"DevOps Odyssey": Day2.1 - Basic Linux Commands

1. Linux:

   • Linux is an open-source operating system that originated from Unix.

   • It provides a robust and stable platform for running various applications and services.

   • Linux is highly customizable and offers a wide range of distributions (e.g., Ubuntu, CentOS, Fedora) to suit different needs.

2. Shell:

   • A shell is a command-line interpreter that acts as a user interface to interact with the operating system.

   • The shell processes commands and executes programs or scripts.

   • In Linux, the default shell is typically Bash (Bourne Again Shell), but there are others like Zsh and Fish.

3. Shell Scripting:

   • Shell scripting involves writing a series of commands in a file (script) that the shell can execute.

   • Shell scripts are written in plain text using a text editor.

   • They can automate repetitive tasks, execute system commands, and combine multiple commands into complex workflows.

4. Script Execution:

   • To execute a shell script, you need to make it executable using the chmod command (e.g., chmod +x script.sh).

   • Run a shell script by entering its filename or path in the terminal (e.g., ./script.sh).

5. Variables:

   • Shell scripts use variables to store data. Variable names are case-sensitive and typically uppercase by convention.

   • You can assign values to variables using the assignment operator (e.g., name="John").

   • Access the value of a variable using the $ symbol followed by the variable name (e.g., $name).

6. Control Structures:

   • Shell scripts support control structures like conditional statements (if-else) and loops (for, while).

   • Conditional statements help make decisions based on certain conditions.

   • Loops allow executing a set of commands repeatedly until a condition is met.

7. Input/Output:

   • Shell scripts can read input from users using the read command.

   • Output can be displayed on the terminal using the echo command.

   • You can redirect input/output using special characters, such as > (output to a file) and | (pipe output to another command).

8. Command Substitution:

   • Command substitution allows capturing the output of a command and using it as a value.

   • It can be done using the $() syntax or backticks () (e.g., result=$(command) or result=command).

9. Functions:

   • Shell scripts can define functions to group a set of commands and reuse them.

   • Functions are declared using the function keyword or just the function name.

   • They can accept arguments and return values.

10. Debugging:

    • You can enable debugging mode in a shell script by adding set -x at the beginning.

    • Debugging mode displays each executed command, which helps in troubleshooting issues.

Basic Commands

Listing Commands

• ls - List the subdirectories and files available in the present working directory.

• ls -l - List the files and directories in long list format with extra information, such as permissions, owner, group, size, and modification date.

• ls -a - List all files and directories, including hidden files and directories (those whose names begin with a dot '.')

• ls *.sh - List all the files having a .sh extension in the present working directory.

• ls -i - List the files and directories along with their index numbers (inodes).

• ls -d */ - List only directories in the present working directory, using a specific pattern to filter them.

  

Directory Commands

• pwd - Print the present working directory, i.e., the directory you are currently in.

• cd path_to_directory - Change the current directory to the specified path.

• cd ~ or just cd - Change the current directory to the home directory.

• cd - - Go to the last working directory (the directory you were in before the last cd command).

• cd .. - Move up one level in the directory hierarchy (change to the parent directory).

• cd ../.. - Move up two levels in the directory hierarchy (change to the grandparent directory).

• mkdir directoryName - Make a new directory with the given name in the present working directory.

• mkdir .NewFolder - Make a hidden directory (beginning with a dot) in the present working directory.

• mkdir A B C D - Make multiple directories with the names 'A', 'B', 'C', and 'D' in the present working directory.

• mkdir /home/user/Mydirectory - Make a new directory named 'Mydirectory' in the specified location (/home/user/ in this example).

• mkdir -p A/B/C/D - Make a nested directory structure (create directories 'A', 'A/B', 'A/B/C', and 'A/B/C/D') even if the parent directories don't exist.