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If a layer 2 managed switch supports management functions and entails a "management plan" it must require an OS.
If it is a typical network operating system, *nix based, does is have a full file system?
Would I be able to write an application and run it on the cpu of the switch?
You are correct that switches run an OS (Cisco IOS, HP Comware...) however they do not normally expose any means for the end-user to write and deploy additional apps.
This is true even when the underlying OS is *nix based (such as Cisco NX-OS for its Nexus datacenter switches).
I would imagine that this approach is intended to guarantee switch reliability / performance, not to mention that layer 2 switches often have anemic general purpose system resources since almost everything is happening in hardware.
I think it depends what brand and what OS it is running.
Cisco devices run their own proprietary version of IOS, which as far as I know doesn't support custom applications.
Juniper, on the other hand, is running JunOS based on FreeBSD, so there might be some chance, but I think it will be limited at best.
Some small companies may use something like embedded linux, since they probably won't develop their own system.
To sum it up, it depends on the manufacturer and what OS are they using. But even if you can develop application, it will have very limited resources, as network devices are designed specifically for their function and many features use special hardware instead of software (especially on switch).
As a philosophy ARISTA encourages customers and partners to run standard Linux applications on their switches, publishing all necessary APIs etc.
As for resources, ARISTA is making very low latency-switches with high port density for 10G/40G/100G, and therefore high CPU power and ample memory is provided. One example of such an integrated application is Splunk for network monitoring/statistics.
