The `tryexec` program

tryexec executes into a command line, with a fallback.

Interface

In an execlineb script:

     tryexec [ -n ] [ -c ] [ -l ] [ -a argv0 ] { prog1... } prog2...

tryexec reads prog1... in a block. It then executes into it, completely forgetting prog2...
If for some reason the execve() fails - for instance, a non-executable prog1 - then tryexec executes into prog2... instead.

-n : reverse prog1... and prog2...'s role. The latter becomes the main execution path and the former becomes the fallback.

The -c, -l and -a options have the same semantics as with the exec program.

tryexec prog1... "" prog2... would be equivalent to sh -c 'exec prog1... || exec prog2...', if such a shell construct existed. Unfortunately, the shell language does not offer that functionality.