add initial 3 lessons

lesson1.md

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+# Basic Linux Commands (Bash)
+
+Almost every software engineer, sysadmin, and DevOps person uses the terminal daily.
+This lesson takes you from never having used Linux to being comfortable navigating
+and managing files on a machine. You can practice on a Raspberry Pi, a cloud VM,
+or any Linux distro — the commands are the same.
+
+## 1. What is a terminal?
+
+A terminal is a text window where you type a command and press Enter. The program
+that reads your commands is called a **shell**. On most Linux systems (including
+Raspberry Pi OS) it's **Bash**.
+
+To open one on the Pi, press `Ctrl+Alt+T` or find "Terminal" in the menu.
+
+Commands in this lesson go in code boxes. Type exactly what's in the box and press
+Enter. Anything in a box by itself is a command; anything after `#` is a comment
+explaining it — don't type that part.
+
+## 2. Your first commands
+
+Run these one by one and read the output:
+
+```
+whoami          # prints your username
+pwd             # prints your current folder (your "address")
+ls              # lists the files in your current folder
+date            # prints the date and time
+```
+
+Linux organizes everything as files and folders in one big tree. The very top
+folder is `/` (the root), and your personal folder (like `/home/pi`) is your home.
+
+## 3. Moving around
+
+```
+ls            # what's in this folder
+ls -la        # everything, including hidden files, with details
+cd downloads  # go into a folder (no space between cd and the name)
+cd ..         # go up one level
+cd ~          # go to your home folder (cd with no argument does the same)
+```
+
+Tip: type the first few letters of a folder name and press **Tab** — Linux
+autocompletes it for you. Use Tab constantly.
+
+## 4. Files and folders
+
+```
+touch notes.txt                 # create an empty file
+mkdir projects                  # create a folder
+mkdir -p projects/web/app       # create nested folders in one go
+cp notes.txt backup.txt         # copy a file
+mv notes.txt projects/          # move a file (or rename it)
+rm backup.txt                   # delete a file
+rm -r projects                  # delete a folder and everything inside it
+```
+
+Warning: `rm` does not ask "are you sure?" and there is no trash bin. Deleted is
+deleted. Read your command carefully before pressing Enter — especially `rm -rf`,
+which deletes recursively without mercy.
+
+## 5. Looking at files
+
+```
+cat notes.txt     # print the whole file
+head notes.txt    # first 10 lines
+tail notes.txt    # last 10 lines
+less notes.txt    # scroll through a big file (press q to quit)
+```
+
+## 6. Writing to files
+
+```
+echo "hello" > notes.txt        # write (overwrites the file if it exists)
+echo "second line" >> notes.txt # append (>> adds to the end)
+```
+
+## 7. Editing files: nano
+
+`nano` is the standard beginner editor. Open a file with:
+
+```
+nano notes.txt
+```
+
+Now you can edit. The keys you need are shown at the bottom of the screen:
+
+- `Ctrl+O` then `Enter` — save (it calls saving "Write Out")
+- `Ctrl+X` — quit
+
+If it asks "Save modified buffer?" when you quit, press `y`, then `Enter`.
+
+## 8. Searching and pipes
+
+```
+grep "hello" notes.txt    # print only the lines containing "hello"
+ls | grep notes           # list only files whose name contains "notes"
+```
+
+That `|` is called a **pipe**. It takes the output of the command on the left and
+feeds it as input to the command on the right. Piping small tools together is one
+of the core ideas of Linux — small commands that each do one thing, chained up.
+
+## 9. Getting help
+
+```
+ls --help     # short summary of how to use a command
+man ls        # full manual page (press q to quit)
+```
+
+When you don't know a command, `man` is the first place to look. It's on the
+machine itself, so it works even without internet.
+
+## 10. Installing software: apt
+
+On Raspberry Pi OS (and Ubuntu/Debian), software comes in packages managed by
+`apt`. `sudo` means "run this as the administrator" and it will ask for your
+password:
+
+```
+sudo apt update              # refresh the list of available packages
+sudo apt install -y curl     # install a package (the -y means "yes to prompts")
+sudo apt remove curl         # remove a package
+```
+
+## 11. About your system
+
+```
+uname -a      # what machine and kernel this is
+df -h         # how much disk space is left
+free -h       # how much memory is in use
+history       # everything you've typed so far
+clear         # clear the screen
+```
+
+## 12. Shortcuts that feel like magic
+
+- **Tab** — autocomplete a command or filename
+- **Up/Down arrows** — cycle through your previous commands
+- **Ctrl+C** — stop whatever is currently running
+- **Ctrl+L** — clear the screen (same as `clear`)
+- **Ctrl+R** — search your command history: start typing the old command, keep
+  pressing Ctrl+R to cycle through matches, Enter to run it
+
+## Practice
+
+Try these from memory (scroll up only when you're stuck):
+
+1. Create a folder called `lesson1` and go into it.
+2. Create a file `hello.txt` containing the line `hello world`.
+3. Append a second line: `second line`.
+4. Print only the lines containing `hello`.
+5. Copy `hello.txt` to `hello-backup.txt`, then delete the original.
+6. Print the first line of `hello-backup.txt`.
+
+If you can do all six without looking, you know the essentials: `pwd`, `ls`, `cd`,
+`touch`, `nano`, `cat`, `rm`, `cp`, `mv`, `grep`, and pipes. That's the toolkit
+you'll reach for on any Linux machine.