;
                    last if $. == 4; # line number before change

            my $line = <$in>;
            $line =~ s/\b(perl)\b/Perl/g;
            print $out $line;

            while( <$in> )   # print the rest of the lines
                    print $out $_;

    To skip lines, use the looping controls. The "next" in this example
    skips comment lines, and the "last" stops all processing once it
    encounters either "__END__" or "__DATA__".

            while( <$in> )
                    next if /^\s+#/;             # skip comment lines
                    last if /^__(END|DATA)__$/;  # stop at end of code marker
                    print $out $_;

    Do the same sort of thing to delete a particular line by using "next" to
    skip the lines you don't want to show up in the output. This example
    skips every fifth line:

            while( <$in> )
                    next unless $. % 5;
                    print $out $_;

    If, for some odd reason, you really want to see the whole file at once
    rather than processing line-by-line, you can slurp it in (as long as you
    can fit the whole thing in memory!):

            open my $in,  '<',  $file      or die "Can't read old file: $!"
            open my $out, '>', "$file.new" or die "Can't write new file: $!";

            my @lines = do { local $/; <$in> }; # slurp!

                    # do your magic here

            print $out @lines;

    Modules such as "File::Slurp" and "Tie::File" can help with that too. If
    you can, however, avoid reading the entire file at once. Perl won't give
    that memory back to the operating system until the process finishes.

    You can also use Perl one-liners to modify a file in-place. The
    following changes all 'Fred' to 'Barney' in inFile.txt, overwriting the
    file with the new contents. With the "-p" switch, Perl wraps a "while"
    loop around the code you specify with "-e", and "-i" turns on in-place
    editing. The current line is in $_. With "-p", Perl automatically prints
    the value of $_ at the end of the loop. See perlrun for more details.

            perl -pi -e 's/Fred/Barney/' inFile.txt

    To make a backup of "inFile.txt", give "-i" a file extension to add:

            perl -pi.bak -e 's/Fred/Barney/' inFile.txt

    To change only the fifth line, you can add a test checking $., the input
    line number, then only perform the operation when the test passes:

            perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt

    To add lines before a certain line, you can add a line (or lines!)
    before Perl prints $_:

            perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt

    You can even add a line to the beginning of a file, since the current
    line prints at the end of the loop:

            perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt

    To insert a line after one already in the file, use the "-n" switch.
    It's just like "-p" except that it doesn't print $_ at the end of the
    loop, so you have to do that yourself. In this case, print $_ first,
    then print the line that you want to add.

            perl -ni -e 'print; print "Put after fifth line\n" if $. == 5'

    To delete lines, only print the ones that you want.

            perl -ni -e 'print unless /d/' inFile.txt

                    ... or ...

            perl -pi -e 'next unless /d/' inFile.txt


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