Using kerl with direnv

30 Sep 2014 00:00 kerl direnv erlang

kerl allows you to easily build and install multiple Erlang/OTP releases. It’s kinda like nvm or rvm, but for Erlang. It doesn’t do everything, and that’s what direnv is for. direnv allows you to run commands upon entering or leaving a particular directory.

Installing direnv

direnv requires make and go to be installed, so on Ubuntu, that’s:

sudo apt-get install make golang

On Mac OS X, that’s:

brew update ; brew install go

Then follow (more-or-less) the instructions at

git clone
cd direnv
sudo make install       # sudo is optional on OS X.

To track the current directory, direnv hooks your shell. On bash, it hooks $PROMPT_COMMAND. To get this to work, add the following near the end of your .bashrc:

type -P direnv &>/dev/null && eval "$(direnv hook bash)"

This is slightly different from the installation instructions on the direnv homepage, because my bash profile is shared between all of the machines I use, and not all of them will have direnv installed.

Installing kerl

See, but it’s essentially this:

curl -O
chmod +x kerl

Note that, if you put kerl in $PATH for your primary user account, it probably won’t be available when attempting to run it via sudo, so it’s probably easier to put it in (e.g.) /usr/local/bin:

sudo mv kerl /usr/local/bin

Using kerl to build and install Erlang

In this example, I’ll show building R16B03-1, with the default settings. This is the version used at Electric Imp at the time of writing.

kerl list releases

kerl build R16B03-1 r16b03-1
kerl list builds

sudo kerl install r16b03-1 /usr/local/erlang-R16B03-1
kerl list installations

Activating an Erlang version using kerl

. /usr/local/erlang-R16B03-1/activate

Combining kerl with direnv

What if your team doesn’t use the same locations for installing Erlang versions with kerl? Then you need the following in your .envrc file:


if has kerl; then
    ERLANG_INSTALLATION=$(kerl list installations | grep "^$ERLANG_VERSION " | cut -d' ' -f2)
    if [ -x "$ERLANG_INSTALLATION/activate" ] ; then
        echo "Using Erlang $ERLANG_VERSION (in $ERLANG_INSTALLATION) via kerl."
        . $ERLANG_INSTALLATION/activate
        echo "Erlang $ERLANG_VERSION not available; using default."
    echo "kerl not available; using default Erlang."