Bash – Why are parameters to Bash’s `command` and `type` optional

This is a bug in older versions of bash, specifically 4.0​β2 patchlevel d to 4.0 patchlevel m. From the changelog from 4.0 to 4.1​α:

n. Fixed the behavior of set -u to conform to the latest Posix interpretation: every expansion of an unset variable except $@ and $* will cause the shell to exit.

This behavior was introduced in ​4.0β2 because the previous (and now again current) behavior was thought to be buggy:

d. Fixed a bug that caused expansions of $@ and $* to not exit the shell if the -u option was enabled and there were no posititional parameters.

Bash built-ins are inconsistent and poorly documented.

Here's an example:

$ help command
command: command [-pVv] command [arg ...]
    Runs COMMAND with ARGS ignoring shell functions.  If you have a shell
    function called 'ls', and you wish to call the command `ls', you can
    say "command ls".  If the -p option is given, a default value is used
    for PATH that is guaranteed to find all of the standard utilities.  If
    the -V or -v option is given, a string is printed describing COMMAND.
    The -V option produces a more verbose description.
$ command; echo $?
0

Even without command the return code $? -eq 0 and there is no error on std err.

Another one:

$ help disown
disown: disown [-h] [-ar] [jobspec ...]
    By default, removes each JOBSPEC argument from the table of active jobs.
    If the -h option is given, the job is not removed from the table, but is
    marked so that SIGHUP is not sent to the job if the shell receives a
    SIGHUP.  The -a option, when JOBSPEC is not supplied, means to remove all
    jobs from the job table; the -r option means to remove only running jobs.
$ disown; echo $?
-bash: disown: current: no such job
1

All the arguments are optional but it returns $? -eq 1 when there are none.

I've even compiled the newest Bash 4.2 and here are my results:

$ help command
command: command [-pVv] command [arg ...]
    Execute a simple command or display information about commands.

    Runs COMMAND with ARGS suppressing  shell function lookup, or display
    information about the specified COMMANDs.  Can be used to invoke commands
    on disk when a function with the same name exists.

    Options:
      -p    use a default value for PATH that is guaranteed to find all of
        the standard utilities
      -v    print a description of COMMAND similar to the `type' builtin
      -V    print a more verbose description of each COMMAND

    Exit Status:
    Returns exit status of COMMAND, or failure if COMMAND is not found.
$ command; echo $?
0

There's a new section "Exit Status" and command is still an optional argument. Even worse than 3.x. The same for other built-ins.

So, you're right. Bash built-ins are a mess and should be fixed.