Which tree to use?
In almost all cases, new material that you install should go into the “local” tree of your (La)TeX installation. (A discussion of reasons not to use the local tree appears below.)
On a Unix(-alike) system, using TeX Live or teTeX, the root
directory will be named something like /usr/share/texmf-local/
or /usr/local/share/texmf/
You can ask such a system where it believes a local tree should be:
kpsewhich -var-value TEXMFLOCAL
the output being the actual path, for example (on the workstation this author is using today):
/usr/local/share/texmf
In a MiKTeX installation, the location will in fact typically be
something you specified yourself when you installed MiKTeX in the
first place, but you may find you need to create one. The MiKTeX
“Settings” window ( Start 
Programs
MiKTeX Settings) has a tab Roots;
that tab gives a list of current TDS roots (they're typically not
called texmf-anything). If there's not one there with
local in its name, create an appropriate one (see below),
and register it using the window's Add button.
The MiKTeX FAQ
suggests that you should create C:\\Local TeX Files, which is good if you
manage your own machine, but often not even possible in corporate, or
similar, environments — in such situations, the user may have no
control over the hard disc of the computer, at all.
Ce n'est plus vrai, MiKTeX propose maintenant de s'installer sous C:\Users\…\AppData\Local\Programs.
So the real criterion is that your local tree should be somewhere that you, rather than the system, control. Restrictive systems often provide a “home directory” for each user, mounted as a network drive; this is a natural home for the user's local tree. Other (often academic) environments assume the user is going to provide a memory stick, and will assign it a defined drive letter — another good candidate location. Note that the semantics of such a tree are indistinguishable from those of a "home" TEXMF tree.
You might not wish to use the 'local' tree:
- if the package, or whatever, is “personal” (for example, something commercial that has been licensed to you alone, or something you're developing yourself), it should go in your "home" TEXMF tree;
- if you know that the package you are installing is a replacement for the copy on the TEXMF tree of your (La)TeX distribution; in this case it is reasonable to replace the existing copy in the TEXMF tree.
If the system is upgraded (or otherwise re-installed), a copy made in the TEXMF tree will probably be overwritten or deleted. This may be what you want, but otherwise it's a powerful incentive to use a tree that is not “part of the installed system”.
The reason one might place upgrades the distribution's main tree is to avoid confusion. Suppose you were to place the file on the local tree, and then install a new version of the distribution — you might have an effect like:
- distribution comes with package version n;
- you install package version n+1 on the local tree; and
- the updated distribution provides package version n+2.
In such a situation, you could find yourself using version n+1 (from the local tree) after the new distribution has been installed.
If you install in the local tree, the only way to avoid such problems is to carefully purge the local tree when installing a new distribution. This is tedious, if you're maintaining a large installation.
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