Hardware

Command	Shows
lscpu	CPU model, cores, threads and virtualization support.
cat /proc/cpuinfo	Detailed information per CPU thread.
free -h	Memory and swap usage.
cat /proc/meminfo	Detailed memory statistics.
lspci -tv	PCI devices as a tree; lspci -k adds the kernel driver.
lsusb -tv	USB devices as a tree.
lsblk	Block devices and partitions.
sudo dmidecode -t memory	RAM modules from the firmware tables (-t system for the model).
sudo dmesg -w	Kernel messages, live. Shows hardware errors and newly plugged devices.
sensors	Temperatures and fan speeds (package lm_sensors).

Disks

Command	Effect
sudo smartctl -a /dev/sda	SMART health data and error log (package smartmontools).
sudo smartctl -t short /dev/sda	Starts a short self-test; results appear in -a afterwards.
sudo hdparm -tT /dev/sda	Measures read speed.
sudo badblocks -sv /dev/sda	Read-only scan for unreadable blocks.

Tip

In free, the available column is what matters. Linux uses idle memory as a disk cache (buff/cache) and releases it when programs need it, so low free memory is normal.

Danger

badblocks -w overwrites the whole disk. Without -w (or with -n, non-destructive read-write) the data stays intact.