loading nix-env automatically

1 aug 2013

motivation

using my-env you create a custom environment, see [1] where you will find a ~/.nixpkgs/config.nix example script. a couple of times i was discussing with others that it would be cool to have that environment loaded automatically when entering that directory. some say that the ruby devs did astonishing things in that regard. reading [2] finally inspired me to write this example how this could be done.

so assuming you have installed ‘nix-env -i env-sdl’, this posting describes what to do in order to get it loaded automatically.

the shell extension(s)

default shell (cd overloading)

i put this into my ~/.zshrc:

function cd {
    # loads environments automatically when .load-env file exists with the project name <name> in it
    # load-env-<name> must of course exist, example: load-env-sdl where .load-env would contain sdl
    builtin cd "$@"
    if [ -f ".load-env" ] ; then
        echo "Note: found '.load-env' file, automatically loading the 'load-env-${a}' environment."
        # display its contents
        a=$(cat .load-env) 
        load-env-${a}
        echo "Note: You left the 'load-env-${a}' environment. Type: load-env-${a} to reload it"
    fi
}

afterwards type:

zsh

load-env-sdl shell

to make changes to the spawned shell, edit $NIXREPOS/nixpkgs/pkgs/misc/my-env/default.nix

usage

to load the environment manually:

% load-env-sdl
env-sdl loaded
 
sdl:[joachim@lenovo-t530:~/projects/myproject]$

to add a environemnt loader when entering the directory:

% cd ~/projects/myproject
% echo "sdl" > .load-env

this creates a .load-env file containing ‘sdl’ in it.

afterwards the environment should be loaded automatically when entering ~/projects/myproject

% cd ~/projects/myproject
env-sdl loaded
 
sdl:[joachim@lenovo-t530:~/projects/myproject]$

summary

there is currently no support for environment nesting but i doubt this is needed anyway. whenever you want to leave the environment simply type ‘exit’ or hit ‘ctrl+d’.