Shell Bash Ksh

The shell is an interactive command interpreter. The shell is independent of the operating system, which gives the user the flexibility to choose the right shell for them. The shell lets you type commands at the command line, which is interpreted by the shell and routed to the operating system (kernel) for execution.
The Shell is a command processor-a program that reads in and interprets the commands you enter. In addition to being a command interrupt, the shell is a programming language. You can write programs that the shell can interpret (known as source programs), which can contain shell programming commands and so on.   In addition to explaining commands, the shell has other tasks, which can also be configured and programmed.
the shell has its own language that allows the user to write programs and run in a complex way. The Shell programming language has many common programming language features, such as looping and control structures. Users can generate shell programs that are as complex as other applications.
here is a summary of the shell features:
find the location of the command and execute the associated program;
assigns a new value to the shell variable;
perform command substitution;
handles I/O redirection and piping functions;
provides an explanatory programming language interface, including statements such as tests, branches, and loops.

BASH is the acronym for the borne again shell, which is a shell, and the default on Linux is bash.
When you enter a bash command on the command line, it is equivalent to entering a bash environment, and if it is a bash environment, it is a child bash environment (equivalent to a child process).

Enter Ksh carriage return in bash, enter Ksh, equivalent to child shell, exit can use Ctrl+d, return bash, enter bash under Ksh can switch to bash, of course, exit is also ctrl+d.

Shell Bash Ksh