In mksh (and pdksh), 'exec' looks up shell functions and builtins before
external commands, and if it finds one it appears to do the equivalent
of running the function or builtin followed by 'exit'. This turns out[1]
to be a bug in POSIX terms; 'exec' is supposed to launch a program that
overlays the current shell[2], implying the program launched by 'exec'
is always external to the shell.
This means that
(exec commandname arguments ...)
is a POSIXly correct way of guaranteeing the execution of an external
command without specifying the path.
In pdksh this is definitely a bug as this behaviour differs from ksh88
which pdksh is supposed to be a clone of. The POSIX spec is also based
on ksh88 behaviour.
This behaviour also appears to be contrary to the documentation in
mksh(1) ("The command is executed without forking, replacing the shell
process"). The test script below demonstrates that neither "exec"
replaces the shell process as the first executes a shell function and
the second (within the function) executes a mksh builtin.
testFn() {
exec print "this shell execs both functions and builtins"
}
PATH=/dev/null
exec testFn
Expected output: something like "testFn: not found".
Actual output: "this shell execs" etc.
- M.
[1] https://www.mail-archive.com/austin-group-l@opengroup.org/msg01469.html
[2]
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_20_14
