The question mark key and the tab key are your friends.
Those are the keys in most CLIs for help and command-completion.
If you know what you're doing (and not only what command to type in) then it is relatively easy to figure out what command to type on any box. All routers (and most serious switches) have the same concepts of an interface or a port, a routing protocol, tty or ssh access, etc. They just use slightly different syntax. Question mark and tab will help you find out quickly what you need.
E.g. on a Nokia router you type "admin display-config" in stead of "show running-config". But when you configure BGP, you still type "router bgp" and "neighbor 1.2.3.4". (BTW, Nokia routers are the former Alcatel-Lucent routers. High-end routers, used by providers, not so much be enterprise networks).
There might be places where they use only cisco equipment in the network. But I don't think there are that many places. Most ISPs, and I think also many enterprise networks too, prefer to use a "second source". Meaning that even when they are happy with products from vendor X, they still want products from a second vendor, vendor Y too. Otherwise vendor X might not give them any discounts. Or it might become impossible to switch to another vendor in a few years if it turns out vendor X's products don't have the necessary features or performance (compared to other or new vendors).
And even if you work with only cisco equipment, cisco has 3 major Operating Systems. IOS-XE for enterprise routers, IOS-XR for provider routers and Nexus-OS for data-center routers/switches. And then there are the catalyst switches which have their own OS. All the CLIs are slightly different. You might have to get used to all 4. And then it is not so hard to get used to Juniper's, Nokia's or Huawei's CLI syntax too.
Don't worry about your CCNA, CCNP, etc. Those will be very valuable. If you have those certificates, and have that knowledge, you can use it to quickly convert to other devices too. Try to understand the technology and the protocols and how they are used. Don't memorize the exact commands. You have question-mark and tab for that.