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


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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' inFile.txt

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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are not necessarily experts in every domain where Perl might show up,
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