Input from a file as a command line argument

[Teemu Luojola]

I have read through Python documentation and tried several searches, but 
I can't find a simple answer to this question.

If I make an executable script in linux environment, how can I pass 
input to that script from a file on command line? Like the following:

$ script.py input.txt

or perhaps

$ script.py < input.txt

I make now a comparison to a Perl script. In Perl I would write the 
following to handle the input (line by line) from a file:

while (<>) {
	## (do something to $_)
}

and I can execute the script simply with

$ script.perl < input.txt

and it would process each line in input.txt.

How would a similar Python code look like?

Teemu Luojola