Touch up README for comprehension and clarity (#2051)

# ruby-build

ruby-build is a command-line utility that makes it easy to install virtually any

version of Ruby, from source.

ruby-build is a command-line tool that simplifies installation of any Ruby version from source on Unix-like systems.

It is available as a plugin for [rbenv][] that

provides the `rbenv install` command, or as a standalone program.

It is available as a plugin for [rbenv][] as the `rbenv install` command, or as a standalone program as the `ruby-build` command.

## Installation

### Homebrew package manager

```sh

# Using Homebrew on macOS

$ brew install ruby-build

# As an rbenv plugin

$ mkdir -p "$(rbenv root)"/plugins

$ git clone https://github.com/rbenv/ruby-build.git "$(rbenv root)"/plugins/ruby-build

brew install ruby-build

```

# As a standalone program

$ git clone https://github.com/rbenv/ruby-build.git

$ PREFIX=/usr/local ./ruby-build/install.sh

Upgrade with:

```sh

brew update && brew upgrade ruby-build

```

### Upgrading

### Clone as rbenv plugin using git

```sh

git clone https://github.com/rbenv/ruby-build.git "$(rbenv root)"/plugins/ruby-build

```

Upgrade with:

```sh

# Via Homebrew

$ brew update && brew upgrade ruby-build

git -C "$(rbenv root)"/plugins/ruby-build pull

```

# As an rbenv plugin

$ git -C "$(rbenv root)"/plugins/ruby-build pull

### Install manually as a standalone program

First, download a tarball from https://github.com/rbenv/ruby-build/releases/latest. Then:

```sh

tar -xzf ruby-build-*.tar.gz

PREFIX=/usr/local ./ruby-build-*/install.sh

```

## Usage

$ ruby-build 2.2.0 ~/local/ruby-2.2.0  # installs Ruby 2.2.0 to ~/local/ruby-2.2.0

```

ruby-build does not check for system dependencies before downloading and

attempting to compile the Ruby source. Please ensure that [all requisite

libraries][build-env] are available on your system.

> **Warning**  

> ruby-build mostly does not verify that system dependencies are present before downloading and attempting to compile Ruby from source. Please ensure that [all requisite libraries][build-env] such as build tools and development headers are already present on your system.

Basically, what ruby-build does when installing a Ruby version is this:

- Downloads an official tarball of Ruby source code;

- Extracts the archive into a temporary directory on your system;

- Executes `./configure --prefix=/path/to/destination` in the source code;

- Runs `make install` to compile Ruby;

- Verifies that the installed Ruby is functional.

Depending on the context, ruby-build does a little bit more than the above: for example, it will try to link Ruby to the appropriate OpenSSL version, even if that means downloading and compiling OpenSSL itself; it will discover and link to Homebrew-installed instances of some libraries like libyaml and readline, etc.

### Advanced Usage

#### Custom Build Definitions

If you wish to develop and install a version of Ruby that is not yet supported

by ruby-build, you may specify the path to a custom “build definition file” in

place of a Ruby version number.

To install a version of Ruby that is not recognized by ruby-build, you can specify the path to a custom build definition file in place of a Ruby version number.

Use the [default build definitions][definitions] as a template for your custom

definitions.

Check out [default build definitions][definitions] as examples on how to write definition files.

#### Custom Build Configuration

#### Applying Patches

Both `rbenv install` and `ruby-build` support the `--patch` (`-p`) flag to apply

a patch to the Ruby (/JRuby/Rubinius/TruffleRuby) source code before building.

Patches are read from `STDIN`:

Both `rbenv install` and `ruby-build` commands support the `-p/--patch` flag to apply a patch to the Ruby source code before building. Patches are read from standard input:

```sh

# applying a single patch

#### Checksum Verification

If you have the `shasum`, `openssl`, or `sha256sum` tool installed, ruby-build will

automatically verify the SHA2 checksum of each downloaded package before

installing it.

All Ruby definition files bundled with ruby-build include checksums for packages, meaning that all externally downloaded packages are automatically checked for integrity after fetching.

Checksums are optional and specified as anchors on the package URL in each

definition. All definitions bundled with ruby-build include checksums.

See the next section for more information on how to author checksums.

#### Package Mirrors

Be sure to include the full build log for build failures.

  [rbenv]: https://github.com/rbenv/rbenv

  [rbenv]: https://github.com/rbenv/rbenv#readme

  [definitions]: https://github.com/rbenv/ruby-build/tree/master/share/ruby-build

  [wiki]: https://github.com/rbenv/ruby-build/wiki

  [build-env]: https://github.com/rbenv/ruby-build/wiki#suggested-build-environment