- A shell program is a Spark-manipulating control program from the Beast Wars and Beast Machines portions of the Generation One continuity family.
A shell program is a Predacon-designed personality altering program that can be installed within a Transformer to change the way they act. A shell program can even compel a Transformer to behave in ways diametrically opposed to what they would do without the shell program.
Beast Wars cartoon
Upon the landing of each stasis pod, the Maximals and Predacons would both rush to the scene in hopes of recruiting the protoform inside to their team. The protoforms were Maximals by default, but the Predacons were able to turn some by implanting shell programs before waking the protoform up. Blackarachnia and Inferno joined the Predacons through this scheme, but Quickstrike was not directly reprogrammed- although his command code was changed by remote while his security programming was deactivated due to damage his pod sustained during landing-, Rampage only joined due to him being tortured into submission, and Dinobot II was a clone of the original Dinobot created specifically to be a Predacon. Rhinox's brief stint as a Predacon was not explicitly explained, but may have also been the result of a shell program.
Eventually, despite her shell program, Blackarachnia decided to side with the Maximals, although she ws still motivated primarily by self-interest rather than Maximal altruism. When hidden code in her shell program began to threaten her life, the Maximals attempted to remove it entirely, a procedure which was very dangerous for her. Rhinox then conducted a strange kind of virtual, video game-style "search and destroy" mission within Blackarachnia to accomplish this. Sadly, a safeguard that Tarantulas included in the program terminated her before Rhinox could finish the removal (She was then revived "magically" by the Transmetal driver completing her original attempt to use it to upgrade herself, triggering a mutation that incorporated fragments of the shell program while making her more fundamentally Maximal).
Beast Machines Cartoon
Megatron utilized the sparks of Silverbolt, Waspinator, and Rhinox to power shell programs giving life to the original three Vehicon generals. Each of these programs seemed to have personalities the exact opposite of the spark, such as Thrust being a dark, brooding type as opposed to Waspinator's wimpy personality, the psychotic Jetstorm versus the noble Silverbolt and the extremely dim Tankor contrasting to Rhinox's intelligence (Some sources speculate that Rhinox's turn to villainy was caused by the Maximals reactivating Rhinox's personality without disabling the Tankor shell program). Obsidian and Strika, however, did not appear to be altered in this manner.