Why are some Bash commands both built-in and external?

Some commands are internal built-in Bash commands while others are external (other programs). I see why certain commands need to be built-in. Some of the reasons are:

1. If a command needs to change the internal state of the shell process.
2. If a command performs a very basic operation in the shell.
3. If a command is called often and needs to be made fast. An external command is executed by loading an external program and hence is slower.

But why are some commands both built-in and external, for example echo and test? I understand echo is used a lot and thus is built-in (Reason 3). But then why also have it as an external command and have a binary for it in /bin/echo? The built-in version of echo will always take precedence over the external version and thus, the external version is hardly ever used. So, why then have an external version of it at all?

Answer

It’s exactly your point 3. When a command does very little (echo is a good example), spawning a new process dominates the run time behavior. With growing disks and bandwidth and code bases you always reach a spot when you have so much data and so many files (our code base at work has 100k files!!) that one less spawn per file makes a difference of minutes.

That’s also why the typical built-in is a drop-in replacement which takes (perhaps a superset of) the same arguments as the binary.

You also ask why the old binary is still retained even though Bash has it as a built-in — the answer is that a lot of programs rely on the existence of that /bin/echo. It’s actually standardized.

Bash is only one of many user interfaces and offline command interpreters. They all have different sets of built-ins. Some shells are purposefully small and rely a lot on what you could call “legacy” binaries. One example is ash and its successor, Dash. Dash is now the default /bin/sh in Ubuntu and Debian due to its speed, and is popular for embedded systems due to its small size. (But even Dash has builtins for echo, test and dozens of other commands, and provides a command history for interactive use.)