modules-update: команда не найдена

1. man man

2. man apropos

DESCRIPTION
       apropos  searches a set of database files containing short descriptions
       of system commands for keywords and displays the result on the standard
       output.

Применительно к конкретному примеру:

$ apropos modules
/etc/modules.autoload [modules] (5)  - kernel modules to load at boot time
...
depmod               (8)  - handle dependency descriptions for loadable kernel modules
depmod               (8)  - program to generate modules.dep and map files
...
lsmod                (8)  - list loaded modules
lsmod                (8)  - program to show the status of modules in the Linux Kernel
modprobe             (8)  - high level handling of loadable modules
modprobe             (8)  - program to add and remove modules from the Linux Kernel
modules.conf [modules] (5)  - configuration file for loading kernel modules
modules.dep [modules] (5)  - List of module dependencies
pam                  (3)  - Pluggable Authentication Modules Library
pam [PAM]            (8)  - Pluggable Authentication Modules for Linux
pango-querymodules   (1)  - Module registration utility
pcimodules           (8)  - List kernel driver modules available for all currently plugged in PCI devices
...
query_module         (2)  - query the kernel for various bits pertaining to modules
rmmod                (8)  - unload loadable modules
tcmodinfo            (1)  - Get and Set information in filter modules
update-modules       (8)  - (re)generate module config files in /etc/
...

ЗЫ: А ещё настоятельно рекомендую почитать про grep.