How to tell if you’re using a bash builtin in Linux
If you’re not sure if you’re running a Linux command or you’re using a bash builtin, don’t stress, it isn’t all that obvious. In fact, you can get very used to commands like cd without realizing that they’re part of your shell, unlike commands like date and whoami that invoke executables (/bin/date and /usr/bin/whoami).
Builtins in general are commands that are built into shell interpreters, and bash is especially rich in them, which is a good thing because built-ins by their very nature run a bit faster than commands which have to be loaded into memory when you call them into play.