The basic things to look for:
CPU, memory, disk.
A small monitoring solution:
If you insist of writing shell scripts, look at my Nagios plugins. The plugin scripts can be run standalone (without Nagios), you can adapt the output to suit your needs.
Only recently i learned that NMON is available for Linux too (i used to use it on AIX systems only). It has an interactive mode but also a daemon mode and its output can be analysed with the "NMON Analyser". Both NMON and NMON Analyser is downladable freely.