Docker for MAC - Mkmf.Rb Can't Find Header Files for Ruby

Docker for Mac - mkmf.rb can't find header files for ruby

For Xcode 11 on macOS 10.14, this can happen even after installing Xcode and installing command-line tools and accepting the license with

sudo xcode-select --install
sudo xcodebuild -license accept

The issue is that Xcode 11 ships the macOS 10.15 SDK which includes headers for ruby2.6, but not for macOS 10.14's ruby2.3. You can verify that this is your problem by running

ruby -rrbconfig -e 'puts RbConfig::CONFIG["rubyhdrdir"]'

which on macOS 10.14 with Xcode 11 prints the non-existent path

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/include/ruby-2.3.0

However, Xcode 11 installs a macOS 10.14 SDK within /Library/Developer/CommandLineTools/SDKs/MacOS10.14.sdk. It isn't necessary to pollute the system directories by installing the old header files as suggested in other answers. Instead, by selecting that SDK, the appropriate ruby2.3 headers will be found:

sudo xcode-select --switch /Library/Developer/CommandLineTools
ruby -rrbconfig -e 'puts RbConfig::CONFIG["rubyhdrdir"]'

This should now correctly print

/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/include/ruby-2.3.0

Likewise, gem install should work while that SDK is selected.

To switch back to using the current Xcode 11 SDK, use

sudo xcode-select --switch /Applications/Xcode.app

Error while installing json gem 'mkmf.rb can't find header files for ruby'

Modern era update, as stated by mimoralea:

In case that you are using ruby 2.0 or 2.2 (thanks @patrick-davey).
sudo apt-get install ruby2.0-dev
sudo apt-get install ruby2.2-dev
sudo apt-get install ruby2.3-dev

or, generic way:

sudo apt-get install ruby-dev

sudo apt-get install ruby`ruby -e 'puts RUBY_VERSION[/\d+\.\d+/]'`-dev

The first link you’ve posted is exactly your case: there is no ruby development environment installed. Development env is needed to compile ruby extensions, which are mostly written in C. Proxy has nothing to do with the problem: everything is downloaded fine, just compilation fails.

I would suggest you to install ruby-dev (ruby-devel for rpm-based distros) package onto you target machine.

gcc package might be needed as well.

Try:

$ sudo apt-get install ruby-dev

Or, for Redhat distro:

$ sudo yum install ruby-devel

Or, for [open]SuSE:

$ sudo zypper install ruby-devel

I can't run my Ruby program and I see errors in the terminal. I can't find the program

On Fedora & CentOS, the Ruby installation is split into many smaller packages. If you want to run irb, you'll also have to install the ruby-irb package using dnf install ruby-irb.

Can't find the 'libpq-fe.h header when trying to install pg gem

It looks like in Ubuntu that header is part of the libpq-dev package (at least in the following Ubuntu versions:
11.04 (Natty Narwhal), 10.04 (Lucid Lynx), 11.10 (Oneiric Ocelot), 12.04 (Precise Pangolin), 14.04 (Trusty Tahr) and 18.04 (Bionic Beaver)):

...
/usr/include/postgresql/libpq-fe.h
...

So try installing libpq-dev or its equivalent for your OS:

For Ubuntu/Debian systems: sudo apt-get install libpq-dev
On Red Hat Linux (RHEL) systems: yum install postgresql-devel
For Mac Homebrew: brew install postgresql
For Mac MacPorts PostgreSQL: gem install pg -- --with-pg-config=/opt/local/lib/postgresql[version number]/bin/pg_config
For OpenSuse: zypper in postgresql-devel
For ArchLinux: pacman -S postgresql-libs

Errors when installing cocoapods with gem

The answer for this can be found here here although it is unrelated to this question. Below is a snippet of the answer which works best.