From Bash Manual:
command [-pVv] command [arguments ...]
Runs command with arguments ignoring any shell function named command. Only shell builtin commands or commands found by searching the PATH are executed. If there is a shell function named ls, running ‘command ls’ within the function will execute the external command ls instead of calling the func- tion recursively. The -p option means to use a default value for PATH that is guaranteed to fi nd all of the standard utilities. The return status in this case is 127 if command cannot be found or an error occurred, and the exit status of command otherwise.
Does the manual explain why it fails when command is assignment or begins with assignment (for environment variables)?
$ command aaa=1 aaa=1: command not found $ command aaa=1 echo hello aaa=1: command not found