It's best to search distribution's manual/official help pages before trying commands found on Internet, including ones here. Certain seemingly short commands can instantly break your installation and - rarely - command parameters differ, leading to behavior different from expected.

In all examples, cat could be replaced with less with few crucial considerations:

less opens file in interactive browser (and is not always installed in container images) - suitable for browsing content of file by human
cat prints entire content of file in terminal (potentially locking it and erasing commands history if file is long) - suitable for use both in scripts and manually

Notes regarding easy of use

human-friendly - can be interpreted at a glance (the default)
script-friendly - have somewhat stable formatting making it easy to parse
verbose - sometimes hard to read manually

General info

Getting information about shell built-in functions and commands.

help

Shows help page for built-in help command

man

Shows manual entry page for command man command

Persistent sessions

Preventing programs terminating due to connectivity issues.

tmux

Open tmux session (programs running in it will continue to do so even if your ssh connection is interrupted and ssh session terminates) tmux
List open tmux sessions tmux ls
Reattach to open tmux session (to resume work) tmux a
Reattach to open tmux session SESSION_ID (to resume work) tmux a -t SESSION_ID

Disk and space

Checking space usage and available disks.

Information about disk devices (script-friendly, verbose)

cat /proc/diskstats

df

Lists disk usages (sizes in human-friendly format, file system names included) df -hT
Lists disk usages (sizes in human-friendly format, file system names included, some formats excluded and sorted) df -hTx tmpfs -x vfat|sort -r

lsblk

Lists block devices excluding loop device (for example, snap devices) lsblk -e 7 Lists block devices with serial number (if any) lsblk -o +SERIAL

System info

Shows OS information less /etc/os-release
Memory info (script-friendly, verbose - 55 lines) cat /proc/meminfo
swap info (script-friendly) less /proc/swaps
Load average info (script-friendly, somewhat verbose - 5 values; first 3 are jobs that are in the run queue - state R - or waiting for disk IO - state D averaged over 1, 5 and 15 minutes then number of processes/threads that are currently being executed - up to number of vCPUs slash and total numer of processes/threads then PID of the process most recently created on the system) cat /proc/loadavg
CPU info (script-friendly, verbose) cat /proc/cpuinfo

htop

Start program listing system CPU/memory loads and list of processes htop

free

Memory and swap info (sizes in human-friendly format) free -h

swapon

swap info (human-friendly) swapon --show

uptime

Current hour, uptime then load averages over 1/5/15 minutes uptime

dmidecode

DMI - Desktop Management Interface (SMBIOS - System Management BIOS) table content in human-readable format (description of hardware components, serial numbers, BIOS revision sudo dmidecode Display value of DMI string identified by KEYWORD: -s, --string (skipping KEYWORD prints available options). Most of those can be read directly from sysfs at /sys/devices/virtual/dmi/id sudo dmidecode --string KEYWORD Display entries of type TYPE: -t, --type (skipping TYPE prints available options) sudo dmidecode --type TYPE DMI types:

Type	Information
0	BIOS
1	System
2	Baseboard
3	Chassis
4	Processor
5	Memory Controller
6	Memory Module
7	Cache
8	Port Connector
9	System Slots
10	On Board Devices
11	OEM Strings
12	System Configuration Options
13	BIOS Language
14	Group Associations
15	System Event Log
16	Physical Memory Array
17	Memory Device
18	32-bit Memory Error
19	Memory Array Mapped Address
20	Memory Device Mapped Address
21	Built-in Pointing Device
22	Portable Battery
23	System Reset
24	Hardware Security
25	System Power Controls
26	Voltage Probe
27	Cooling Device
28	Temperature Probe
29	Electrical Current Probe
30	Out-of-band Remote Access
31	Boot Integrity Services
32	System Boot
33	64-bit Memory Error
34	Management Device
35	Management Device Component
36	Management Device Threshold Data
37	Memory Channel
38	IPMI Device
39	Power Supply
40	Additional Information
41	Onboard Devices Extended Information
42	Management Controller Host Interface

Keyword - type table:

Keyword	Type
bios	0, 13
system	1, 12, 15, 23, 32
baseboard	2, 10, 41
chassis	3
processor	4
memory	5, 6, 16, 17
cache	7
connector	8
slot	9

Logs

Searching for things

find

Helps find file based on its properties or name.

List all files in current directory and its subdirectories find . -type f | less
List all files in /path directory and its subdirectories with names ending with .html find /path -name *.pdf -type f | less
List all files and directories in current directory and its subdirectories that are not owned by USER find . \! -user USER
Execute COMMAND with relative path to all files and directories in current directory and its subdirectories find . -exec COMMAND {} \;

grep

Helps locate file containing specific content.

Finds FOOBAR in all files in current directory grep -s "FOOBAR" *
Finds FOOBAR in all files in current directory or its subdirectories grep -rs "FOOBAR" *
Finds all instances of FO followed by any number of any characters (new lines included) then AR in all files in current directory or its subdirectories grep -rse "FO.*AR" *
Finds all instances of FO followed by any number of most characters (new lines not included) then AR in all files in current directory or its subdirectories grep -rse "FO*AR" *
Finds all instances of FO followed by any two characters (number of dots, new lines included) then AR in all files in current directory or its subdirectories grep -rse "FO..AR" *

Other

Sends GET to address and prints response curl https://hekkaryk.eu
Sends GET to address and saved response wget hekkaryk.eu
Sends GET to address and saved response (recursively - tries to scrape website - do not use unless allowed by site owner - does not work most of the time anyway): wget -r hekkaryk.eu
Reboots system to UEFI/BIOS systemctl reboot --firmware-setup
Minimal image viewer sudo apt install feh feh PATH
Minimal PDF viewer sudo apt install evince evince PATH
Minimal video player sudo apt install ffmpeg ffplay PATH