Linux Essentials — getting started with Linux

Linux Essentials covers the terminal, files, permissions, packages, processes, services and networking. Based on the attached 28-slide presentation, with expanded explanations and commands for independent practice.

1. Unix, GNU, Linux and distributions

Unix is the historical foundation; GNU provides tools and free software; Linux is the kernel managing CPU, memory, disks, devices and networking. A distribution combines the kernel, system tools, packages and configuration into a usable system. Ubuntu is based on Debian; Fedora and RHEL belong to the Red Hat family. Drivers connect the kernel to hardware.

• Commercial software is sold for payment and can also be open source. Freeware is free to use but not necessarily open source; shareware is a trial or limited version.
• Open source provides source-code access under a license. FSF promotes user freedoms; OSI defines open-source principles. GPL requires preserving its license terms when distributing covered derivative works.

User → Terminal → Shell (Bash) → Programs → Linux kernel → Hardware
Linux distribution
├── Kernel + drivers
├── GNU and other system tools
├── Package manager (apt / dnf)
└── Configuration and applications

2. Terminal, shell and help

The terminal is the window; the shell interprets commands and launches programs. Bash is common; zsh and fish are alternatives. /etc/shells lists registered shells. Unlike CMD, Linux paths use / and names are case-sensitive: Notes.txt and notes.txt differ. uname -r shows the kernel version; /etc/os-release identifies the distribution.

uname -r
cat /etc/os-release
cat /etc/shells
whoami
id
ls --help
man ls

3. Paths, files and directories

pwd shows the current directory; ls -la includes hidden files, owners and permissions; cd changes directories. A path starting with / is absolute; other paths are relative to the current directory. . means current, .. parent and ~ home. Quote names containing spaces.

mkdir -p ~/linux-practice/project/logs ~/linux-practice/backup
cd ~/linux-practice
pwd
ls -la
mkdir "My notes"
touch project/notes.txt
cp project/notes.txt project/notes-copy.txt
mv project/notes-copy.txt project/renamed.txt
cp -r project backup/
ls backup/project
rm -i project/renamed.txt

• mkdir -p creates parent directories. touch creates an empty file if absent, otherwise updates timestamps. cp copies; mv moves or renames.
• If backup already exists, cp -r project backup/ creates backup/project. Repeating it may overwrite files, so inspect the destination first.
• /home holds user homes; /etc configuration; /var/log logs; /tmp temporary files; /root the root user's home. A name starting with a dot is hidden from ordinary ls output.

4. Editing, searching and output redirection

nano notes.txt opens the editor if installed: Ctrl+O then Enter saves; Ctrl+X exits. Use cat for a short file and less for paged reading (q exits). grep searches text inside files; find searches for files by attributes. grep -i, -n, -r mean ignore case, show line numbers and search recursively.

cd ~/linux-practice
printf "INFO: started\nERROR: test\n" > project/logs/app.log
printf "INFO: finished\n" >> project/logs/app.log
cat project/logs/app.log
grep -in "error" project/logs/app.log
grep -rin "error" project/logs/
find . -type f -name "*.log"
head -n 5 project/logs/app.log
tail -n 5 project/logs/app.log
cat project/logs/app.log | grep "ERROR"
tail -f project/logs/app.log

• > replaces existing content; >> appends. | sends output to the next command. && runs the next command only if the previous one succeeded.
• tail -f waits for new lines; stop it with Ctrl+C. The grep example should show 2:ERROR: test. Quoting *.log prevents the shell from expanding the pattern before find runs.

5. Users, permissions and sudo

ls -l shows type, permissions and owner. The three permission groups are owner, group and others: r=4 read, w=2 write, x=1 execute. On directories, x permits traversal; deleting a file depends on parent-directory permissions. chmod changes permissions; chown changes owner and group.

-rw-r-----  →  640
-   regular file (d = directory)
rw- owner: 4 + 2 = 6
r-- group: 4     = 4
--- others:     = 0

600 → private file
644 → owner reads/writes; others read
700 → private directory
755 → owner has full access; others read/execute

cd ~/linux-practice
touch project/private.txt
chmod 600 project/private.txt
ls -l project/private.txt
whoami
id

6. Packages and processes

On Debian/Ubuntu, apt update refreshes package information, not installed programs. apt install installs; apt upgrade upgrades installed packages; apt remove removes them. Review proposed changes before confirming. ps aux lists processes and PIDs; top shows live resource use (q exits). kill PID sends SIGTERM for graceful termination; kill -9 sends SIGKILL as a last resort.

sudo apt update
sudo apt install tree
tree ~/linux-practice
ps aux
top

7. Services, logs, networking and SSH

A service is a background program. systemctl status checks state; start starts it now; enable configures boot-time startup but does not necessarily start it immediately. journalctl reads systemd logs. The examples require an SSH server: Ubuntu/Debian use openssh-server and the ssh service; Fedora/RHEL usually use sshd.

systemctl status ssh
journalctl -u ssh -n 20 --no-pager
ip a
ip r
ping -c 4 8.8.8.8
getent hosts example.com

ip a shows addresses and interfaces; ip r shows routes and the gateway. ping checks ICMP responses; no response does not prove a server is down. getent hosts checks name resolution using system settings, including /etc/hosts and DNS. For an authorized server, substitute the user and address in ssh user@server-ip; verify the service, port, credentials or key and server fingerprint.

• No such file: check pwd, path and case. command not found: check spelling, installation and PATH.
• Permission denied: check id and ls -l; for SSH also check user and authentication. Connection refused: check the listening service and port; a firewall may also reject the connection.

8. Independent practice — 15 minutes

In the practice VM, create a workspace, log, backup and private file. Run once; if you already followed the examples above, inspect files first to avoid overwriting important content.

mkdir -p ~/linux-practice
cd ~/linux-practice
mkdir -p project/logs backup
printf "INFO: started\nERROR: test\n" > project/logs/app.log
grep -in "error" project/logs/app.log
cp -r project backup/
find . -type f -name "*.log"
touch project/private.txt
chmod 600 project/private.txt
ls -l project/private.txt

• grep prints 2:ERROR: test; find locates the log and its backup copy.
• ls -l shows -rw------- for private.txt. Your user owns it; the empty file has size 0.
• Explain what changes if you replace >> with >, and why cp differs from mv. Test only on a disposable file.

9. Your first Bash script

In your practice directory, open nano hello.sh, enter the following text and save. The first line selects the interpreter; printf prints text and a newline.

#!/bin/bash
printf "Hello, Linux!\n"

After saving, grant the owner execute permission and run it. ./ refers to the current directory; expected output is Hello, Linux!.

chmod u+x hello.sh
./hello.sh