Bash is used as the base configuration language for FireHOL since it is the common denominator for a language that all UNIX system administrators and developers should know and understand.
The fact that FireHOL uses Bash for its configuration, allows development of add-ons and enables FireHOL to use programs to access SQL databases, directory structures, DBM or other files, web front ends or other means for the rules of the firewall.
The only Bash commands a FireHOL configuration should never use are trap and exit.
Traps are used by FireHOL for cleaning up all temporary files, and possibly restoring the previously running firewall in case FireHOL execution breaks, and the exit command will not just exit the configuration file, it will exit FireHOL. FireHOL has disabled these features by default, so that you will not be able to use them, unless you specifically enable them.
Since a FireHOL configuration script runs inline with FireHOL, all variables and function names defined within the configuration file overwrite the ones defined by FireHOL so you should avoid some names.
Avoid using variables that start with FIREHOL, work, server, and client as many such variables are used by FireHOL internally.
There are also a number of functions names you should avoid, but there is no generic pattern at the moment. I suggest you should avoid defining functions with the names of FireHOL commands (interface, router, client, server, etc) and functions starting with rules.
Note however that you may wish to overwrite a few variables and functions if you want to modify FireHOL services. See the Adding Services guide for details.
To learn Bash scripting I suggest the following documents: