Inserting headers into multiple files

I found some command line with Perl that inserts headers into my files without going through the tedious process of inserting them one by one. Can someone walk me through the Perl aspect of this command line? I'm new to this and can't seem to find the right explanations for what I wrote.

cat header.txt | perl -0 -i -pe 'BEGIN{$h = <STDIN>}; print $h' 1*

Solution

-e rather than provide a script in a xxxx.pl file, provide it on the command line

-p makes it iterate over filename arguments somewhat like sed but also prints the contents of $_ at the end of the script.

the two above are combined in -pe

-i indicate you want to edit the file in place and write the output to the same file. In practice, Perl renames the input file and reads from this renamed version while writing to a new file with the original name

-0 redefines the end of record character (\n by default) so that you can read the entire input file as a single line

1* is the command line argument to your script, so I guess you are modifying any file with a name that starts with 1 (you could have used *.c, or whatever depending on the type of files you are trying to modify)

print $h prints the variable $h that is the "main" of your script. if it was initialized with the content of the header file (the intent of this one-liner) then it will print the header file

BEGIN{ some code here } this is stuff you execute before the script starts. this is where I'm stumped. this doesn't seem like valid perl code

so basically:

this will supposedly slurp the entire header file (because of -0) in the BEGIN block and store it in the variable $h
iterate over all the files specified by the wildcards at the end of the command line
for each file: print the header (print $h) then print hte file itself (because of -pe)

so it's equivalent to spelling the script out:

$h = gets content of the entire header file
while (<>){ #loop implied by -pe, iterates over all the 1* files
    # the main contents of the "-e" script are inserted below as part of executing -pe
    print h$; #print the header we saved 
    print $_; # implied by -pe, and since we are using -0, this prints the entire content in one shot
    # end of the "-e" script. again it was a single print $h statement, the second print is implied by -pe
}

It's a bit hard to explain, take a look at the perlrun documentation for details (run man perlrun).

This is not 100% complete explanation because I don;t think the BEGIN block is right. I tried it on my ubuntu machine and it complained about its syntax too