What does it mean in linux scripts? #!/usr/bin/python -tt

Question

I know that in the begining of .sh bash scripts is

#!/bin/bash

which points to the command interpeter executable.

But during watching Google Python Class http://www.youtube.com/watch?v=tKTZoB2Vjuk I noticed that for python they use

#!/usr/bin/python -tt

. Surfing the Internet I also have found such styles of this notation:

#!/usr/local/bin/python

and even

#!/usr/bin/env python

.

So, I'm new with Python and I'm ordinary Linux user and I have a few questions about this "magic" line:

1. First of all, what is the right form of this line? and why?
2. What does -tt key means in #!/usr/bin/python -tt ?
3. What program is parsing this line in Linux?
4. What syntax of this line for any script?
5. Why this line is so necessary if each file have it's extension?
6. And what about that in each computer interpreter for some kind of scripts will be stored in different place than in another? And script couldn't be run.

It's really interesting to me. What's this line? Why this line? How to write this line? Why in such a way?...

Answer 1

Question #1) The line is called a shebang, and there's no right form that works universally. e.g.

#!python
#!/usr/bin/python
#!/usr/local/bin/python
#!/usr/bin/python -t

are all valid/acceptable forms, but may not work on all systems:

#!python will work only if the python executable is somewhere in your shell's PATH

#!/usr/bin/python only works if the python binary is actually in /usr/bin

#!/usr/local/bin/python also only works if python is in /usr/local/bin

Question #2)

#!/usr/bin/python -tt is passing the -tt option to python, as if you'd done:

$ python -t somescript.py

at the shell prompt. You can pass arbitary command line arguments to the interpreter on the shebang line.

Question #3)

The line is interpreted by the OS kernel and the shell you're currently using. The stuff after the #! simply tells the OS which program should be fired up to "execute" the rest of the script.

Question #4)

The script syntax depends on the language you're using. E.g. a PHP shell script must take the form of

#!/usr/bin/php
<?php
  ... php code here ...

A #!/usr/bin/perl perl script must use Perl syntax, etc... If you put PHP code with a Perl shebang, you'll just have Perl barf up the script with syntax errors, as PHP code is NOT perl code

Question #5)

Shebangs are for Unix systems, where file extensions were never really used to identify file types to the OS. A .c file was understood to be a C language source code file, but that's merely a convention. You could put a Bash shell script into a .c file, make it executable, and with the #!/bin/bash shebang, it would execute as a Bash script.

Determining executable types by file extension is more of a Windows thing.

Question #6)

That goes back the stuff in question #1 - if the shebang claims the interpreter is at some OTHER path than where it is, this particular script can't be executed until the shebang is fixed, or the interpreter is moved. Shebangs are very handy, but not infallible.

Thankfully, most interpreters are installed in fairly standard locations these days, so it'd be somewhat unusual to find (say) Perl installed at /some/wonky/weird/path instead of /usr/bin