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 -u variable" to perform variable substitution.