The `forx` program

forx runs a loop.

Interface

In an execlineb script:

     forx [ -p | -x breakcodes ] variable { args... } loop...

forx reads a block and unquotes it. That block contains a list of args.
For each argument x in args..., forx runs loop as a child process, with variable=x added to its environment.
forx then exits 0.

-p : run in parallel. Do not wait for an instance of loop... to exit before spawning the next one. forx will still wait for all instances of loop to terminate before exiting, though.
-x breakcodes : breakcodes must be a comma-separated list of exit codes. If the -p flag hasn't been given and loop exits with one of the codes in breakcodes, forx will not run the following instances of the loop, but exit immediately with the same exit code.

You can start loop with "import -u variable" if you want variable substitution.