$)$ if the command is "exit"

then exit the program $($to be terminated$)$

else if the command is "listall"

then you replace the input by $"$ls $-$la$"$

else if the command is "compile program.c$"$ where program.c is hello.c

then you replace the input by $"$gcc hello.c $-$o hello"

$(3)$ let child process to run the input command via exec call

repeat the loop