bash and ash shell returns different error message for same command

Question

I'm using ash and bash shell in my embedded system, I got following error messages for same command using both shell

For ash shell
$ kill -9
sh: you need to specify whom to kill

For bash shell
$ kill -9
kill: usage: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]

So,My question is why there are two different error messages for same command in two different shell? My understanding is error message return from command not from shell.

Answer 1

My question is why there are two different error messages for same command in two different shell?

Because kill is a shell-builtin (at least on bash). This implies that is using bash, saying kill ... would execute the shell builtin and not the binary that might reside in /bin or /usr/bin.

$ echo $SHELL
/bin/bash
$ type kill
kill is a shell builtin
$ kill
kill: usage: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]
$ which kill
/bin/kill
$ /bin/kill
usage: kill [-s signal_name] pid ...
       kill -l [exit_status]
       kill -signal_name pid ...
       kill -signal_number pid ...

You could disable the shell-builtins in bash by making use of the enable builtin:

$ enable -n kill
$ kill
usage: kill [-s signal_name] pid ...
       kill -l [exit_status]
       kill -signal_name pid ...
       kill -signal_number pid ...

(Invoking kill after disabling the builtin invokes the system /bin/kill instead.)