Why Does Unix Have Separate /bin, /usr/bin, and /opt? The Historical Reason Explained

Unix beginners often wonder why the filesystem contains multiple directories for binaries such as /bin, /usr/bin, /usr/local/bin, and /opt/bin. The answer lies in the historical development of Unix.

In 1969 Ken Thompson and Dennis Ritchie created Unix on a PDP‑7, and in 1971 they moved to a PDP‑11. The system used an RK05 storage disk of about 1.5 MB. As the operating system grew, a single disk could no longer hold all files, so a second RK05 disk was added. The first disk was dedicated to system programs and was mounted as the root directory /, while the second disk held user programs and was mounted as /usr. The directory structures on both disks were identical, so directories like /bin, /sbin, /lib, and /tmp appeared under /usr as well.

When the second disk filled up, a third RK05 disk was introduced and mounted as /home. At that point, /usr was used for user‑installed programs, and /home stored user data. This three‑disk layout became the basis for the Unix filesystem hierarchy that continues to this day, with each directory’s purpose becoming clearer as storage capacities increased.

/ : stores system programs (original AT&T Unix)</code><code>/usr : stores programs provided by Unix vendors (e.g., IBM, HP)</code><code>/usr/local : stores programs installed by the user</code><code>/opt : stores third‑party vendor programs ("optional" software)

Source: https://www.ruanyifeng.com/blog/2012/02/a_history_of_unix_directory_structure.html