2. Console

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Command	Description
Quick system information
hostnamectl	shows the host name and basic system data (virtual/hardware, hostname)
uname -a	information about the Linux kernel (version, architecture).
lsb_release -a	Ubuntu distribution version (release, codename)
uptime	how long the server is running + current load average.
w	who is logged in and what they are doing (sessions, load, commands).
who	list of logged in users
id	UID/GID of the current user and his group.
date	current date/time.
timedatectl	time, time zone, NTP synchronization.
who -b	date and time of last download
uptime -s	date and time of last download
last -x | grep boot | head -n 1	date and time of last download
Users and Groups
adduser vit	creates a user (with a home folder and basic settings).
deluser vit	deletes the user (usually does not touch the home folder without options).
passwd vit	sets/changes the user password.
usermod -aG sudo vit	adds the user to the sudo group (gives sudo rights).
chage -l vit	Shows password policy (validity/expiration).
groupadd devs	creates the group devs.
usermod -aG devs vit	adds the user to the devs group.
Permissions and Ownership
chmod 640 file	changes access rights (example: owner rw, group r, others without rights).
chown user:group file	changes the owner and group of a file.
getfacl file	shows ACL rights (extended rights besides chmod).
setfacl -m u:vit:rwx dir	issues ACL rights to the user vit on the directory.
Sudo (admin access)
sudo -i	open root-shell (almost like a full root login).
sudo -l	shows what commands the current user can run through sudo.
visudo	secure editing /etc/sudoers (checks syntax before saving).
journalctl _COMM=sudo	shows events related to sudo (who/when called).
systemd/systemctl - service management
systemctl status nginx	service status: whether it is active, PID, last lines of logs.
systemctl start nginx	starts the service.
systemctl stop nginx	stops the service.
systemctl restart nginx	restarts the service.
systemctl enable nginx	Enables autorun on boot.
systemctl disable nginx	turns off autorun
systemctl is-enabled nginx	quick answer: autorun is enabled or not.
systemctl list-units --type=service --state=running	list of all running services.
systemctl list-unit-files --type=service	list of units and their autorun mode.
systemctl status nginx -l	status with “long” lines without trimming.
journalctl -u nginx -b	logs of a specific service from the moment of loading.
journalctl -u nginx --since "1 hour ago"	service logs for the last hour.
Processes and load
top	interactively shows processes, CPU, memory, load.
htop	a more convenient analogue of top (if installed).
ps aux --sort=-%cpu | head	top processes by CPU.
ps aux --sort=-%mem | head	top processes by memory.
pidof nginx	PID of the process by name (if one/several, it will show everything).
pgrep -a nginx	PID + run command (more useful than pidof)
kill -TERM <pid>	”gently” asks the process to terminate (preferred).
kill -9 <pid>	“hard” kills (use only if frozen).
nice -n 10 <cmd>	runs a command with reduced CPU priority.
renice 10 -p <pid>	changes the priority of an already running process.
Memory, disk, file systems
free -h	Using RAM and swap in human readable form.
vmstat 1 5	brief CPU/memory/IO statistics every 1 second (5 times).
df -h	busy/free on file systems.
df -i	checking the inode (sometimes there is space, but the inode has run out).
lsblk -f	disks/partitions/FS/UUID (convenient for diagnostics and mounting).
blkid	UUID and file system types.
du -h --max-depth=1 /var | sort -h	which folders in /var are taking up space.
sudo du -xh / | sort -h | tail -n 30	the heaviest directories are on the root.
lsof | grep deleted	Shows processes that keep deleted files open.
Network and Ports
ip a	IP addresses, interfaces, status.
ip r	routing table (where traffic goes by default).
ss -tulpn	which ports are listened to (TCP/UDP) and by which processes.
ss -tpn	active TCP connections and processes.
ping -c 4 1.1.1.1	checking ICMP to IP (whether there is a connection to the Internet/network).
ping -c 4 google.com	DNS + connection check.
curl -I https://example.com	check HTTP(S) headers and availability.
wget -S --spider https://example.com	“check without downloading” shows the server response.
resolvectl status	current DNS settings are systemd-resolved.
resolvectl query example.com	check how the domain resolves.
traceroute 8.8.8.8	the path of packets to the node (where it “dies”).
`mtr -rw 8.8.8.8	traceroute + loss statistics (very convenient).
Firewall (UFW)
ufw status verbose	current rules and operating hours.
ufw allow 22/tcp	allow incoming SSH.
ufw allow 80,443/tcp	allow HTTP/HTTPS.
ufw deny 23/tcp	deny port (example: telnet).
ufw delete allow 80/tcp	delete a specific rule.
ufw enable	enable firewall.
ufw disable	turn off the firewall.
SSH (access and diagnostics)
ssh user@host	connection via SSH.
ssh -v user@host	detailed debugging: keys, algorithms, reasons for failure.
ssh-keygen -t ed25519	create an SSH key.
ssh-copy-id user@host	copy the key to the server (to log in without a password).
systemctl status ssh	sshd status.
journalctl -u ssh -b	sshd logs from the moment of boot.
Packages and Updates (APT/dpkg)
apt update	updates the list of packages from the repositories.
apt upgrade	updates packages without “dangerous” dependency replacements.
apt full-upgrade	updates packages, may remove/replace dependencies (sometimes necessary).
apt install pkg	package installation.
apt remove pkg	removing the package (configs may remain).
apt purge pkg	removing the package along with its configs.
apt autoremove	removes orphaned dependencies.
apt clean	clears the cache of downloaded packages.
apt policy nginx	shows versions (installed/available sources).
apt-cache madison nginx	list of available package versions.
dpkg -l | grep nginx	what nginx packages are installed.
dpkg -S /path/to/file	which package “owns” the file.
File/string search and orientation
find / -name "nginx.conf" 2>/dev/null	search for a file by name (we hide access errors).
grep -R "PermitRootLogin" /etc/ssh -n	search for a line in files recursively + line numbers.
rg "server_name" /etc/nginx	quick search (ripgrep), if installed.
which nginx	path to the executable file that will be launched from the shell.
whereis nginx	where is the binary/sources/man pages (wider than which).
type nginx	will show what it is: alias/function/built-in/file.
Schedulers: cron and systemd timers
crontab -l	list of cron jobs for the current user.
sudo crontab -l	cron jobs for root.
crontab -e	editing cron.
ls -l /etc/cron.*	system cron directories (daily/weekly/monthly).
systemctl list-timers --all	list of systemd timers (what runs and when).
Docker (if used)
docker ps	running containers.
docker logs -f <container>	container logs (monitor in real time).
docker exec -it <container> bash	go inside the container.
docker stats	container load (CPU/RAM/IO).
Quick diagnostic checklist (5 commands)
systemctl status <service> -l	Is the service alive and what does it say?
journalctl -u <service> -b | tail -n 200	service errors.
ss -tulpn	whether the port is listening.
df -h; df -i; free -h	location/inodes/memory.
ip r; resolvectl query google.com; curl -I https://example.com	route/DNS/HTTP.