`cd` is a shell builtin, not a program: it cannot exist as a separate process

Every process has its own working directory, inherited from its parent at fork time. If `cd` were an external program (like `ls` or `grep`), it would change the working directory of a child process and exit: the shell's own directory would remain unchanged. This is why `cd` must be a shell builtin: the shell executes it in its own process, directly calling the `chdir()` syscall on itself. There is no `/bin/cd` on a properly configured system.

# Confirm cd is a builtin, not an external command
type cd        # prints: cd is a shell builtin
which cd       # prints nothing: no binary on PATH

# You can verify with strace: cd calls chdir() directly in the shell process
strace -e trace=chdir bash -c "cd /tmp"
# chdir("/tmp") = 0