The `forbacktickx` program

forbacktickx runs a program and uses its output as loop elements to run another program.

Interface

In an execlineb script:

     forbacktickx [ -p | -x breakcode ] [ -n ] [ -C | -c ] [ -0 | -d delim ] variable { gen... } loop...

forbacktickx reads a block, gen..., and unquotes it.
It runs gen... as a child process. gen's output must not contain a null character.
It reads gen's output as it needs, splitting it automatically.
For every argument x in the split output, forbacktickx runs loop... as a child process, with variable=x added to its environment.
forbacktickx then exits 0.

-p : parallel mode. Do not wait for a loop... instance to finish before spawning the next one. forbacktickx will still wait for all instances of loop to terminate before exiting, though.
-0 : accept null characters from gen's output, using them as delimiters. If this option and a -d option are used simultaneously, the rightmost one wins.
-x breakcodes : breakcodes must be a comma-separated list of exit codes. If at some point loop... exits with a code listed in breakcodes, forbacktickx will not keep looping, but will exit immediately with the same exit code. This doesn't apply if the -p option has been given.
Other options are used to control the substitution mechanism for every x. Of course, you can't split x.

You can start loop... with "import variable unexport variable" to perform variable substitution.