The shell is a Command interpreter that interacts directly with the user. It takes commands from the user and carries them out, one at a time. As soon as the user logs onto the system, the shell becomes active. The shell provides the arrangement for the kernel to run a command. The shell thus forms a layer above the kernel and hides it from the user. It interacts with the kernel by passing commands and other information - arguments and parameters to the kernel. The kernel, in turn, loads programs and commands to run.
Following figure shows the importance of shell -
|Diagram for shell|
The shell reads a command line from its standard input and interprets it according to a fixed set of rules. The standard input and standard output file descriptors for the login shell are usually the terminal on which the user logged in.