Which distribution, which kernel, and how long has it been running?
[uname](/man/uname) -a
[hostnamectl](/man/hostnamectl)
[lsb_release](/man/lsb_release) -a
[cat](/man/cat) /etc/os-release
[uptime](/man/uptime)
Pretty system overviews for screenshots and quick orientation.
[fastfetch](/man/fastfetch)
[neofetch](/man/neofetch)
[inxi](/man/inxi) -Fz
lscpu summarizes model, cores, and features; /proc/cpuinfo has the raw per-core details.
[lscpu](/man/lscpu)
[cat](/man/cat) /proc/cpuinfo
Current CPU load: top in batch mode for a one-shot value, mpstat for per-core statistics, vmstat for a rolling view.
[top](/man/top) -bn1 | [grep](/man/grep) "Cpu(s)"
[mpstat](/man/mpstat) 1 5
[vmstat](/man/vmstat) 1
free -h answers the common question; /proc/meminfo has every detail.
[free](/man/free) -h
[cat](/man/cat) /proc/meminfo
[vmstat](/man/vmstat)
In free, look at the "available" column, not "free": Linux uses spare RAM for disk cache and releases it when programs need it.
Snapshot with ps, live view with top or one of its friendlier successors.
[ps](/man/ps) aux
[ps](/man/ps) aux --sort=-%mem | [head](/man/head)
[top](/man/top)
[htop](/man/htop)
[btop](/man/btop)
[atop](/man/atop)
Free space per filesystem, block devices as a tree, and partition tables (root required for fdisk/parted).
[df](/man/df) -h
[duf](/man/duf)
[lsblk](/man/lsblk) -f
[fdisk](/man/fdisk) -l
[parted](/man/parted) -l
List devices by bus, or get the full inventory including model numbers.
[lspci](/man/lspci)
[lsusb](/man/lsusb)
[lshw](/man/lshw) -short
[dmidecode](/man/dmidecode) -t memory
Temperatures and fan speeds (run sensors-detect once first).
[sensors](/man/sensors)
upower reports detailed battery state; acpi gives the one-line answer. The raw values live in /sys.
[acpi](/man/acpi) -b
[upower](/man/upower) -i $([upower](/man/upower) -e | [grep](/man/grep) BAT)
[cat](/man/cat) /sys/class/power_supply/BAT*/capacity
bluetoothctl is the current tool; hciconfig is deprecated but still found on older systems.
[bluetoothctl](/man/bluetoothctl) show
[bluetoothctl](/man/bluetoothctl) devices
[hciconfig](/man/hciconfig) -a
[ip](/man/ip) a
[ifconfig](/man/ifconfig) -a
dmesg prints the kernel ring buffer (root on most distributions); journalctl -b shows everything logged since this boot.
[dmesg](/man/dmesg)
[dmesg](/man/dmesg) -w
[journalctl](/man/journalctl) -b
[lsmod](/man/lsmod)