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The bash Shell

A command line is a text-based interface which can be used to input instructions to a computer system. The Linux command line is provided by a program called the shell. Various options for the shell program have been developed over the years, and different users can be configured to use different shells. Most users, however, stick with the current default.
The default shell for users in Red Hat Enterprise Linux is the GNU Bourne-Again Shell (bash). Bash is an improved version of one of the most successful shells used on UNIX-like systems, the Bourne Shell (sh).
When a shell is used interactively, it displays a string when it is waiting for a command from the user. This is called the shell prompt. When a regular user starts a shell, the default prompt ends with a $ character. 
[student@host ~]$
The $ is replaced by a # if the shell is running as the superuser, root. This makes it more obvious that it is a superuser shell, which helps to avoid accidents and mistakes which can affect the whole system. 
[root@host ~]#
Using bash to execute commands can be powerful. The bash shell provides a scripting language that can support automation of tasks. The shell has additional capabilities that can simplify or make possible operations that are hard to accomplish efficiently with graphical tools.

Note:

The bash shell is similar in concept to the command line interpreter found in recent versions of Microsoft Windows cmd.exe, although bash has a more sophisticated scripting language. It is also similar to Windows PowerShell in Windows 7 and Windows Server 2008 R2 and later. Administrators using the Apple Mac who use the Terminal utility may be pleased to note that bash is the default shell in macOS.

Shell Basics

Commands entered at the shell prompt have three basic parts:
  • Command to run
  • Options to adjust the behavior of the command
  • Arguments, which are typically targets of the command

The command is the name of the program to run. It may be followed by one or more options, which adjust the behavior of the command or what it will do. Options normally start with one or two dashes (-a or --all, for example) to distinguish them from arguments. Commands may also be followed by one or more arguments, which often indicate a target that the command should operate upon.
For example, the command line usermod -L morgan has a command (usermod), an option (-L), and an argument (morgan). The effect of this command is to lock the password on user morgan's account.

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