Are Linux commands universal across distributions?

From 3 Reasons to Learn Linux Commands in Multiple Linux Distributions - Linux Training Online:

Are you just new to Linux? Are you confused about which Linux distribution to learn for your Linux system administration career?

Here's the great news! Linux commands are virtually identical from one Linux distribution (a.k.a.version, distro) to another.

So, when you learn a Linux command in one Linux distribution, you have learned that command for ALL Linux distributions.

Linux Tips: The Linux commands we're talking about here are the very popular GNU / Linux commands that are common to every Linux distribution. These are the essential Linux commands required for Linux system administration, like the cd, ls, cp, rm, pwd and mkdir commands.