What is Linux?

• Linux is a powerful, open-source operating system that serves as the backbone of countless technologies we use every day.

• It was created by Linus Torvalds in 1991 and is based on the Unix operating system.

• Unlike proprietary operating systems, Linux is freely available for anyone to use, modify, and share.

• At its core, Linux is a kernel - the fundamental part of an OS responsible for managing hardware and system resources.

• Surrounding this kernel are various distributions (or "distros"), such as Ubuntu, Fedora, and CentOS, which package the kernel with user-friendly tools and applications.

• Linux is renowned for its stability, security, and flexibility, making it a favorite among developers, system administrators, and organizations for servers, cloud computing, embedded systems, and more.

Working with THE SHELL

• The Linux Shell is a program that allows text-based interaction between the user and the operating system. It is a command-line interface (CLI) that acts as an interpreter between the user and the operating system.

• This interaction is carried out by typing commands into the interface and receiving response in the same way.

The HOME Directory

• When you log into the shell, the very first directory you are taken to is the home directory.

• The home directory in Linux is a personal space on the file system which allows users to store their personal data in the form of files and directories.

• Each user in the system gets their own unique home directory with complete access to it, to be able to save, retrieve and delete data.

• The home directory is typically located under the /home directory (/home us system create directory). For example:

  • If your username is rohit, your home directory will be /home/rohit.

  • System administrators (root users) usually have their home directory as /root.

• The ~ (tilde)symbol is shorthand for the home directory, so you can reference it like this:

    $ ls ~
  

Command and Arguments

• A command in Linux is an instruction given to the shell to perform a specific action.

  • For example: The echo command is used to print a line of text on the screen.

    $ echo

• An argument acts as an input to the command.

  • For example: To print a hello message type echo hello command.

    $ echo hello

• Many of the commands needs arguments to work, though it is not mandatory. There are several commands that work without arguments too.

  • For example: Type in the command called uptime, this is used to print information about how long a system has been running for since the last reboot along with the other information

    $ uptime

• A command can also have options to modify its behavior in some predetermined way.

• The option, also sometimes referred to as a switch or a flag, is usually start with a - (single character) or -- (full word).

  • For example: To print a same word hello but without a trailing line, use echo command with -n flag.

    $ echo -n hello

Command Types

• Commands in Linux can be generally categorized into two types:

  1. Internal or Build-in Commands:

     • This are part of the shell itself and come bundled with it.

     • There are total of 30 such commands.

     • For example: echo, cd, pwd, set, etc.

  2. External Commands:

     • This are binary programs or scripts which are usually located in distinct files in the system.

     • They either come pre-installed with the distributions package manager or can be created or installed by the user.

     • For Example: mv, date, uptime, cp, etc.

• To determine whether the command is internal or external, use the type command.

    type echo                        # command
    echo is a shell build-in         # output
  
    type mv                          # command
    mv is hashed (/bin/mv)           # output
  

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Basic Linux Commands

Commands:

Directory Navigation Commands

• Directory navigation commands provide shortcuts to navigate to the desired location quickly.

File Commands

• File commands help with file and directory management on the system. Create, delete, move, and modify files and directories from the terminal

System Management and Information Commands

• Use the terminal to manage the system directly. The commands show how to view basic system information, change options, and reboot or restart the system.

Absolute Path and Relative Path

• Absolute Path : An absolute path is defined as specifying the location of a file or directory from the root directory(/).

• Relative Path : Relative path is defined as the path related to the present working directly(pwd).

Alternatives to the cd command

• Alternative to the cd is the pushd\popd command.

• The pushd command is used to save the current directory into a stack and move to a new directory. Furthermore, popd can be used to return back to the previous directory that is on top of the stack.

• To change directory using pushd, run pushd <directory_name>

$ pushd /etc

• You can change to subdirecties under /etc as many times as you wish

$ pushd /var
$ pushd /tmp
$ pwd
/etc/var/tmp

• To return back to origin directory(say your home directory), use the popd command

$ popd

Pagers

• In Linux, a pager is a utility that displays the output of commands or the contents of a file one screen (or page) at a time.

• It’s especially useful when dealing with long outputs that don’t fit entirely on your terminal screen.

Common Pagers in Linux

1. more:

   • With more command, we can view the text file in a scrollable manner.

   • This command loads the entire file at once, and is not the best choice when dealing with large files

       more largefile.txt
     

2. less:

   • Less command, allows you to view the contents of a file and navigate through the file.

       less largefile.txt
     

• Key Controls in Pagers:

Command-line help

• whatis: This command displays a one line description of what a command does.

    $ whatis date
  

• man: Most of the commands internal or external come bundled with man pages which provides information about the command in detail (with examples, usecases and with command options).

    $ man date
  

• --help: Several commands will provide -h or --help to provide users with the options and use cases available in a command.

    $ date -h
    $ date --help
  

• apropos: To search through the man page names and descriptions for instances of the keyword use apropos. This is useful if you want to look for all commands within the system that contains the specific keyword.

    $ apropos directories
  

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Bash Shell

Types of Shell

1. Bourne Again Shell (bash)

   • The most widely used shell on Linux systems which supports command history, functions and auto-completion.

   • Bash supports command history and autocompletion.

       history            # to get a list of previously run commands
     

   • Can set aliases for the actual commands.

       alias dt=date       # setting dt as an alias for date command
     

   • An improved version of the original Bourne Shell (sh).

   • Bash is often the default shell on many Linux distributions.

2. Bourne Shell (sh)

3. C Shell (csh or tcsh)

4. Korn Shell (ksh)

5. Z Shell (zsh)

To Check Your Current Shell

• You can find out which shell you are using by running:

echo $SHELL

How to Switch Between Shells

• To switch to a different shell, type its name in the terminal (if installed). For example:

zsh  # Switch to zsh
bash # Switch to bash

• To make a shell your default, you can use the chsh command:

chsh -s /bin/sh rohit      # change shell from bash to sh for user rohit

Bash Environment Variables

• Environment variables, specifically stores the information about the user’s login session, which is use by the shell when executing commands.

• To see the list of all environment variables.

    env                   # to see the list of all environment variables
  

• To set the environment variable (this set the variable for the current shell and any other process or programs started by the shell)

    export NAME=Rohit     # to set the environment variable (this set the variable for the current shell and any other process or programs started by the shell)
  

• To set the environment variable (this only apply the variable within the shell, the value is not carried forward by any other process)

    NAME=Rohit            # to set the environment variable (this only apply the variable within the shell, the value is not carried forward by any other process)
  

• To make the environment variables persistent over persistent login or reboots (add them to ~/.profile or ~/.pam_environment file.

Path Variable

• When a user issues an external command into the shell, the shell uses path variable to search for these external commands.

• To see the directories defined in path variable. Use the command echo $PATH.

    $ echo $PATH
  

• To check if the location of the command can be identified. Use the which command:

    $ which obs-studio
  

• To define a command in the PATH variable, we can use the export command.

    $ export PATH=$PATH:/opt/obs/bin
    $ which obs-studio
  

Customize Bash Prompt

• Once you login into the shell, the line you see is the bash prompt.

    [~]$        # ~ = present working directory
                # $ = user prompt symbol
  

• The bash prompt is control by a special shell environment variables: PS1

    [~]$ echo $PS1       
    [\W]$                   # \W = ~ = present working directory
                            # $ = user prompt symbol
  

• To change the PS1 to only display the word ubuntu-server: (To customize further, have a look at the below special character).

  

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