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
.
/usr/local
is usually for applications built from source. i.e. I install most of my packages using something like apt
, but if I download a newer version of something or a piece of software not part of my distribution, I would build it from source and put everything into the `/usr/local' hierarchy.
This allows for separation from the rest of the distribution.
If you're developing a piece of software for others, you should design it so that it can be installed anywhere people want, but it should default to the regular FHS specified system directories when they specify the prefix to be /usr
(/etc
, /usr/bin
, etc.)
i.e. /usr/local
is for your personal use, it shouldn't be the only place to install your software.
Have a good read of the FHS, and use the standard Linux tools to allow your source to be built and installed anywhere so that package builders for the various distributions can configure them as required for their distribution, and users can put it into /usr/local
if they desire or the regular system directories if they wish.
Best Answer
It's possible to have multiple versions of Emacs installed on the same machine. Only Emacs 23 would load files from
/usr/share/emacs23/site-lisp/
. All versions of Emacs would load files from/usr/share/emacs/site-lisp/
.Generally, you just use
/usr/share/emacs/site-lisp/
, unless there's some reason the code applies only to a particular version of Emacs.To see where your Emacs is loading code from, type Control+h v
load-path
Enter.