free
free - Display amount of free and used memory in the system
Command manuals for monitoring
htop
htop, pcp-htop - interactive process viewer
iotop
iotop - simple top-like I/O monitor
logrotate
logrotate ‐ rotates, compresses, and mails system logs
multitail
MultiTail - browse through several files at once
nload
nload - displays the current network usage
ps
ps - report a snapshot of the current processes.
tail
tail - output the last part of files
top
top - display Linux processes
uptime
uptime - Tell how long the system has been running.
vmstat
vmstat - Report virtual memory statistics
w
w - Show who is logged on and what they are doing.
who
who - show who is logged on