The user equivalent of the global /opt directory

While both are designed to contain files not belonging to the operating system, /opt and /usr/local are not intended to contain the same set of files.

/usr/local is a place to install files built by the administrator, typically by using the make command (e.g., ./configure; make; make install). The idea is to avoid clashes with files that are part of the operating system, which would either be overwritten or overwrite the local ones otherwise (e.g., /usr/bin/foo is part of the OS while /usr/local/bin/foo is a local alternative).

All files under /usr are shareable between OS instances, although this is rarely done with Linux. This is a part where the FHS is slightly self-contradictory, as /usr is defined to be read-only, but /usr/local/bin needs to be read-write for local installation of software to succeed. The SVR4 file system standard, which was the FHS' main source of inspiration, is recommending to avoid /usr/local and use /opt/local instead to overcome this issue.

/usr/local is a legacy from the original BSD. At that time, the source code of /usr/bin OS commands were in /usr/src/bin and /usr/src/usr.bin, while the source of locally developed commands was in /usr/local/src, and their binaries in /usr/local/bin. There was no notion of packaging (outside tarballs).

On the other hand, /opt is a directory for installing unbundled packages (i.e. packages not part of the Operating System distribution, but provided by an independent source), each one in its own subdirectory. They are already built whole packages provided by an independent third party software distributor. Unlike /usr/local stuff, these packages follow the directory conventions (or at least they should). For example, someapp would be installed in /opt/someapp, with one of its command being /opt/someapp/bin/foo, its configuration file would be in /etc/opt/someapp/foo.conf, and its log files in /var/opt/someapp/logs/foo.access.

This change was introduced by BSD after 1985 (BSD 4.2 was still documenting /usr) and in or before 1988 (BSD 4.3/SunOS 4.1 hier(7) manual page already documents /home). It was quickly followed by Solaris 2.0 (which kind of merged System V and BSD) and was later adopted by most other Unix vendors.

This is from the Solaris 2.0 useradd manual page:

   -D          Display the default values for group, basedir, skel, shell,
               inactive, and expire.  When used with the -g, -b, -f, or -e
               options,  the  -D  option  sets  the default values for the
               specified fields.  The default values are:

                       group       other (GID of 1)
                       basedir     /home
                       skel        /etc/skel
                       shell       /sbin/sh
                       inactive    0
                       expire      Null (unset).

Before that, older Unixes were using either the traditional /usr directory or some variants like /user1 documented in SVR3 and SVR4.0. Unix version 7 hier(7) manual page defines /usr as the default location for user's home directory:

/usr/wd/     initial working directory of a user, typically wd is the
             user's login name

Unix version 6, the first Unix to be widely released outside of the Bell Labs had not the hier manual page yet but was already using and documenting /usr.

There are several reasons that explain the move from /usr to something else, including:

• With some Unix versions, upgrading the OS was blowing away the /usr directory.

• Usernames like tmp, src, bin, local and the likes were forbidden as they clashed with existing directories under /usr.

• Using /usr as an automounter base directory was not possible as it was not empty (Thanks to Johan for pointing this)

• Diskless machines were expected to use a read only NFS share for /usr but read-write home directories