Raspberry Pi Commands Basic

Update

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
sudo rpi-update

Hardware Info

cat /proc/cpuinfo discover information about the Raspberry Pi's CPU
cat /proc/meminfo displays details about the Raspberry Pi's memory.
cat /proc/partitions reveals the size and number of partitions on your SD card or HDD.
cat /proc/version shows you which version of the Pi you are using.
vcgencmd measure_temp reveals the CPU temperature (vital if you're concerned about airflow).
vcgencmd get_mem arm && vcgencmd get_mem gpu will reveal the memory split between the CPU and GPU, which can be adjusted in the config screen.
free -o -h will display the available system memory.
top d1 checks the load on your CPU, displaying details for all cores.

Shutdown

sudo shutdown -h now will commence the shutdown process with immediate effect.
sudo shutdown -h 21:55 Schedule a timed shutdown at 21:55

References

Raspberry Pi Terminal Commands: A Quick Guide for Raspberry Pi Users