Knowing which shell to use is important. Take a quick tour of text input into a terminal gets processed by the shell and sent to the kernel while processing the result and presenting it in a terminal to the user. Most of these shells can be installed on Linux even if Bash is the most popular.
- [Narrator] When you use the keyboard to type in commands into a terminal window, such as GNOME Terminal, these commands are translated by the shell into ones and zeroes and sent to the kernel which then processes them. When the kernel finishes, it sends the data back to the shell which translates the output back into text which is displayed in the terminal emulator and output on the screen. A shell allows you to execute operating system commands. When a command is entered into the terminal, the shell checks to see if it's a built-in first, and if so, executes it.
If it's not built-in, it will see if it's an alias of another command, configured by the OS or another user. If it's not built-in, or an alias, it will look for the command on the disk and try to execute it there. If it doesn't exist, it will say "command not found." There is a standard for Unix-like operating systems called POSIX. If you want to write your shell scripts to be portable, it may be advantageous to write your shell scripts in a POSIX-compliant manner. One of the first shells was the Bourne Shell, which came in out 1977.
It was fairly basic and POSIX-compliant. Bill Joy one of the founders of Sun Microsystems wrote csh or the C Shell, which resembles the C language. The Bourne Shell won out in popularity, and it's not recommended that we write shell scripts in csh anymore. In 1983 ksh or the Korn Shell was introduced adding job control and command history, which we'll talk about later in this course. Later, Bash was introduced as a replacement for Bourne. If started with the correct options, Bash is mostly POSIX compliant.
Bash includes most of the original functionality of the Bourne Shell, most of the additions of Korn Shell, as well as some additional functionality of its own. Bash is probably the default shell on more Unix systems than any other shell. Most systems now run Bash 4. Another popular shell is Dash. Dash is the Debian version of the Ash Shell from NetBSD Unix. Its biggest claim to fame is that it's smaller and uses less memory than Bash. Although it's popular on Debian-based distributions it isn't easily installed on CentOs 7.
If interactivity is your interest you may want to check out Fish. Fish has some interesting features like syntax highlighting on the command line so you know if you've typed in the correct command. It also suggests commands, options and paths. The last shell we'll talk about is Zsh. Zsh has virtually identical features to Bash, but is enhanced in several ways including better command and option completion and better pattern matching. Most of these shells can be installed using yum. For instance, to install Zsh, you'd type into a terminal sudo yum install -y zsh to change your shell, run the change shell command and follow the prompts.
Be sure to type in the whole path to the desired shell when prompted. You type in chsh and hit Enter. If will prompt you for the desired shell. To use the new shell, you'll have to log out and log back in again. There are a lot of interesting shell options in Linux. For interactive use, you should try out the different shells to see what you like. However, if you create shell scripts to automate tasks, the choice of a shell becomes very important as the syntax of built-in design elements like loops, conditionals and arrays will change depending on which shell you use.
Shell scripts written for one shell may not be compatible with another. I would recommend at this time to keep using Bash and later explore other shells.
- Write the command that will take you to the most recent directory.
- Write the command that brings back the arguments and options from the previous line.
- Explain what extended globs can do.
- Identify the access.conf line that will restrict all users from using the cron service except for the user named bob.
- List the line that will get a list of only running services.
- Identify what typing ‘systemctl enable crond’ will do.