Early operating systems were written in assembly language.Now, although
some operating systems are still written in assembly language, most are written
in a higher-level language such as C or an even higher-level language such as
C++. Actually, an operating system can be written in more than one language.
The lowest levels of the kernel might be assembly language. Higher-level
routines might be in C, and system programs might be in C or C++, in
interpreted scripting languages like PERL or Python, or in shell scripts. In
fact, a given Linux distribution probably includes programs written in all of
those languages.