There are two classes of builtins:
Some commands have to be built into the shell program itself because they cannot work if they are external.
cd is one such since if it were external, it could only change its own directory; it couldn't affect the current working directory of the shell. (See also: Why is cd not a program?)
The other class of commands are built into the shell purely for efficiency.
The dash man page has a section on builtins which mentions printf, echo, and test as examples of commands in this class.
Unix systems have always included separate executables for commands in that second class. These separate executables are still available on every Unixy system I've used, even though they're also built into every shell you're likely to use. (POSIX actually requires that these executables be present.)
I believe echo got built into the shell in AT&T Unix System V Release 3.1. I base that on comparisons of two different editions of manuals for AT&Ts 3B1 series Unix systems. Someone has kindly scanned 1986 editions of these manuals and put them online; these correspond to the original release of SVR3. You can see that echo isn't in the list on page 523 of UNIX System V User's Manual, Volume II, where you'd expect it if the command were built into the shell. In my local paper copy of the SVR3.1 manuals from 1987, echo is listed in this section of the manual.
I'm pretty sure this isn't a Berkeley CSRG innovation that AT&T brought back home. 4.3BSD came out the same year as SVR3, 1986, but if you look at 4.3BSD's sh.1 manpage, you see that echo is not in the "Special Commands" section's list of built-in commands. If CSRG did this, that leaves us wanting a documented source to prove it.
At this point, you may wonder if echo was built into the shell earlier than SVR3.1 and that this fact simply wasn't documented until then. The newest pre-SVR3 AT&T Unix source code available to me is in the PDP-11 System III tarball, wherein you will find the Bourne shell source code. You won't find echo in the builtin command table, which is in /usr/src/cmd/sh/msg.c. Based on the timestamps in that file, that proves that echo certainly wasn't in the shell in 1980.
Trivia
The same directory also contains a file called builtin.c which doesn't contain anything on-point for this question, but we do find this interesting comment:
/*
builtin commands are those that Bourne did not intend
to be part of his shell.
Redirection of i/o, or rather the lack of it, is still a
problem..
*/
type tells you what the shell would use. For example:
$ type echo
echo is a shell builtin
$ type /bin/echo
/bin/echo is /bin/echo
That means that if, at the bash prompt, you type echo, you will get the built-in. If you specify the path, as in /bin/echo, you will get the external command.
which, by contrast is an external program that has no special knowledge of what the shell will do. On debian-like systems, which is a shell script which searches the PATH for the executable. Thus, it will give you the name of the external executable even if the shell would use a built-in.
If a command is only available as a built-in, which will return nothing:
$ type help
help is a shell builtin
$ which help
$
Now, let;s look at cat:
$ type cat
cat is hashed (/bin/cat)
$ which cat
/bin/cat
cat is an external executable, not a shell builtin.
Best Answer
You can use the
commandshell built-in to bypass the normal lookup process and run the given command as an external command regardless of any other possibilities (shell built-ins, aliases, etc.). This is often done in scripts which need to be portable across systems, although probably more commonly using the shorthand\(as in\rmrather thancommand rmorrm, as especially the latter may be aliased to something not known likerm -i).This can be used with an alias, like so:
The advantage of this over e.g.
alias time=/usr/bin/timeis that you aren't specifying the full path to thetimebinary, but instead falling back to the usual path search mechanism.The
aliascommand itself can go into e.g. ~/.bashrc or /etc/bash.bashrc (the latter is global for all users on the system).For the opposite case (forcing use of the shell built-in in case there's an alias defined), you'd use something like
builtin time, which again overrides the usual search process and runs the named shell built-in. The bash man page mentions that this is often used in order to provide customcdfunctionality with a function namedcd, which in turn uses the builtincdto do the real thing.