System Information commands

1. File system disk space usage

df -h

will give information using megabytes (M) and gigabytes (G)instead of blocks (-h means "human-readable").

2. Amount of free and used memory

free -m

will give the information using megabytes

3. Displays information running processes and system resources

top

4. Print all system information

uname -a

5. Version information of Linux release

lsb_release -a

Reference: Ubuntu 8.10 Help Center Basic commands