exec doesn't allow command lines which begin with an env variable assignment
The documentation for exec states: "If single string is given as the command, it is taken as a command line that is subject to shell expansion before being executed." However, command lines that begin with environment variable assignments do not seem to work. For example:
$ install/bin/ruby -e 'exec "FOO=1 echo bar"'
exec': No such file or directory - FOO=1 echo bar (Errno::ENOENT) from -e:1:in '
Other examples of shellish syntax (not a filename) for the first word do work.
This same problem seems to also afflict all the other constructs for running an external command: system, backticks, IO.popen, spawn. I'm reporting against 1.9.3, but all previous versions of ruby (s far back as 1.8.6) that I've tried have the problem as well.