The `ifthenelse` program

ifthenelse performs a conditional alternative.

Interface

In an execlineb script:

     ifthenelse [ -X ] [ -s ] { progif... } { progthen... } { progelse... } prog...

ifthenelse reads progif..., progthen... and progelse... in 3 consecutive blocks.
ifthenelse runs progif... as a child process and waits for it to complete.
If progif... crashes (i.e. is killed by a signal), ifthenelse prints an error message, then exits 128 plus the number of the signal that killed progif.
If progif... exits zero, ifthenelse runs progthen... as a child process, else it runs progelse....
ifthenelse waits for its child to complete and puts the exit status in the ? environment variable. It then execs into prog....

-X : if progif crashes, do not exit; proceed as if it had returned false.
-s : magic scoping hack. This option does powerful but ugly things, and is left undocumented on purpose.