Vital components of UNIX system

The Unix system is composed of several components that are normally packaged together. By including — in addition to the kernel of an operating system — the development environment, libraries, documents, and the portable, modifiable source-code for all of these components, Unix was a self-contained software system. This was one of the key reasons it emerged into an important teaching and learning tool and had such a broad influence.

Inclusion of these components did not make the system large — the original V7 UNIX distribution, consisting of copies of all of the compiled binaries plus all of the source code and documentation occupied less than 10Mb, and arrived on a single 9-track magtape. The printed documentation, typeset from the on-line sources, was contained in two volumes.

The names and filesystem locations of the Unix components has changed substantially across the history of the system. Nonetheless, the V7 implementation is considered by many to have the canonical early structure:

Kernel — source code in /usr/sys, composed of several sub-components:
conf — configuration and machine-dependent parts, including boot code
dev — device drivers for control of hardware (and some pseudo-hardware)
sys — operating system "kernel", handling memory management, process scheduling, system calls, etc.
h — header files, defining key structures within the system and important system-specific invariables
Development Environment — Early versions of Unix contained a development environment sufficient to recreate the entire system from source code:
cc — C language compiler (first appeared in V3 Unix)
as — machine-language assembler for the machine
ld — linker, for combining object files
lib — object-code libraries (installed in /lib or /usr/lib) libc, the system library with C run-time support, was the primary library, but there have always been additional libraries for such things as mathematical functions (libm) or database access. V7 Unix introduced the first version of the modern "Standard I/O" library stdio as part of the system library. Later implementations increased the number of libraries significantly.
make - build manager (introduced in PWB/UNIX), for effectively automating the build process
include — header files for software development, defining standard interfaces and system invariants
Other languages — V7 Unix contained a Fortran-77 compiler, a programmable arbitrary-precision calculator (bc, dc), and the awk "scripting" language, and later versions and implementations contain many other language compilers and toolsets. Early BSD releases included Pascal tools, and many modern Unix systems also include the GNU Compiler Collection as well as or instead of a proprietary compiler system.
Other tools — including an object-code archive manager (ar), symbol-table lister (nm), compiler-development tools (e.g. lex & yacc), and debugging tools.
Commands — Unix makes little distinction between commands (user-level programs) for system operation and maintenance (e.g. cron), commands of general utility (e.g. grep), and more general-purpose applications such as the text formatting and typesetting package. Nonetheless, some major categories are:
sh — The "shell" programmable command-line interpreter, the primary user interface on Unix before window systems appeared, and even afterward (within a "command window").
Utilities — the core tool kit of the Unix command set, including cp, ls, grep, find and many others. Subcategories include:
System utilities — administrative tools such as mkfs, fsck, and many others
User utilities — environment management tools such as passwd, kill, and others.
Document formatting — Unix systems were used from the outset for document preparation and typesetting systems, and included many related programs such as nroff, troff, tbl, eqn, refer, and pic. Some modern Unix systems also include packages such as TeX and GhostScript.
Graphics — The plot subsystem provided facilities for producing simple vector plots in a device-independent format, with device-specific interpreters to display such files. Modern Unix systems also generally include X11 as a standard windowing system and GUI, and many support OpenGL.
Communications — Early Unix systems contained no inter-system communication, but did include the inter-user communication programs mail and write. V7 introduced the early inter-system communication system UUCP, and systems beginning with BSD release 4.1c included TCP/IP utilities.

The 'man' command can display a 'man page' for every command on the system, including itself.
The 'man' command can display a 'man page' for every command on the system, including itself.

Documentation — Unix was the first operating system to include all of its documentation online in machine-readable form. The documentation included:
man — manual pages for each command, library component, system call, header file, etc.
doc — longer documents detailing major subsystems, such as the C language and troff