The install command is not embedded into rbenv, it comes from the ruby-build plugin. You can install it using the command:

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

On Mac OS X you can install it through homebrew:

brew install ruby-build

On Debian (version >= 7) and Ubuntu (version >= 12.10) both rbenv and ruby-build can be installed using apt-get (or aptitude):

sudo apt-get update
sudo apt-get install rbenv ruby-build

On FreeBSD ruby-build is available in the Ports Collection, it can be install both as a binary package or build from the port:

# Using pkgng rbenv will be installed
pkg install ruby-build

# Building ruby-build form Ports will install rbenv only if the RBENV option is set
cd /usr/ports/devel/ruby-build
make install

I found that when using rbenv from a global directory, it's necessary to export the RBENV_ROOT variable, otherwise it won't load the plugins.

export RBENV_ROOT="/usr/local/rbenv"
if [ -d "${RBENV_ROOT}" ]; then
  export PATH="${RBENV_ROOT}/bin:${PATH}"
fi

As everyone mentioned problem is missing ruby-build. For older versions of OS ruby-build may not be available as an apt package. In that case install using original instructions, which should've omitted the word Optional in this:

5. (Optional) Install ruby-build, which provides the rbenv install command that simplifies the process of installing new Ruby versions.

git clone [email protected]:rbenv/ruby-build.git ~/.rbenv/plugins/ruby-build
# OR use http
git clone  ~/.rbenv/plugins/ruby-build

Simply install ruby-build in ubuntu:

sudo apt-get install ruby-build

And add

eval "$(rbenv init -)"

To your ~/.bashrc

I had installed the ruby-build plugin before and installed ruby 1.9.3-p327 using

$ rbenv install 1.9.3-p327

A few days later I tried to install ruby 2.0.0-p247 using

$ rbenv install 2.0.0-p247

but I received the error message

rbenv: no such command 'install'

All I had to do was to run

$ exec $SHELL -l

and that fixed the problem.

Answered on 2021

If you're getting that error, it's very likely you have accidentally skipped one of the installation instructions:

git clone  ~/.rbenv/plugins/ruby-build
echo 'export PATH="$HOME/.rbenv/plugins/ruby-build/bin:$PATH"' >> ~/.bashrc
exec $SHELL

Replace .bashrc with whatever shell you're using, for example .zshrc, or just manually access your shell config file and paste this line:

export PATH="$HOME/.rbenv/plugins/ruby-build/bin:$PATH"

I strong advise against re-installing/installing Ruby via brew or apt-get just to get around this issue. The whole point of using rbenv is to make your life easier in future when there is ruby version upgrade, or when you're working on different rails projects that require different version of ruby.

It looks like ruby-build is not present. Run this command :

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

Anyone finding their way here with this issue on OSX and already having installed ruby-build via homebrew (like me), you may solve this by just upgrading ruby-build:

brew update
brew upgrade ruby-build

This fixed the problem for me.

This issue also happens in Linux when doing a stand-alone installation of ruby-build, if the ruby-build executable is not found in the path. If installing under /usr/local, try for example:

PATH=/usr/local/bin:$PATH /usr/local/bin/rbenv install ...

I came to this solution, but looking for a macOS solution that uses MacPorts.
So, here the same command using MacPorts:

➜ ~ sudo port install ruby-build
Password:
--->  Computing dependencies for ruby-build
--->  Fetching archive for ruby-build
--->  Attempting to fetch ruby-build-20210804_0.darwin_19.noarch.tbz2 from 
--->  Attempting to fetch ruby-build-20210804_0.darwin_19.noarch.tbz2.rmd160 from 
--->  Installing ruby-build @20210804_0
--->  Activating ruby-build @20210804_0
--->  Cleaning ruby-build
--->  Scanning binaries for linking errors
--->  No broken files found.
--->  No broken ports found.

➜ ~ curl -fsSL  | bash

Checking for `rbenv' in PATH: /opt/local/bin/rbenv
Checking for rbenv shims in PATH: OK
Checking `rbenv install' support: /opt/local/bin/rbenv-install (ruby-build 20210804)
Counting installed Ruby versions: none
  There aren't any Ruby versions installed under `/Users/user/.rbenv/versions'.
  You can install Ruby versions like so: rbenv install 3.0.2
Checking RubyGems settings: OK
Auditing installed plugins: OK
➜ ~ rbenv install 3.0.2
Downloading openssl-1.1.1k.tar.gz...
-> 
Installing openssl-1.1.1k...