(This is the official perlfaq answer, minus any subsequent edits)
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> )
{
s/\b(perl)\b/Perl/g;
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' inFile.txt
To delete lines, only print the ones that you want.
perl -ni -e 'print unless /d/' inFile.txt