a list. However, knowing that a parameter exists and might relate to
something you are looking for can assist you in tracking down more
information because it gives you terms to enter into an Internet search
engine such as Google.
sysrq.txt—If you have ever wondered what that key on your
keyboard marked SysRq is used for, this file has the answer. Briefly, it is
a key combination hardwired into the kernel that can help you recover
from a system lockup. Ubuntu disables this function by default for
security reasons. You can reenable it at a root prompt by entering the
command # echo “1” > /proc/sys/kernel/sysrq, and you
can disable it by echoing the value 0 rather than 1.In the other directories under Documentation, you find similar text files
that deal with the kernel modules for CD-ROM drivers, file system drivers,
game port and joystick drivers, video drivers (not graphics card drivers; those
belong to X11R6 and not to the kernel), network drivers, and all the other
drivers and systems found in the Linux operating system. Again, these
documents are usually written for programmers, but they can also provide
useful information to intermediate and advanced Linux users.
The directory named scripts contains many of the scripts that make uses.
It really does not contain anything of interest to anyone who is not a
programmer or a kernel developer (also known as a kernel hacker).
After a kernel is built, all the compiled files wind up in the arch directory
and its subdirectories. Although you can manually move them to their final
location, you learn later in this chapter how the make scripts will do it for
you. In the early days of Linux, this post-compilation file relocation was all
done by hand; you should be grateful for make.
NOTE
The make utility is a complex program. You can find complete
documentation on the structure of make files, as well as the arguments that
it can accept, at www.gnu.org/software/make/manual/make.html.The remaining directories in /usr/src/linux-4.15.3 contain the
source code for the kernel and the kernel drivers. When you install the kernel
sources, these files are placed there automatically. When you patch kernel
sources, these files are altered automatically. When you compile the kernel,
these files are accessed automatically. Although you never need to touch the
source code files, they can be useful. The kernel source files are nothing more
than text files with special formatting, which means you can look at them and
read the programmer comments. Sometimes, a programmer writes an