[[kerl]]
== OTP version management

Erlang.mk comes with integrated support for
https://github.com/kerl/kerl[Kerl], a shell script that
automates the downloading, building and installing of
Erlang/OTP. It can be used to easily build a specific
Erlang/OTP version (with or without custom build options)
or maintain different versions side by side.

=== Erlang versions

Erlang.mk uses the Git tags from Erlang/OTP to identify
OTP versions. The most recent tag at the time of writing
is `OTP-20.0.4`, which is a patch release of `OTP-20.0`.
A patch release is a non-official release containing a
few fixes on top of the official release.

Older versions used a slightly different versioning scheme
and tag format, for example: `OTP_R16B03`. Beware though,
there also was an `OTP_R16B03-1` release that fixed a
critical issue in the initial release.

The README file for all official Erlang/OTP releases can
be found on http://www.erlang.org/downloads[erlang.org].
To obtain information about patch releases when they are
released you need to be subscribed to the
http://erlang.org/mailman/listinfo/erlang-questions[erlang-questions mailing list].

[[otp_version_pinning]]
=== OTP version pinning

Erlang.mk can use a specific version of Erlang/OTP when
interacting with your project. This can be very useful
when you are working with a team because you can define
the version you need in the Makefile and Erlang.mk will
ensure this version is used by everyone in your team.

To pin the version all you need to do is to set the
`ERLANG_OTP` variable in your Makefile before including
Erlang.mk. For example, to use `OTP-20.0.4`:

[source,make]
----
ERLANG_OTP = OTP-20.0.4

include erlang.mk
----

The next time you run `make` Erlang.mk will build and
use the version you configured.

Note that there has been reports that this functionality
is not compatible with the `.ONESHELL` feature from
GNU Make.

=== Continuous integration

Erlang.mk can automatically test your project against
many different Erlang/OTP versions. This functionality
is documented in the xref:ci[Continuous integration] chapter.

=== Configuring Kerl

All of the Kerl variables can be configured directly in the
Makefile. All you need to do is to export them. For example,
to change the installation directory for the Erlang/OTP
versions managed by Kerl, you could add the following to
your Makefile:

[source,make]
export KERL_INSTALL_DIR = $(CURDIR)/erlang

When configuring paths like this, always make sure to provide
an absolute path in the value. Erlang.mk will NOT expand them
automatically for you.