Question

Is it possible to take a device, say, a PDA, and wipe an software off of it and install your own?

For example, could I take a mac terminal program and install it onto a PDA (with wifi) and do SSHing and such?

And what language would / could it be in?

Answer 1

The language this could be in is not really the issue; it is, mostly, a matter of system compatibility.

Software applications do not run in a vacuum: they rely on the underlying operating system or for the least some form of virtual environment or a runtime such as Java, Silverlight etc.

Before one can re-purpose a PDA or other similar device, he/she need to install some system / host-software of sorts on it, and doing so can be rather complicated because of the proprietary and dedicated nature of many of the hardware subsystems therein.

General purpose systems such as Linux or Windows can be installed on various hardware platforms (including appliances) provided that:
– said hardware subsystems (CPU, keyboard/input devices, display device, storage devices…) comply to some specification, and
– the corresponding device drivers are available.

In the case of PDA, GPS appliances, smartphones and various other hardware platforms (and while many such platforms run on custom versions of Windows, Linux, Android etc.), there is typically enough proprietary differences, custom hardware and other deviations from specifications that installing alternative operating systems or runtimes is typically a challenge. Lack of documentation can also be a limiting factor.

Many such devices however host some form of runtime atop the system (Java in many cases), and rather than installing anew a alternative operating system, it is possible, in some cases, to install and run applications written in these hosted languages.
Even though, uninstalling existing applications (say to make room) and installing new applications may be a challenge as well. Difficulties arise because of
– purposeful “locking in” of the appliances (the manufacturers purposely prevent such re-purposing, using various forms of encryption, undocumented features and the like)
– intrinsic limitations of the runtime (whereby only a subset / sandboxed version of the language features is available).

In short, the specific approach for re-purposing appliances depends on:

• the specific appliance/device: make, version etc.
• the intended purpose: which particular uses are desired for the new device
• the technical expertise and patience of the implementers 😉
  In general this is far from trivial: beginners beware! (*)

(*) BTW, the relative lack of sophistication apparent in the question seem to indicate the OP may not have the necessary skills involved in this kind of “hacking”. It can however, be a very fun and rewarding learning experience.