Market leading networking vendors, such as Cisco, Juniper, and Arista, have all developed their own operating systems that allow network operators to issue a series of commands to network devices via a command-line interface (CLI).
Each vendor's CLI is run from their bespoke operating systems:
Cisco Ios and Nxos
Juniper Junos
Arista Eos
All of these operating systems have meant that it has become easier to programmatically control switches, routers, and security devices provided by these vendors, as they seek to simplify operating network devices.
The rise of DevOps in industry has also meant that it is no longer acceptable to not provide programmatic APIs or SDK to aid automation, with networking vendors now integrating with configuration management tooling, such as Puppet, Chef, Ansible, and Salt, to plug into DevOps tool chains.