Linux Upskill Challenge

https://linuxupskillchallenge.org
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Linux Upskill Challenge is a month-long free and open-source course teaching practical Linux server...
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### Run nmap from different installation paths

Source: https://linuxupskillchallenge.org/17

This example shows how to execute the `nmap` utility by specifying its full path, demonstrating how the system resolves commands based on the PATH environment variable. It allows testing different versions of the same utility installed in separate locations.

```bash
/usr/bin/nmap -V
/usr/local/bin/nmap -V
```

--------------------------------

### System Crontab Configuration Example

Source: https://linuxupskillchallenge.org/10

This example shows the structure of the system-wide crontab file located at /etc/crontab. It includes comments explaining the time fields and demonstrates how to define cron jobs, specifying the user (e.g., 'root') that will execute the command. It also shows examples of hourly, daily, weekly, and monthly job execution.

```bash
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
# You can also override PATH, but by default, newer versions inherit it from the environment
#PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name command to be executed
17  *  *  *  *  root      cd / && run-parts --report /etc/cron.hourly
25  6  *  *  *  root      test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47  6  *  *  7  root      test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52  6  1  *  *  root      test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

```

--------------------------------

### Install Software Packages with APT

Source: https://linuxupskillchallenge.org/04?q=

This command installs a specified package from the online repositories. It requires root privileges, hence the use of 'sudo'. The system will download and configure the package and its dependencies.

```bash
sudo apt install mc
```

--------------------------------

### Vim: Running Vim Tutor

Source: https://linuxupskillchallenge.org/06

This command initiates the Vim tutorial, a guided exercise designed to teach the fundamentals of Vim. It's recommended for new users to build proficiency.

```bash
vimtutor
```

--------------------------------

### Install rsyslog

Source: https://linuxupskillchallenge.org/08

Installs rsyslog to enable logging on minimal Ubuntu installations. This command ensures the availability of the `/var/log/auth.log` file, which is crucial for the lesson's examples. It addresses a common issue in minimal Ubuntu versions.

```Bash
sudo apt install rsyslog
```

--------------------------------

### Search for Software Packages with APT

Source: https://linuxupskillchallenge.org/04?q=

This command searches the online repositories for packages matching a given keyword. It's the first step before installing any new software on Debian-based systems like Ubuntu. The output lists available packages and their descriptions.

```bash
apt search "midnight commander"
```

--------------------------------

### Install UFW Firewall (Linux)

Source: https://linuxupskillchallenge.org/09

Installs the Uncomplicated Firewall (UFW) utility on Ubuntu-based Linux systems. UFW provides a simpler interface for managing netfilter firewall rules compared to raw iptables commands. This command requires root privileges.

```bash
sudo apt install ufw
```

--------------------------------

### Launch Midnight Commander (mc)

Source: https://linuxupskillchallenge.org/04?q=

This command starts the Midnight Commander file manager. It provides a text-based interface for navigating the file system, copying, moving, and editing files. It's useful for exploring directories and viewing file contents.

```bash
mc
```

--------------------------------

### Install and Use 'at' for One-Shot Tasks (Ubuntu)

Source: https://linuxupskillchallenge.org/10

This snippet demonstrates how to install the 'at' package on Ubuntu and schedule a one-time command to execute in the future. It shows how to pipe a command to 'at' and verifies the output.

```bash
sudo apt update
sudo apt install at

vagrant@ubuntu2204:~$ tty
/dev/pts/0

vagrant@ubuntu2204:~$ echo 'echo "Greetings $USER!" > /dev/pts/0' | at now + 1 minutes
warning: commands will be executed using /bin/sh
job 2 at Sun May 26 06:30:00 2024

...
vagrant@ubuntu2204:~$ Greetings vagrant!

```

--------------------------------

### Getting Concise Command Help with `tldr`

Source: https://linuxupskillchallenge.org/02

The `tldr` command provides simplified, example-driven explanations for commands, making it easier to quickly understand common use cases. It's useful when man pages are too verbose.

```bash
$ tldr pwd
pwd
Print name of current/working directory.More information: https://www.gnu.org/software/coreutils/pwd.

 - Print the current directory:
   pwd

 - Print the current directory, and resolve all symlinks (i.e. show the "physical" path):
   pwd -P
```

--------------------------------

### View APT Repository Sources

Source: https://linuxupskillchallenge.org/04?q=

This command uses Midnight Commander to view the contents of the Ubuntu sources list file. This file specifies the locations from which APT downloads packages, often including mirror sites.

```bash
mc /etc/apt/sources.list.d/ubuntu.sources
```

--------------------------------

### Cron Job Script Example for Daily Tasks

Source: https://linuxupskillchallenge.org/10

This script, located in `/etc/cron.daily/dpkg`, is an example of a task that runs daily. It checks if systemd is running and exits if it is. Otherwise, it executes the `dpkg-db-backup` command. This illustrates how scripts in the `/etc/cron.daily` directory are executed automatically.

```bash
#!/bin/sh

# Skip if systemd is running.
if [ -d /run/systemd/system ]; then
  exit 0
fi

/usr/libexec/dpkg/dpkg-db-backup

```

--------------------------------

### Getting Help for Shell Builtin Commands with `help`

Source: https://linuxupskillchallenge.org/02

The `help` command is used to display information about shell builtin commands, which are commands implemented directly within the shell itself and may not have a corresponding man page.

```bash
$ man export
No manual entry for export

$ help export
export: export [-fn] [name[=value] ...] or export -p]
    Set export attribute for shell variables.

    Marks each NAME for automatic export to the environment of subsequently
    executed commands.  If VALUE is supplied, assign VALUE before exporting.

    Options:
      -f        refer to shell functions
      -n        remove the export property from each NAME
      -p        display a list of all exported variables and functions

    An argument of `--' disables further option processing.

    Exit Status:
    Returns success unless an invalid option is given or NAME is invalid.
```

--------------------------------

### View File Hierarchy Manual

Source: https://linuxupskillchallenge.org/04?q=

This command displays the manual page for the 'hier' command, which explains the standard Linux directory structure. Understanding this hierarchy is crucial for system administration.

```bash
man hier
```

--------------------------------

### Update and Search File Index with updatedb and locate

Source: https://linuxupskillchallenge.org/17

This snippet demonstrates how to update the file index using `sudo updatedb` and then search for specific files, such as 'bin/nmap', using the `locate` command. This is useful for finding files quickly, especially after manual installations.

```bash
sudo updatedb
locate bin/nmap
```

--------------------------------

### Create a Compressed Home Directory Backup with Date

Source: https://linuxupskillchallenge.org/10

This command manually creates a compressed tar archive of the /home directory, including the current date in the filename. It uses the date -I command to get the current date in ISO 8601 format, which is then embedded in the archive name.

```bash
sudo tar -czvf /var/backups/home.$(date -I).tar.gz /home/
```

--------------------------------

### Basic Vim File Operations

Source: https://linuxupskillchallenge.org/06

These commands demonstrate the initial steps for opening, copying, and editing a file using Vim. It covers navigating directories, copying files, and launching Vim for editing.

```bash
cd
pwd
cp -v /etc/services testfile
vim testfile
```

--------------------------------

### Create and Manage Executable Scripts in Bash

Source: https://linuxupskillchallenge.org/20

This section outlines the process of creating a script file named 'topattack' using vim, setting execute permissions with chmod, and moving it to the system's binary directory using mv. It's a fundamental workflow for deploying custom scripts.

```bash
vim topattack
chmod +x topattack
mv topattack /usr/local/bin
```

--------------------------------

### Create and Configure Admin User Account

Source: https://linuxupskillchallenge.org/00-VPS-small

Create a new user account and grant it administrative privileges. This involves adding the user to the 'admin' and 'sudo' groups, allowing them to execute commands with root privileges using the 'sudo' command. Replace 'snori74' with your desired username.

```shell
adduser snori74
```

```shell
usermod -a -G admin snori74
```

```shell
usermod -a -G sudo snori74
```

--------------------------------

### Vim: Searching and Navigating File

Source: https://linuxupskillchallenge.org/06

Illustrates how to navigate to the beginning ('gg') and end ('G') of a file, and how to search for text using the '/' command followed by the search term. The 'n' key is used to step through search results.

```vim
G
gg
/sun
n
```

--------------------------------

### Upgrade Ubuntu Packages

Source: https://linuxupskillchallenge.org/00-VPS-big

Upgrades installed packages to their latest versions on an Ubuntu system. The '-y' flag automatically confirms any prompts. This command requires administrator privileges.

```bash
sudo apt upgrade -y
```

--------------------------------

### Get VM IP Address

Source: https://linuxupskillchallenge.org/00-Local-Server

This command is used to display network interface information, including the IP address assigned to the virtual machine. It is essential for establishing remote connections.

```bash
ip address

```

--------------------------------

### Searching for Commands by Keyword with `apropos` and `man -k`

Source: https://linuxupskillchallenge.org/02

Both `apropos` and `man -k` allow you to search for commands based on keywords in their descriptions. This is helpful when you know what you want to do but not the exact command name.

```bash
$ apropos "working directory"
git-stash (1)        - Stash the changes in a dirty working directory away
pwd (1)              - print name of current/working directory
pwdx (1)             - report current working directory of a process

$ man -k "working directory"
git-stash (1)        - Stash the changes in a dirty working directory away
pwd (1)              - print name of current/working directory
pwdx (1)             - report current working directory of a process
```

--------------------------------

### Bash Shebang Line

Source: https://linuxupskillchallenge.org/20

The shebang line, starting with '#!', specifies the interpreter for the script. For bash scripts, it typically points to '/bin/bash'. This line is crucial for the system to execute the script correctly.

```bash
#!/bin/bash
```

--------------------------------

### Connect to Server via SSH with Private Key (Linux/macOS)

Source: https://linuxupskillchallenge.org/00-Local-Server

This command establishes an SSH connection to a remote server using a specific private key file. It's a more secure alternative to password authentication, especially when the private key is passphrase-protected.

```bash
ssh -i private_key username@ip_address

```

--------------------------------

### Vim: Saving Changes

Source: https://linuxupskillchallenge.org/06

Demonstrates commands for saving changes to a file. ':w' writes the changes and stays in Vim, while ':wq' writes the changes and then quits the editor.

```vim
:w
:wq
```

--------------------------------

### Enable UFW Firewall (Linux)

Source: https://linuxupskillchallenge.org/09

Activates the Uncomplicated Firewall (UFW) on a Linux system, applying the configured rules. Once enabled, UFW starts enforcing the defined policies for incoming and outgoing network traffic. This command requires root privileges.

```bash
sudo ufw enable
```

--------------------------------

### Accessing Manual Pages with `man`

Source: https://linuxupskillchallenge.org/02

The `man` command is used to display the manual page for a given command, providing detailed syntax and usage information. It's the primary tool for in-depth documentation.

```bash
man pwd
man cp
man mv
man grep
man ls
man man
```

--------------------------------

### Exit Midnight Commander

Source: https://linuxupskillchallenge.org/04?q=

Pressing F10 within Midnight Commander exits the application. On some systems, like macOS Terminal, you might need to use ESC+0 as an alternative.

```bash
F10
```

--------------------------------

### Understanding Crontab Syntax

Source: https://linuxupskillchallenge.org/10

This snippet provides a clear breakdown of the standard crontab syntax, explaining each field (minute, hour, day of month, month, day of week) and the meaning of common operators like '*', ',', '-', and '/'.

```text
* * * * * command to be executed
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

```

--------------------------------

### Pipe and Filter with grep

Source: https://linuxupskillchallenge.org/08

Piping allows the output of one command to be used as input for another. This example chains `cat` and `grep` to search for a specific string within a file. This is useful for combining commands to refine search results.

```Bash
cat /var/log/auth.log  | grep "authenticating"
```

--------------------------------

### Filter with grep and cut

Source: https://linuxupskillchallenge.org/08

This example combines `grep` and `cut` to extract specific fields from lines matching a pattern.  The `cut` command is used to select specific columns based on a delimiter. This is useful for extracting structured data from text.

```Bash
grep "authenticating" /var/log/auth.log| grep "root"| cut -f 10- -d" "
```

--------------------------------

### Connect to Server via SSH (Linux/macOS)

Source: https://linuxupskillchallenge.org/00-Local-Server

This command initiates an SSH connection to a remote server using the provided username and IP address. It's the standard method for remote server management on Unix-like systems.

```bash
ssh username@ip_address

```

--------------------------------

### Invert Selection with grep -v

Source: https://linuxupskillchallenge.org/08

The `-v` option inverts the `grep` selection, showing lines that do *not* match the given pattern. This is useful for excluding specific entries from the output. This example excludes lines containing "root".

```Bash
grep "authenticating" /var/log/auth.log| grep -v "root"| cut -f 10- -d" "
```
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