Start with program shellex.c from figures $8\mathrm{.}22-8\mathrm{.}24$ and create your own shell to

$1.$ Maintain variables using a syntax set abc$=$xyz

$2.$ Report all variables by using the command set with not arguments.

$3.$ Use the value of variable PATH as the path of one additional directory where executable

files may be found.

$4.$ Maintain information on all processes running in the background.