This is the error that I got.
modpath: Command not found.
I'm suspecting that it's because modpath is a csh command but not the right syntax for bash. So, I want to know what's its equivalent in bash. Thanks!
modpath - change global search path for dynamically loadable kernel modules
modpath allows users with appropriate privilege to modify the global search path used to locate object files for dynamically loadable kernel modules. The search path modifications take effect immediately and affect all subsequent loads for all users on the system.
pathname may be either a colon-separated list of absolute pathnames or NULL. If the former, these path names represent directories which should be searched for all autoloads of loadable kernel modules and for demand loads (see modload(2)) where the module is given by a simple file name. This list of directories will be prepended to the existing list of directories and so will be searched before any directories given in previous calls to modpath and before the default location which is always searched last. The directories do not have to exist on the system at the time modpath is called, or when a load actually takes place. If pathname is equal to NULL, the global search path is set back to its initial default value, /stand/dlkm/mod.d.
Notes: modpath is currently implemented as a macro.
[source: http://modman.unixdev.net/?sektion=2&page=modpath&manpath=HP-UX-11.11]
Based on the documentation now included in the question, modpath has nothing to do with $PATH, and it's not specific to csh, bash, or any other shell.
On my system (Ubuntu 16.10), there is no command, system call, or library function by that name, and there appears to be no installable package that provides it. The documentation you quoted is for HP-UX, which is a different flavor of UNIX. I suspect it's a system call that exists only on HP-UX.
Since you've been able to invoke modpath from csh, there's probably a command that's some kind of wrapper around the system call, though the documentation is for the system call itself, not for the command.
If such a command exists, you should be able to find it by typing
which modpath
from csh. If that gives you the full path to the command, then from bash you can either use that full path to invoke the command, or you can add the appropriate directory to your $PATH. (You very likely have a different setting for $PATH in csh vs. bash.)