The `background` program

background launches a command in the background, then goes on with the execution flow.

Interface

     background [ -d ] ~prog1... ; prog2...

or in an execlineb script:

     background [ -d ] { prog1... } prog2...

background reads a prog1... command in a block and unquotes it.
It spawns a child executing prog1....
It sets the LASTPID environment variable to the pid of the prog1... process.
It then execs into prog2....

-d : doublefork. If the -d option is set, prog1... will run as a grandchild of background. You can use this option together with pgrphack on prog1... to completely separate it from the main execution flow.

background ~prog1... ; prog2... is equivalent to sh -c 'prog1... & ; exec prog2...'.