almquist shell ancient unix systems application software bash bell labs berkeley software distribution bill joy bourne shell c shell command-line interface comparison of command shells computer david korn debian debian almquist shell es shell filename extension freebsd friendly interactive shell functional programming gnome gnu gentoo linux graphical user interface internet john mashey kde korn shell linux ms-dos mud microsoft windows netbsd open source operating system posix pwb shell perl scsh shell account shell script symbolic link syntax thompson shell tom duff unix unix-like user interface windowing system windows nt windows powershell xfce z shell