UNIX for Dummies Questions & Answers

i'm trying to figure out a script that uses sed, and i'm not totally sure if it does what I think it does.
The script...
- takes in 3 inputs, $1, $2 are names. $3 is a file.
- filename is a file.

Here is what I'm trying to figure out:

cat $3 | grep "id17" > var2
sed "s|@@.*||g" var2 > var3
sed "s/^/filename $1 $2 /g" var3 > $3x
chmod a+x $3x
$3x

so does this mean that...
1. it will cat $3 and then pipe it into grep. Grep will find all the lines that contain id17, and then output it to file var2.
2. sed will substitute all words in var2 that contain @@ in the word. It will do this for the entire line. Then it will output the results to var3.
3. sed will substitute all words in var3 ,that begin with $1 and $2, with the filename. It will do this for the entire line. Then it will output the results to a file that has the same name as file $3, but with an x in it.
4. the file $3x will be chmod to be executable.
5. run file $3x.

thanks,
gammaman

sed "s/^/filename $1 $2 /g" var3 > $3x

sed will replace the beginning of the line with "filename $1 $2 ". In other words, this gets prepended to the line.