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Mritunjay Singh
Mritunjay Singh

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Deep dive into Linux

What is Linux

Linux is an open-source operating system modeled on UNIX. It was created by Linus Torvalds in 1991 and has grown to become one of the most popular operating systems worldwide, especially for servers and development environments.

Windows vs Linux

Linux is known for its stability, security, and open-source nature, while Windows is more user-friendly with a large variety of software and hardware support. Linux is preferred in server environments and by developers, while Windows is common on desktops.

What is Kernel

The kernel is the core part of the operating system that manages hardware resources and facilitates communication between software and hardware.

Details of Basic to Advance commands

What is Shebang (#!)?

Shebang is a special character in Linux used to specify which shell to use to execute the script. It is placed in the very first line of the script, followed by the absolute path of the interpreter that will be used to execute the script.

Example:

#!/bin/bash
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Types of Shells

  1. Bash
  2. Sh
  3. Ksh

How to check your current shell?

To check the current shell, run the following command:

echo $SHELL
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To Change Shell

To change the shell:

chsh -s /bin/bash
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What Does ./ Do?

The ./ tells Linux to look in the current directory. Without it, Linux only looks for commands in predefined system paths ($PATH).

Why is ./ Needed to Run Scripts?

By default, Linux does not execute files from the current directory unless you explicitly tell it to.

Example: Trying to run a script without ./:

script.sh
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❌ Error: Command not found

bash: script.sh: command not found
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What If I Don’t Use ./?

  1. Option 1: Use Full Path 
/home/user/myscript.sh
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  1. Option 2: Add the Script to $PATH If you want to run myscript.sh without ./, move it to /usr/local/bin: 
sudo mv myscript.sh /usr/local/bin/myscript
chmod +x /usr/local/bin/myscript
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Now, you can run:

myscript
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from anywhere without ./.

What Does "Adding a Script to $PATH" Mean?

The $PATH variable is a list of directories where Linux looks for executable commands.

Why Add a Script to $PATH?

If you place your script in one of these directories, you can run it from anywhere without using ./.

Example:

  1. Move your script to /usr/local/bin/: 
sudo mv myscript.sh /usr/local/bin/myscript
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  1. Make it executable: 
chmod +x /usr/local/bin/myscript
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  1. Now you can run it from anywhere: 
myscript
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How Linux Finds and Executes Commands

Example 1: Running ls Command

When you type ls, Linux needs to find the ls executable file and run it.

  1. Check if ls is a built-in command (e.g., cd is a built-in shell command).
  2. Check $PATH directories for ls.

To check where ls is located:

which ls
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Output:

/usr/bin/ls
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$PATH and Why It Matters

The $PATH variable contains a list of directories separated by : where Linux searches for commands.

Example output:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
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Finding Where a Command is Located

Use the following commands:

  • which → Shows the full path of an executable 
which vim
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Output:

/usr/bin/vim
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  • type → Tells if a command is built-in or external 
type cd
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Output:

cd is a shell builtin
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  • command -v → Also finds the location of a command 
command -v node
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Output:

/home/mritunjay/.nvm/versions/node/v20.18.2/bin/node
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How to Check If a Script is in $PATH

Use which to check:

which myscript
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If no output is returned, the script is not in a $PATH directory.

History Command in Linux – Deep Dive with Practical Examples

Basic Usage of history

Simply type:

history
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Output:

1  ls
2  cd /var/log
3  cat syslog
4  nano file.txt
5  history
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Search Command History

Press Ctrl + R to search interactively:

  • Press Ctrl + R
  • Start typing a command (e.g., nano)
  • It will show the most recent match.
  • Press Enter to execute it or Esc to edit.

How to Write a Simple Shell Script?

  1. Create a file: 
vim sample-script.sh
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  1. Add the following script: 
#!/bin/bash
# Create a folder
mkdir learning-shell-script
cd learning-shell-script
# Create two files
touch f1 f2
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  1. Save and exit: 
:wq!
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  1. Make the script executable: 
chmod 777 sample-script.sh
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  1. Run the script: 
./sample-script.sh
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You will see the folder learning-shell-script containing f1 and f2.

!q vs !wq

  • !q: Quits without saving.
  • !wq: Saves and quits.

Other System-Related Commands

  • nproc: Shows the number of processing units available.
  • free: Displays memory usage.
  • top: Displays system tasks and resource usage.

Role of Shell Scripting in DevOps

Shell scripting plays a crucial role in DevOps automation, allowing developers to write scripts for tasks such as deployment, monitoring, backups, and configuration management.

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