Shell Expansion

One small complication when running programs from within a shell is that the shell will sometimes substitute a pattern for a set of arguments. For example, consider this run in the Bash shell:

$ ./command.py *.txt

You might expect the following output:

Executable: ./command.py
Arg: *.txt

but this isn't what you get. Instead the output depends on the number of .txt files in the current working directory. When I run it I get this:

Executable: ./command.py
Arg: errors.txt
Arg: new.txt
Arg: output.txt

The Bash shell substitutes the *.txt command for the filenames of all the .txt files in the current directory so your program recieves more arguments than you might have expected.
You can disable shell expansion by quoting the argument, but actually most of the time it is a very useful feature once you are aware of it.

$ ./command.py "*.txt"
Executable: ./command.py
Arg: *.txt