Key Differences Between Unix and Linux Every Developer Should Know

Shell differences

POSIX defines sh as the standard command interpreter on all Unix-like systems. On many Linux distributions sh is a symbolic link to bash, which enables Bash‑specific extensions (e.g., [[ … ]], source built‑in) that are not required by POSIX and therefore may not exist on BSD, macOS, or other Unix variants.

Option syntax

Traditional Unix utilities support only short options (a single dash followed by a single character, e.g., -c). GNU implementations commonly add long options (double dash, e.g., --count) that are not defined by POSIX. Scripts that rely on long options may fail on systems that provide only the original utilities, such as BSD or minimal embedded environments.

Make implementations

The POSIX make specification defines a limited feature set (basic pattern rules, simple variable substitution). GNU make adds many extensions: conditional directives ( ifeq, ifneq), functions ( $(wildcard …)), parallel execution ( -j), and pattern-specific variables. When portability is required, a common strategy is to move complex logic into a configure script that generates a GNU‑compatible Makefile, or to require GNU make on the target platforms (e.g., macOS and Linux).

C and C++ compiler variables

In Makefiles, use the standard variables $(CC) for the C compiler and $(CXX) for the C++ compiler. On most BSD‑derived systems the default compiler is clang, and the executable named gcc may be absent. The generic driver commands cc and c++ invoke the system’s default C and C++ compilers and work across POSIX platforms.

GNU userland vs. Linux distributions

Not every Linux distribution ships the full GNU toolchain. For example, Alpine Linux uses the musl C library and BusyBox for core utilities, providing a much smaller footprint but lacking many GNU‑specific options. Portable scripts should therefore rely on POSIX‑defined interfaces rather than GNU extensions, even when the target is a Linux system.

UNIX trademark

The all‑caps term “UNIX” is a registered trademark of The Open Group. Only systems that have passed the Open Group’s certification may use the trademark (e.g., macOS). Most BSD‑derived systems and GNU tools aim for compliance with the UNIX specification, but the trademark distinction does not affect technical compatibility.

Original discussion: https://akr.am/blog/posts/unix-is-not-linux