Shell is a user interface for access to an operating system‘s services. Shells use either a command-line interface (CLI) or graphical user interface (GUI), depending on a computer’s role and particular operation.
When we speak of the command line, we are really referring to the shell.
Shell was founded by Stephen R. Bourne and It is named a shell because it is the outermost layer around the operating system kernel. Shell accepts your instruction and command in English and translate it into Computer native binary Language.
The shell is a program that provide interface to the user and takes keyboard commands and passes them to the operating system to carry out. Almost all Linux distributions supply a shell program from the GNU Project called bash. The name “bash” is an acronym for “Bourne Again SHell”, a reference to the fact bash is an enhanced replacement for sh.
When using a graphical user interface, we need another program to interact with the shell, called Terminal Emulators. If we look through our desktop menus, we will probably find one. KDE uses konsole and GNOME uses gnome-terminal, though it’s likely called simply “terminal” on our menu. There are a many other terminal emulators are available for Linux, which provides access to the shell.
After logging in to the shell using username and password above prompt will come, it means shell is ready to take input. If the last character of prompt is “#” rather than a dollar sign, it shows user has logged in via root or have superuser privileges.
Some Simple Commands
A related command is cal which, by default, displays a calendar of the current month.
To display the amount of free memory, enter the free command.