Awk: I want to use the input filename to generate an output file with same name different extension - awk

Question

I have a script that looks like this:
#! /bin/awk -f
BEGIN { print "start" }
{ print $0 }
END { print "end" }
Call the script like this: ./myscript.awk test.txt
Pretty simple - takes a file and adds "start" to the start and "end" to the end.
Now I want to take the input filename, lets call it test.txt, and print the output to a file called test.out.
So I tried to print the input filename:
BEGIN { print "fname: '" FILENAME "'" }
But that printed: fname: '' :(
The rest I can figure out I think, I have this following to print to a hard-coded filename:
#! /bin/awk -f
BEGIN { print "start" > "test.out" }
{ print $0 >> "test.out" }
END { print "end" >> "test.out" }
And that works great.
So the questions are:
how do I get the input filename?
Assuming somehow I get the input file name in a variable, e.g. FILENAME which contains "test.txt" how would I make another variable, e.g. OUTFILE, which contains "test.out"?
Note: I will be doing much more awk processing so please don't suggest to use sed or other languages :))

Answer 1

Try something like this:
#! /bin/awk -f
BEGIN {
file = gensub(".txt",".out","g",ARGV[1])
print "start" > file
}
{ print $0 >> file }
END {
print "end" >> file
close(file)
}
I'd suggest to close() the file too in the END{} statement. Good call to Sundeep for pointing out that FILENAME is empty in BEGIN.

Answer 2

$ echo 'foo' > ip.txt
$ awk 'NR==1{op=FILENAME; sub(/\.[^.]+$/, ".log", op); print "start" > op}
{print > op}
END{print "end" > op}' ip.txt
$ cat ip.log
start
foo
end
Save FILENAME to a variable, change the extension using sub and then print as required
From gawk manual
Inside a BEGIN rule, the value of FILENAME is "", because there are no input files being processed yet

Answer 3

If you're using GNU awk (gawk), you can use the patterns BEGINFILE and ENDFILE
awk 'BEGINFILE{
outfile=FILENAME;
sub(".txt",".out",outfile);
print "start" > outfile
}
ENDFILE{
print "stop" >outfile
}' file1.txt file2.txt
You can then use the variable outfile your the main {...} loop.
Doing so will allow you to process more that 1 file in a single awk command.