UNIX source vs. binary (NOT A LEGAL ARGUMENT)

[Rob Warnock]

+---------------
| Sorry, that won't work on v7 systems, where you can't execute a shell
| file with exec(2), which is what login uses (execlp, actually).
| Dave Sherman | {utzoo pesnta nrcaero utcs}!lsuc!dave
| {allegra decvax ihnp4 linus}!utcsri!lsuc!dave
+---------------

Well, I know that in 4.1bsd "execlp" will run a shell script
(and on my F-box here, too), but where is it documented? A look
at "exec(2)" in a V.7 manual reveals:

	Execlp and execvp are called with the same arguments as
	execl and execv, but duplicate the shell's actions in
	searching for an executable file in a list of directories.

I have always read this to mean that in addition to using "$PATH",
execlp/execvp also duplicates the shell's check for legal "magic
numbers" (see "a.out(5)") and assumes anything else is a shell
script (the 4.?bsd "#!" cookie is treated as a magic number).
Checking "sh(1)" we see:

	Execution
	...If the file has execute permission but is not an "a.out"
	file, it is assumed to be a file containing shell commands.

But having my curiosity aroused, I went back to "exec(2)", read further
on and found:

	BUGS
	If execvp is called to execute a file that turns out to be
	a shell command file, and if it is impossible to execute the
	shell, the values of argv[0] and argv[-1] will be modified
	before return.

To be sure, there are clearer ways to say it explicitly (rather than
as a side effect of describing a bug), but that pretty much settles it --
Version 7 "execlp/execvp" DOES work (exec a shell) for shell scripts.

(Additional minor quibble: given the amount of diddling around it does,
execlp/execvp should probably be with the other library routines as
"exec(3)", rather than with the "pure" system calls in "exec(2)".)


Rob Warnock
Systems Architecture Consultant

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