Description of how to use the Nix expressions

README.md

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+# Overview
+
+This package contains a collection of Nix expressions created by
+SHARCNET staff for various software packages.  Once copied or linked
+into your *~/.nix-defexpr* directory they will allow you to install
+the associated package into your Nix environment.
+
+These directions assume you have loaded the Nix module (this enabes
+your Nix environment and makes the various Nix commands avaialble)
+
+
+```sh
+module load nix
+```
+
+# Obtaining the latest expressions
+
+If you don't yet have a copy of *~/nix-nixpkgs-sharcnet*, you will
+need to clone a copy (use your SHARCNET credentials)
+
+```sh
+cd ~
+git clone https://git.sharcnet.ca/tyson/nix-nixpkgs-sharcnet.git
+```
+
+If you already have a copy, you can update to the latest version
+
+```sh
+cd ~/nix-nixpkgs-sharcnet
+git pull
+```
+
+## Installing an expression
+
+To use any of the provided Nix expressions, you need to copy it to or
+link it into your *~/.nix-defexpr* directory (replace *package.nix*
+with the name of the Nix expression you wish to use)
+
+```sh
+ln -s ../nix-nixpkgs-sharcnet/package.nix ~/.nix-defexpr/
+
+```
+
+Using a symbolic link has the advantage that it automatically gets
+updated whenever you update *~/nix-nixpkgs-sharcnet*.
+
+# Using an expression
+
+Once you have the expression in your *~/.nix-defexpr* you can use the
+`nix-env` command to install (or update) the associated package in
+your Nix environment.
+
+## Installing and updating
+
+To install or update the associated package (replace *package* with
+the base name of the Nix expression file you copied to
+*~/.nix-defexpr*)
+
+```sh
+nix-env --install --attr package
+```
+
+## Removing
+
+To remove a package you first need to figure out its name (generally,
+but not nessesarily, the same as the name of the Nix expression file)
+
+```sh
+nix-env --query
+```
+
+Then you (replace *package* with the package name)
+
+```sh
+nix-env --uninstall package
+```
+
+## Reverting
+
+To undo the last install/update/remove command
+
+```sh
+nix-env --rollback
+```