Why is cd not a program?

The cd command modifies the "current working directory", right?

"current working directory" is a property that is unique to each process.

So, if cd was a program it would work like this:

1. cd foo
2. the cd process starts
3. the cd process changes the directory for the cd process
4. the cd process exits
5. your shell still has the same state, including current working directory, that it did before you started.

cd in addition to being a shell builtin, is actually also a program on POSIX compliant OSes. They must provide independent executables for regular utilities, like cd. This is for example the case with Solaris, AIX, HP-UX and OS X.

Obviously, a builtin cd is still mandatory as its external implementation doesn't change the current shell directory. However, the latter can still be useful. Here is an example showing how POSIX envision how this cd command could be used:

find . -type d -exec cd {} \;

On a POSIX system, this oneliner will report an error message for all directories you aren't allowed to cd in. On most Gnu/Linux distributions, it fails with that error message though:

find: `cd': No such file or directory

And here is the answer to your question, "Why is cd not a program?" by one of the original Unix co-author. On a very early Unix implementation, cd (spelled chdir at that time) was an external program. It just stopped working unexpectedly after fork was first implemented.

Quoting Dennis Ritchie:

In the midst of our jubilation, it was discovered that the chdir (change current directory) command had stopped working. There was much reading of code and anxious introspection about how the addition of fork could have broken the chdir call. Finally the truth dawned: in the old system chdir was an ordinary command; it adjusted the current directory of the (unique) process attached to the terminal. Under the new system, the chdir command correctly changed the current directory of the process created to execute it, but this process promptly terminated and had no effect whatsoever on its parent shell! It was necessary to make chdir a special command, executed internally within the shell. It turns out that several command-like functions have the same property, for example login.

Source: Dennis M. Ritchie, “The Evolution of the Unix Time-sharing System”, AT&T Bell Laboratories Technical Journal 63(6), Part 2, Oct. 1984, pp.1577–93

Unix Version 1 (March 1971) chdir manual page states:

Because a new process is created to execute each command, chdir would be ineffective if it were written as a normal command. It is therefore recognized and executed by the Shell.

From the Bash introduction (What is a shell?):

Shells also provide a small set of built-in commands (builtins) implementing functionality impossible or inconvenient to obtain via separate utilities. For example, cd, break, continue, and exec) cannot be implemented outside of the shell because they directly manipulate the shell itself. The history, getopts, kill, or pwd builtins, among others, could be implemented in separate utilities, but they are more convenient to use as builtin commands. All of the shell builtins are described in subsequent sections.