# 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.