Why nano and rnano are different?

Question

I tried to edit files by using nano and rnano. The latter is really in restrict mode, but they are exactly same binary files. Why do they behave differently?

# which nano rnano
/usr/bin/nano
/bin/rnano

# md5sum /usr/bin/nano /bin/rnano
fa670e309a033718bad4b2051f5974fd  /usr/bin/nano
fa670e309a033718bad4b2051f5974fd  /bin/rnano

(In ubuntu 12.04 x64 LTS)

Answer 1

They behave differently because of the argument vector (argv), whose first element (argv[0]) contains the name of the file being executed.

Because rnano is a separate file from nano (even though it is just a symbolic link), it has its own, separate argv[0].

You can see this check in nano.c's main function:

/* If the executable filename starts with 'r', enable restricted
 * mode.
 */
if (*(tail(argv[0])) == 'r')
    SET(RESTRICTED);

You can also test this with a simple shell script. Create a shell script with only one statement, echo $0. Then, create a symbolic link to it with a separate name. Observe the difference.