A shell is a command interpreter. This is not limited to Unix. The programs COMMAND.COM or CMD.EXE are also shells in the DOS/Windows environments. The program EXPLORER.EXE is also a shell, though with a graphical interface.
There is no "default" Unix shell. Different Unix vendors shipped different shells.
William Holliker has written: 'UNIX Shell commands quick reference' -- subject(s): UNIX (Computer file), UNIX Shells
Backticks in PHP are simply shorthand for calling shell_exec on their contents, as in most Unix shells. See the related link.
See related link
The X Windowing System is the "de facto" standard for graphical shells in Unix and Linux. Desktops use it as a client as well as all the graphical applications.
Different shells are not required in Unix; they happen to be available to users to switch to if they wish. Different shells have different environments, depending on what type of user you are. Some users prefer one shell environment over another but it is merely personal choice, not a requirement.
Douglas W. Topham has written: 'A system V guide to UNIX and XENIX' -- subject(s): UNIX System V (Computer file), XENIX (Computer file) 'The first book of UNIX' 'UNIX and XENIX' -- subject(s): UNIX (Computer file), XENIX
1. System V 2. BSD 3. countless unix-like, unix-based, unix-compatible, unix-inspired systems (linux, AIX, Sinix, Xenix, Dynix, Solaris, MacOs etc)
Phyllis Eve Bregman has written: 'UNIX System V performance management' -- subject(s): UNIX System V (Computer file)
Rachel Morgan has written: 'Why Doesn't Mommy Love Me?' 'Introducing UNIX System V' -- subject(s): UNIX System V (Computer file)
Don Crabb has written: 'Macspeed' 'Running UNIX so it doesn't run you' -- subject(s): Computer programs, UNIX Shells, UNIX device drivers (Computer programs) 'Getting the Most Out of Ninja'
System V