Glances is a curses-based system monitoring tool written in python that can display in any terminal size.
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Pro Important information all in one place
Unlike the default system monitors found in Ubuntu or Gnome Shell, Glances shows a lot of information all in one place instead of having to click through tab after tab. This makes cursory glances at one's stats a lot easier and less time-consuming.
Pro Includes network bandwidth
Pro Works in a terminal
Works perfectly well even if you remote access a server via SSH.
Pro Easily installed
Glances is a popular system monitor and in being so is often found in most distros repositories, so it is easily installed.
Pro Web interface and remote monitoring features
A Web interface based on a Restful JSON API is also available and allows remote monitoring.
Pro Shows only necessary details
Most performance monitors overwhelm you with too much detail. Glances almost always shows exactly what you were interested in immediately.
Con High cpu usage (on RPi), broken after a few hours of running
it's based on python.