# OS-9: Linux Client Tools

Status: not started

Domain:
- 1.0 Operating Systems

Objective alignment:
- 1.9 Linux client tools

## What You Need To Know

Linux questions on Core 2 usually test:
- Basic file navigation
- Permissions and ownership
- Important configuration files
- Package managers
- Network commands
- Process and disk usage commands
- The difference between normal user and root/admin actions

## Memory Tricks

Command buckets:
- **Files**: `ls`, `pwd`, `mv`, `cp`, `rm`, `find`, `cat`
- **Permissions**: `chmod`, `chown`, `sudo`, `su`
- **Network**: `ip`, `ping`, `curl`, `dig`, `traceroute`
- **System**: `top`, `ps`, `df`, `du`, `mount`, `fsck`
- **Help**: `man`

Key files:
- `/etc/passwd`: user account list
- `/etc/shadow`: password hashes
- `/etc/hosts`: local name-to-IP mappings
- `/etc/resolv.conf`: DNS resolver settings
- `/etc/fstab`: file systems mounted at boot

Memory hook:
- **PASS users, SHADOW passwords, HOSTS names, RESOLV DNS, FSTAB mounts.**

## Linux Concepts

Root:
- The all-powerful administrative account.
- User ID `0`.

`sudo`:
- Runs one command with elevated privileges.
- Safer than staying logged in as root.

`su`:
- Switches to another user, often root.
- You remain that user until you exit.

Kernel:
- Core of the operating system.
- Manages hardware, memory, and processes.

Bootloader:
- Starts the operating system during boot.

systemd:
- System and service manager.
- Starts and manages services, login sessions, logging, and other system processes.

## Commands To Enter

Safe commands:

```bash
pwd
```

What it does:
- Prints the current working directory.

```bash
ls
```

What it does:
- Lists files and directories.

```bash
ls -l
```

What it does:
- Lists files with permissions, owner, group, size, and date.

```bash
cat /etc/os-release
```

What it does:
- Shows Linux distribution details.

```bash
cat /etc/passwd
```

What it does:
- Shows local user account entries.
- Each line includes username, UID, GID, home directory, and shell.

```bash
cat /etc/hosts
```

What it does:
- Shows local hostname-to-IP mappings.

```bash
cat /etc/resolv.conf
```

What it does:
- Shows DNS resolver settings.

```bash
cat /etc/fstab
```

What it does:
- Shows file systems configured to mount at startup.

```bash
grep root /etc/passwd
```

What it does:
- Searches `/etc/passwd` for lines containing `root`.

```bash
find . -name "*.txt"
```

What it does:
- Finds `.txt` files under the current directory.

```bash
ip addr
```

What it does:
- Shows network interfaces and IP addresses.

```bash
ip route
```

What it does:
- Shows routes, including the default gateway.

```bash
ping -c 4 127.0.0.1
```

What it does:
- Sends four pings to the local loopback address.

```bash
curl https://example.com
```

What it does:
- Retrieves data from a URL.

```bash
dig example.com
```

What it does:
- Queries DNS for detailed domain information.
- If `dig` is not installed, try `nslookup example.com`.

```bash
traceroute example.com
```

What it does:
- Shows the route packets take to a destination.
- If not installed, use `tracepath example.com` if available.

```bash
top
```

What it does:
- Shows live process and resource usage.
- Press `q` to quit.

```bash
ps aux
```

What it does:
- Shows running processes.

```bash
df -h
```

What it does:
- Shows mounted file systems and free space in human-readable units.

```bash
du -h
```

What it does:
- Shows disk usage for files/directories.

```bash
man grep
```

What it does:
- Opens the manual page for `grep`.
- Press `q` to quit.

## Practice File Commands

Use these in a temporary folder:

```bash
mkdir linux-practice
cd linux-practice
echo "Core 2 Linux practice" > notes.txt
cp notes.txt copy.txt
mv copy.txt renamed.txt
ls -l
grep Linux notes.txt
chmod u+x renamed.txt
ls -l
cd ..
rm -r linux-practice
```

What they do:
- `mkdir` creates a directory.
- `cd` changes directory.
- `echo ... > file` writes text to a file.
- `cp` copies a file.
- `mv` moves or renames a file.
- `grep` searches inside a file.
- `chmod u+x` adds execute permission for the owner.
- `rm -r` removes a directory and its contents.

## Admin Commands To Know

Do not run these casually on important systems:

```bash
sudo chown user:group file
```

What it does:
- Changes file owner/group.

```bash
sudo apt update
sudo apt install package-name
```

What it does:
- Updates package lists and installs software on Debian/Ubuntu-based systems.

```bash
sudo dnf install package-name
```

What it does:
- Installs software on Fedora/Red Hat-based systems.

```bash
sudo fsck /dev/device
```

What it does:
- Checks and repairs a file system.
- Usually run on unmounted or read-only volumes.

```bash
sudo mount /dev/device /mnt
```

What it does:
- Mounts a storage device to a directory.

## Windows Comparisons

- `ls` is like `dir`.
- `pwd` is like checking your current path in Command Prompt/PowerShell.
- `top` and `ps` are like Task Manager process views.
- `df -h` is like checking drive free space.
- `fsck` is like `chkdsk`.
- `traceroute` is like Windows `tracert`.
- `dig` is like `nslookup`, but usually more detailed.

## Mini Lab

Goal:
- Practice common Linux commands safely.

Linux:
1. Run `cat /etc/os-release`.
2. Run `pwd`.
3. Run `ls -l`.
4. Run `cat /etc/passwd`.
5. Run `cat /etc/hosts`.
6. Run `cat /etc/resolv.conf`.
7. Run `ip addr`.
8. Run `ip route`.
9. Run `df -h`.
10. Run `ps aux`.
11. Run `top`, then press `q`.
12. Create and remove the `linux-practice` folder from the practice command section.

Record:
- Distribution:
- Current directory:
- Current user:
- DNS server:
- Default gateway:
- Root filesystem free space:
- One running process:
- What permission changed after `chmod u+x`:

Windows comparison:
1. Run `dir`.
2. Run `taskmgr`.
3. Run `tracert example.com`.
4. Run `nslookup example.com`.
5. Record which Linux commands match those Windows tools.

## Quick Check Before Quiz

You are ready for the OS-9 quiz when you can answer these without looking:
- Which file lists user accounts?
- Which file stores password hashes?
- Which command changes file permissions?
- Which command shows live process/resource usage?
- Which command shows disk free space?
- Which package manager is common on Ubuntu/Debian?
- Which command gives help/manual pages?