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texcavator

eTTYmology 1 min read

USR(7) USR(7)
NAME
/usr — the second disk that became half of Unix
SYNOPSIS
/bin /usr/bin /usr/lib /usr/share
FROM
  1. "user"
  2. home directories on disk two
  3. programs spill over
  4. /usr today
SEE ALSO
hier(7), The /usr merge
TexcavatorUSR(7)

Why /usr exists

The first Unix disk on the PDP-11 filled up, so the system spilled onto a second one. That is where /bin and /usr/bin come from.

/usr sounds like it should hold user files. It did, once. Then it became the second half of the operating system.

Two disks

Early Unix on the PDP-11 lived on small disks. When the system outgrew the first one, the developers mounted a second disk and kept going. Home directories lived there, under /usr. As the system kept growing, programs moved onto the second disk too, and /usr/bin appeared beside /bin.

Why it stuck

The split outlived the hardware. For decades /bin held what you needed to boot and repair the machine, and /usr held everything else, even when both lived on the same disk.

Modern distributions have mostly merged them: /bin is now a symlink to /usr/bin.

$ ls -ld /bin
lrwxrwxrwx 1 root root 7 /bin -> usr/bin

Draft: the details of the PDP-11 disk layout need checking against the sources below.

Sources

unixfilesystems