Editing Draft:Elixir/OS processes (section)

==== BEAM internal and external processes ====
[[W:BEAM (Erlang virtual machine)|BEAM]] applications are built out of supervision trees and excel at managing huge numbers of parallel actor processes, all scheduled internally.  Although the communities' mostly share a philosophy of running as much as possible inside of the VM because it builds on this strength, and simplifies away much interface glue and context switching, on many occasions it will still start an external OS process.  There are some straightforward ways to simply run a command line, which might be familiar to programmers coming from another language: <code>[https://www.erlang.org/doc/apps/kernel/os.html#cmd/2 os:cmd]</code> takes a string and runs the thing.  At a lower level, external programs are managed through a [https://www.erlang.org/doc/system/ports.html Port] which is a flexible abstraction allowing a backend driver to communicate data in and out, and to send some control signals such as reporting an external process's exit and exit status.

When it comes to internal processes, BEAM is among the most mature and robust, achieved by good isolation and by its hierarchical [https://www.erlang.org/doc/system/sup_princ supervisors] liberally pruning entire subprocess trees at the first sign of going out of specification.  But for external processes, results are mixed.  Some programs are twitchy and crash easily, for example <code>cat</code>, but others like the BEAM itself or a long-running server are built to survive any ordinary I/O glitch or accidental mashing of the keyboard.  Furthermore, this will usually be a fundamental assumption of that program and there will be no configuration to make the program behave differently depending on stimulus.