Shell

The command-line interpreter

What is Shell

A shell in a computing context refers to a command-line interface that allows users to interact with the operating system by entering commands. It serves as a user interface to the underlying kernel of the operating system, facilitating communication between the user and the computer’s resources. Shells interpret the commands provided by users and execute them, enabling tasks such as file manipulation, process management, and system configuration.

In addition to command-line functionality, shells often offer features like scripting capabilities, allowing users to automate sequences of commands. One of the most widely used shells in Unix-like operating systems is Bash (Bourne Again SHell), known for its powerful scripting capabilities and extensive support in various Linux distributions. Shells provide a flexible and efficient way for users to interact with the system, offering both a simple command-line interface for quick tasks and a scripting environment for more complex automation and customization. Overall, the shell is a fundamental component of the Unix/Linux philosophy, emphasizing the power and simplicity of command-line interfaces.

How to install a shell

On most system it is installed by default, but if you want to install some other ones here how it's done i debian

Install bash
sudo apt install bash
Install zsh
sudo apt install zsh
Install fish
sudo apt install fish
Install dash
sudo apt install dash
Install ksh
sudo apt install ksh

Termomonlogy and Concept

Shell:
- Definition: A shell is a command-line interpreter that provides a user interface for interacting with the operating system. It allows users to enter commands, which it then interprets and executes. Shells play a crucial role in facilitating communication between users and the operating system’s kernel.
Command-Line Interface (CLI):
- Definition: A command-line interface is a text-based interface that allows users to interact with a computer or software by entering commands. Shells provide a CLI, where users can input textual commands to perform various tasks such as file manipulation, process control, and system configuration.
Terminal:
- Definition: A terminal is a program or device that provides a text-based interface for users to interact with the shell. It serves as a window into the shell environment, allowing users to enter commands and view the resulting output. Terminals can be physical devices or software applications.
Prompt:
- Definition: A prompt is a symbol or text displayed by the shell to indicate that it is ready to accept user input. It typically includes information such as the current working directory, username, hostname, and other details. Users enter commands at the prompt to interact with the shell.
Scripting:
- Definition: Scripting refers to the process of writing and executing scripts, which are sequences of commands that can be stored in a file. Shells support scripting, allowing users to automate repetitive tasks, create complex workflows, and customize their interaction with the system. Scripting in shells often involves using control structures, variables, and conditionals.

Folder/files structure and configuration

Shell Configuration Files:
- Location: The main configuration file for the shell is typically found in the user’s home directory and is named according to the shell being used. For example, in Bash, it’s commonly ~/.bashrc or ~/.bash_profile. Other shells may have different configuration files.
Environment Variables:
- Definition: Environment variables are settings that define the behavior of the shell and user environment. They can control aspects such as the default editor, language settings, and search paths for executable files.
- Configuration: Environment variables are often configured and exported in shell configuration files, such as .bashrc or .bash_profile.
Aliases:
- Definition: Aliases are user-defined shortcuts or abbreviations for longer commands. They help customize and simplify the command-line experience.
- Configuration: Aliases are typically configured in shell configuration files. For example, in Bash, you might define an alias in ~/.bashrc like alias ll='ls -l'.
Shell Scripts:
- Definition: Shell scripts are files containing sequences of shell commands that can be executed as a program.
- Configuration: Users often create and customize shell scripts for automating tasks or configuring the environment. These scripts can be placed in directories included in the PATH variable.
Command History:
- Definition: The command history keeps a record of previously entered commands, allowing users to recall and reuse them.
- Configuration: The configuration of command history, including the number of commands to retain and whether to ignore duplicates, is managed through environment variables like HISTSIZE and HISTCONTROL.

Commands

Some commonly used commands used for working with the shell

Edit own crontabs crontab -e
Lists all cron jobs crontab -l
Multiple cronjobs in textfile crontab /path/to/the/file/containing/cronjobs.txt
Deletes all cron jobs crontab -r
Manage other crontabs(may require elevated privileges) crontab -u otherusername -e

DESCRIPTION	COMMAND
Current shell	echo $SHELL echo $0 ps -p $$
List all installed shells	cat /etc/shells
Change shell (May need restart or new terminal)	sudo chsh -s /path/to/shell
Last 20 executed commands	history 20
Repeat last command	!!
System Default Command-line Shell	ls /bin/sh

FAQ

Some commonly used commands used for working with the shell

~/.bashrc
- Purpose: This file is executed every time a new interactive shell (non-login shell) is started. It is used to set up shell-specific settings, environment variables, aliases, and functions that are available for interactive use
- When it is used
  - Typically used for interactive shells (e.g., when you open a new terminal session or run a new bash shell).
  - Not used when logging in via a remote login shell (like SSH) unless explicitly sourced.
- Common Contents:
  - Aliases (alias ll='ls -l')
  - Environment variables (e.g., export PATH=...)
  - Functions
  - Custom prompt settings (e.g., PS1)
  - Shell options and settings (e.g., shopt)
~/.bash_profile
- Purpose: This file is executed once at login (when you log into your system via a console or remotely using SSH). It is generally used to configure settings that should only be set once for the user session, such as environment variables and startup programs.
- When it is use
  - Only used for login shells (e.g., when you log in from a terminal or via SSH).
  - Typically not sourced by non-login shells (like when opening a new terminal window).
- Common Contents:
  - Setting user-specific environment variables (e.g., export PATH=...)
  - Executing commands that need to run once at login (e.g., starting a background process).
  - Sourcing ~/.bashrc to ensure interactive shell settings are applied even in a login shell.
~/.profile
- Purpose: This file is used by sh-compatible shells (like bash, sh, dash, etc.) for setting up environment variables and other settings. It is executed when a login shell starts (for any shell, not just bash).
- When it is used:
  - Used by login shells for all shell types, not just bash. This means it’s used by any shell that is compatible with the sh syntax, such as sh, dash, or bash.
  - It’s not specific to bash, so if you’re using a shell other than bash (like dash or sh), it will still be sourced.
- Common Contents:
  - Setting environment variables (e.g., export PATH=...)
  - Executing startup commands, or sourcing other configuration files like ~/.bash_profile or ~/.bashrc for bash users.

Key Differences:

Feature	`~/.bashrc`	`~/.bash_profile`	`~/.profile`
Shell Type	Bash interactive shells only	Bash login shells only	Used by any login shell, including non-bash shells
When it’s Executed	Every time a new interactive shell is started	Once, when a login shell (e.g., SSH, terminal login) starts	Once, for login shells (sh, bash, dash, etc.)
Common Use	Aliases, functions, prompt settings, interactive shell setup	Environment variables, startup programs, calling `~/.bashrc`	Environment variables, commands that work for any shell
Compatibility	Specific to Bash	Specific to Bash	Works with any login shell (bash, sh, dash, etc.)

Summary:

~/.bashrc: Used for interactive shells, contains aliases, functions, and environment settings.
~/.bash_profile: Used for login shells, typically sources ~/.bashrc to ensure settings from both files are applied.
~/.profile: Used for login shells in any shell type (including non-Bash shells), typically sources ~/.bashrc for Bash users.