direnv
After you've installed direnv, any directory containing a .envrc file will be executed when you enter that directory in your shell. Pairs well with basic commands for managing programming language environments.
This is super handy for loading secret values as environment variables in your programming projects.
For example, say you have an AWS token for your project that you want to keep secret, but make available to your program; you create a .envrc file in that directory with direnv edit ., and fill it in with these contents:
export AWS_TOKEN=DcCca124cedc212
save the file, exit your editor, and direnv will load that variable into the shell for you.
$ direnv edit .
direnv: loading /home/llimllib/test/.envrc
direnv: export +AWS_TOKEN
when you leave the directory, direnv will remove the variable from your environment:
$ cd ~
direnv: unloading
$ echo "aws token is now empty: <$AWS_TOKEN>"
aws token is now empty: <>
For security reasons, direnv won't execute a .envrc file until you tell it to do so with direnv allow:
$ mkdir test
$ echo "export AWS_TOKEN=blahblah" > test/.envrc
$ cd test
direnv: error /tmp/test/.envrc is blocked. Run `direnv allow` to approve its content
$ direnv allow
direnv: loading /tmp/test/.envrc
direnv: export +AWS_TOKEN
What direnv is actually doing is (roughly) starting up a new shell, executing the .envrc file, comparing the environment variables between the two, and importing the variables into your current session that are new.
That means that it is not executing the .envrc file in your current shell session, which means that running a program that modifies your environment, like activating a python virtualenv might not work exactly like you expect it to.
If you try to activate a python virtualenv in a .envrc, activate will modify your environment to insert its own python on your PATH - this will work because direnv will import the update path. However, it also tries to modify your command line prompt via $PS1, which is a local variable, and as such will not propagate back to your shell.
The direnv wiki lists some workarounds, but they're fairly gross. I just live without the updated PS1.
There are some common programming environment layouts available - for example if you add layout node to your .envrc, direnv will put $PWD/node_modules/.bin on your PATH when you enter the directory.