Unix vs Linux
What Unix Is and Where It Came From
Unix vs Linux comes down to family and lineage. Unix is the original family of operating systems, developed at AT&T Bell Labs from the late 1960s. Linux is a Unix-like system written from scratch in the 1990s, which borrowed the design ideas and command conventions of Unix without copying its code. Linux itself is covered in its own article, What Is Linux?
Unix was commercial software, and over the following decades it split into many variants. The best-known lineages are System V, developed at AT&T, and the Berkeley Software Distribution (BSD) from the University of California, Berkeley. Commercial vendors shipped their own versions, which made software compatibility a constant problem for system administrators.
POSIX, Compatibility and Shared Behavior
To reduce that fragmentation, the IEEE standardized POSIX (Portable Operating System Interface). POSIX is a set of rules covering system calls, shell behavior and standard command-line tools. A system that follows POSIX lets many programs run unchanged across different machines.
Linux follows POSIX closely, so a script written for one Unix-like system often works on another with little change. Most Linux distributions are not certified as UNIX under The Open Group's trademark program, however, so the term Unix-like is more accurate. Certification is about a formal test suite, not the everyday experience of using the system.
Compatibility has limits. Option flags, file paths and less common utilities can differ between systems, so a script that works on one machine should be tested on the other before it is trusted.
Licensing and Ownership Differences
The biggest practical difference is licensing. Traditional commercial Unix systems are proprietary: the source code belongs to a vendor and is licensed to customers under restrictive terms. Linux is released under the GNU General Public License, which lets anyone run, study, modify and redistribute it, provided derivative works are shared under the same terms.
BSD-derived systems use permissive licenses that allow reuse in proprietary products without requiring changes to be published. The FreeBSD family is compared with Linux in its own article, FreeBSD vs Linux.
A Backup Script Across Two Systems
A system administrator writes a backup script that uses standard tools such as tar, the shell and common text utilities. Because these tools follow POSIX behavior, the script can often run on a Linux server and on a commercial Unix machine with few changes. Differences still appear, though, so testing on the target system remains essential.
Frequently asked questions
Is Linux a version of Unix?
Not in the strict sense. Linux was written independently and does not contain the original Unix source code. It is usually described as Unix-like, meaning it follows many of the same design principles and interfaces. macOS, by contrast, is formally certified as a UNIX system.
Why does the Unix background matter for a website owner?
It matters mostly for compatibility and support. Scripts, server software and documentation are written with Unix-like systems in mind, so many tools work across Linux and other Unix-like servers. Most hosting plans run Linux, and the Unix background explains why common commands and file layouts look familiar on both.