Tools / Commands for monitoring Memory Usage in ubuntu / Debian
free - Display amount of free and used memory in the system.
watch -n 5 free -m - Real time memory usge

Awareness of what's happening on your network, and why, makes the difference between average and excellent network administration. Great tools are needed to obtain this knowledge quickly and accurately. Fortunately, some of the best network monitoring tools are free. This article lists the top five.

System Administrators (SAs) have a tough job. To assist those weary SAs in their quest to conquer their Linux systems, I've devised this list of 12 native Linux system monitoring tools that are always at my fingertips.

Network UPS Tools is a collection of programs which provide a common interface for monitoring and administering UPS hardware.

When it comes to monitoring your Linux system, few tools can rival Conky. This lightweight system monitoring utility can help you to keep an eye on virtually any aspect of your system, and it offers a huge list of options to choose from.

Linux is an excellent platform for network administration. If you want to monitor your network traffic, you can find many tools -- some accessible from a Web interface, others using a graphical interface -- but nothing beats the speed of the command line.

ZABBIX is an enterprise-class distributed monitoring solution designed to monitor and track performance and availability of network servers, devices, and other IT resources. It supports distributed and Web-based monitoring, auto-discovery, real-time monitoring, SLA assurance, trending, and more.

Network Security Toolkit is one of many live CD Linux distributions focusing on network monitoring, analysis, and security. NST was designed to give network security administrators easy access to a comprehensive set of open source network applications, many of which are among the top 100 security tools recommended by insecure.org.

Nagios is a host and service monitor designed to inform you of network problems before your clients, end-users or managers do. It has been designed to run under the Linux operating system, but works fine under most *NIX variants as well.