The Format of the #! Line

We can say for certain that the first 2 characters must be "#!". Or can we? Many systems, it seems, are willing to delete leading white space. My recommendation is to start with #!. It's traditional.

Next there may be an optional space. Some documentation says this space is required but as far as anyone can determine the only Unix release to require the space was a snapshot release of BSD 4.1... this was not a general release). Actually, it appears that you may have several spaces if you want. And some testing with TAB characters has been done and seems to work. My recommendation is to stay with zero or one spaces.

Next comes the full path to the interpreter and like all full paths, it must start with a /. Oops, another exception... The Linux kernel (at least version 2.0.34) is willing to accept a relative path. My recommendation is don't do that.

We may be done. Or we may have optional white space which lead to our single argument. Except that some versions of FreeBSD handle multiple arguments.

Most versions of BSD and HP-UX will strip trailing white space. Other versions of Unix treat trailing white space as valid characters. And a few versions of BSD can accept a trailing comment delimited by a # character.

How long can the line be? A few versions of Unix set the limit as low as 32 characters. FreeBSD can apparently handle 8192 characters.

At least the line always ends with the Unix standard \n character, right? Well, not always. Some versions of Unix will tolerate a \r\n ending and strip off the \r while others won't do that.

This is not as standard as it could be...

Argument 0 of The Process, Not The Script

There is another way that implementations may differ. Consider the perl script that I ran ar the end of part 1. My shell did the approximate equivalent of
execl("./perlargs", "./perlargs", "one", "two", "three", (char *) NULL)
and the kernel transformed it into the approximate equivalent of
execl("/usr/local/bin/perl", "/usr/local/bin/perl", "-w", "./perlargs", "one", "two", "three", (char *) NULL)

Highlighted in red is argument zero which by convention is the same as the path of the program being executed. A notable execution is that the login program will set it to stuff like "-ksh". Originally, executable shell scripts had the argument 0 set to the name of the script rather than the name of the interpreter. These days, the name of the interpreter is common. The last hold-out I know of is HP-UX which sets argument 0 to the name of the script.