You probably use some bash builtins fairly often whether or not you think of them as builtins or simply as commands. After all, bash builtins are commands, but not implemented as separate executables. Instead, they are part of the bash executable. In other words, they are “built into” bash, thus the term “bash builtins”.
If you’re looking for a particular builtin, the which command isn’t going to find it for you because it only looks through a collection of executables. This includes system commands like /bin/echo as well as scripts for which you have execute permission. Here’s an example of which not finding anything:
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Source:: Network World – Data Center