Linux

Hostname	hostname
Username	whoami
User context	id id <USERNAME>
Operating System, Version	cat /etc/issue cat /etc/*-release
Kernel, Version and Architecture	uname -a uname -r (only kernel version) arch (only architecture) lsb_release -a (distribution) cat /proc/version
Other users and groups	cat /etc/passwd cat /etc/group groups groups <USER> (current user's groups) getent group <GROUP> (users in that group)
Connected users	who (w) finger
Last user login	lastlog
Network information	ifconfig ip a arp -a route routel ss -anp netstat -tulpn /etc/hosts /etc/resolv.conf (DNS) /etc/network/interfaces (if you make changes: sudo systemctl restart networking)
Open port	netstat -atunp
Rule iptables	cat /etc/iptables/rules.v4
Active Directory	realm list ps -ef | grep -i "winbind\|sssd" (others like sssd or winbind)
Installed applications	dpkg -l apt list --installed | tr "/" " " | cut -d" " -f1,3 | sed 's/[0-9]://g' | tee -a installed_pkgs.list
Binaries	ls -l /bin /usr/bin/ /usr/sbin/
Service list	systemctl list-units --type=service (currently active) systemctl list-unit-files --type=service (all, active and non-active)
Running processes	ps aux top (dynamic)
Scheduled tasks	ls -lah /etc/cron* crontab -l (current user's job) cat /etc/crontab ls /etc/cron.d cat /var/spool/cron/crontabs/root
Disks/Drives mounted	df -h (-t ext4 for only /dev/sda) mount cat /etc/fstab (drives mounted at boot time) cat /etc/fstab | grep -v "#" | column -t (unmounted file systems) lsblk (all disks available)
Drivers and Kernel modules	lsmod /sbin/modinfo <MODULE_NAME> (module taken from the command above)
Various listing	lsblk (block devices) lsusb (dispositivi usb) lsof (open file) lspci (PCI device) lscpu (CPU)
History	history cat ~/.*history
Environment variables	env cat /proc/self/environ (file containing environment variables)
PATH variable	$PATH

Warnings

• cat /var/log/syslog | grep <STRING> It allows you to search the logs, check why something is going wrong, what is blocking it etc.