Unix/Linux specific non-Perl solution

If you are using Linux or Unix the cat command can be used together with the >> redirection:

cat 1.txt 2.txt 3.txt >> target.txt

This will work regardless if target.txt existed before or not.

Platform independent solution with Perl

If you need to combine several files, and you want to make sure this will work on other platforms as well (most importantly MS Windows), then you can read the files in memory and then write them out appending to the target file.

Even that we should probably do line-by-line, in order to preserve memory. Instead of reading the input files in memory at once, we will read one line, the print it out to the target file. Then we read in another file and print it out. This will work even with huge file. File way bigger than the available memory in the computer.

examples/merge_files.pl

use strict;
use warnings;

my $target = pop @ARGV;
my @sources = @ARGV;

die "Usage: $0 in in ... in  out\n" if not @sources;

open my $out, '>>', $target or die "Could not open '$target' for appending\n"; 
foreach my $file (@sources) {
    if (open my $in, '<', $file) {
        while (my $line = <$in>) {
            print $out $line;
        }
        close $in;
    } else {
        warn "Could not open '$file' for reading\n";
    }
}
close $out;

print "done\n";

If we run this script using perl merge_files.pl it will print out: Usage: merge_files.pl in in ... in out

Maybe not the best way to indicate, but the idea is that the user has to provide on the command-line a list of input files and as the last element, the name of the output file.

so let's run perl merge_files.pl 1.txt 2.txt 3.txt target.txt (Note, we don't need the >&ht; on the command-line here.)

This will do the job.

In the script, after adding the safety net the first thing we do is pop the last element of the @ARGV array that holds the content of the command-line. This is the name of the $target file. Then we copy all the other values to the @sources array. We don't have to do this, we could have used @ARGV directly, but I thought it is clearer if we have our data in descriptive variable names.

die ... checks if there is at least one element in the @sources array. If there is non, it will print out the usage message and quit. We don't need to check separately if $target has a value, because if $target was empty then surely @sources will be empty too.

The next step is to open the target (or output file) for appending. We use the >> here.

Then we iterate over the elements of the @sources array, each file we open for reading, read the lines one-by-one, from the $in file-handle, and write them out one-by-one to the $out file-handle.

Calling close is not really required, but at least the close $out is strongly recommended. Even without that Perl will write out everything to the disk, but only when it feels like it. If you want to make sure everything is written to the disk before you reach the print "done\n"; statement, then you have to call close $out;.

Why the different error handling?

You might have noticed that in the first call to open I used the open ... or die ... construct and it the second call to open, that will be executed multiple times, I used if (open ...) { } else { warn ... } construct.

The reason for this difference is that if we cannot open the target file, we don't have any more business running this code, so I through an exception using die. This will end the program right there.

On the other hand, if one of the input files is missing, or not readable that should probably not terminate the whole process. Therefore if one of those calls to open failed then we should just report about it calling warn, and we should start working on the next file.

Of course, what exactly should happen in such case will be dependent on your requirements, this is just an example that fits one use-case.

Extra safety - error checking

If you want to be cautious, you could, and probably should check if every write operation and if every close was successful by writing print $out $line or die ... and close $out or die ..., or by using autodie.

This will help you catch the cases when the disk got full or when someone removed the external hard-disk or pen-drive while you were trying to write to it.