Userdir handles situations that can be difficult or not possible at all with /etc/passwd

• I want to allow 'bob' to change his GECOS information but not his login shell.

  chown bob /etc/users/bob/GECOS && \
      chmod 0644 /etc/users/bob/GECOS
  chown root /etc/users/bob/SHELL && \
      chmod 0444 /etc/users/bob/SHELL
                      

• I want to allow bob's boss, 'amelia', to have root-like control over his account.

  chown -R amelia /etc/users/bob/

  (And you have even more power and flexibility if your system has ACLs)

• I want to know how many users on my system are using the zsh as their login shell.

  grep -l /usr/local/bin/zsh /etc/users/*/SHELL | wc -l

• I don't want to use NIS/YP.

  Use NFS:

  mount some.other.machine:/etc/users /etc/users

  (or better yet, use a union filesystem mount to mesh local and remote accounts)

• I'm using Bernstein's qmail, daemontools and ucspi-tcp packages and want to run SMTP service under qmaild's account.

  Tcpserver's -U option switches its uid and gid to the environment variables $UID and $GID, respectively:

  exec envdir /etc/users/qmaild tcpserver -U 0 smtp qmail-smtpd

• I don't want it to be possible to change root's shell.

  chflags schg,sunlink /etc/users/root/SHELL && \
      sysctl kern.securelevel=4

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