getting command not found when middleman is called - ruby

Question

Hi I have installed ruby in my ubuntu 16.04 and successfully installed, and also installed middleman to check my project its worked properly, when I reconnect to middleman I got error middleman: command not found I am not getting what the mistake is. when I try to re initialise it, it asks to install ruby. please help someone to know the exact error.
urujada#gurujada-To-be-filled-by-O-E-M:~$ cd sunil
gurujada#gurujada-To-be-filled-by-O-E-M:~/sunil$ atom .
gurujada#gurujada-To-be-filled-by-O-E-M:~/sunil$ middleman
middleman: command not found
gurujada#gurujada-To-be-filled-by-O-E-M:~/sunil$ middleman
middleman: command not found
gurujada#gurujada-To-be-filled-by-O-E-M:~/sunil$ middleman init
middleman: command not found
gurujada#gurujada-To-be-filled-by-O-E-M:~/sunil$ cd Home
bash: cd: Home: No such file or directory
gurujada#gurujada-To-be-filled-by-O-E-M:~/sunil$ cd..
cd..: command not found
gurujada#gurujada-To-be-filled-by-O-E-M:~/sunil$ cd ..
gurujada#gurujada-To-be-filled-by-O-E-M:~$ cd Home
bash: cd: Home: No such file or directory
gurujada#gurujada-To-be-filled-by-O-E-M:~$ mkdir example
gurujada#gurujada-To-be-filled-by-O-E-M:~$ cd example
gurujada#gurujada-To-be-filled-by-O-E-M:~/example$ middleman init
middleman: command not found
gurujada#gurujada-To-be-filled-by-O-E-M:~/example$ cd ..
gurujada#gurujada-To-be-filled-by-O-E-M:~$ gem install middleman
The program 'gem' is currently not installed. You can install it by typing:
sudo apt install ruby
gurujada#gurujada-To-be-filled-by-O-E-M:~$ sudo apt install ruby
[sudo] password for gurujada:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
fonts-lato javascript-common libjs-jquery libruby2.3 rake ruby-did-you-mean
ruby-minitest ruby-net-telnet ruby-power-assert ruby-test-unit ruby2.3
rubygems-integration
Suggested packages:
apache2 | lighttpd | httpd ri ruby-dev bundler
The following NEW packages will be installed:
fonts-lato javascript-common libjs-jquery libruby2.3 rake ruby
ruby-did-you-mean ruby-minitest ruby-net-telnet ruby-power-assert
ruby-test-unit ruby2.3 rubygems-integration
0 upgraded, 13 newly installed, 0 to remove and 11 not upgraded.
Need to get 6,042 kB of archives.
After this operation, 27.5 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 fonts-lato all 2.0-1 [2,693 kB]
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Get:5 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 ruby-did-you-mean all 1.0.0-2 [8,390 B]
Get:6 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 ruby-minitest all 5.8.4-2 [36.6 kB]
Get:7 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 ruby-net-telnet all 0.1.1-2 [12.6 kB]
Get:8 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 ruby-power-assert all 0.2.7-1 [7,668 B]
Get:9 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 ruby-test-unit all 3.1.7-2 [60.3 kB]
Get:10 http://in.archive.ubuntu.com/ubuntu xenial-updates/main amd64 libruby2.3 amd64 2.3.1-2~16.04 [2,957 kB]
Get:11 http://in.archive.ubuntu.com/ubuntu xenial-updates/main amd64 ruby2.3 amd64 2.3.1-2~16.04 [40.9 kB]
Get:12 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 ruby all 1:2.3.0+1 [5,530 B]
Get:13 http://in.archive.ubuntu.com/ubuntu xenial/main amd64 rake all 10.5.0-2 [48.2 kB]
Fetched 6,042 kB in 25s (239 kB/s)
Selecting previously unselected package fonts-lato.
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gurujada#gurujada-To-be-filled-by-O-E-M:~$ gem install middleman
Fetching: bundler-1.12.5.gem (100%)
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /var/lib/gems/2.3.0 directory.
gurujada#gurujada-To-be-filled-by-O-E-M:~$ cd example
gurujada#gurujada-To-be-filled-by-O-E-M:~/example$ middleman init
middleman: command not found
gurujada#gurujada-To-be-filled-by-O-E-M:~/example$ cd ..
gurujada#gurujada-To-be-filled-by-O-E-M:~$ ruby -v
ruby 2.3.1p112 (2016-04-26) [x86_64-linux-gnu]
gurujada#gurujada-To-be-filled-by-O-E-M:~$ whereis ruby
ruby: /usr/bin/ruby /usr/bin/ruby2.3 /usr/lib/ruby /usr/share/man/man1/ruby.1.gz
gurujada#gurujada-To-be-filled-by-O-E-M:~$

Answer 1

From your log:
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /var/lib/gems/2.3.0 directory.
Middleman was not successfully installed because your user can't write to the directory it should be installed in.
You have two easy options:
run sudo gem install middleman
run sudo gem install bundler, and then in your project directory, create a Gemfile with the following content:
source "https://rubygems.org"
gem "middleman", "~> 4.1"
and run bundle install in the project directory.
You should really do option 2 so you can keep multiple version of gems installed.
You could also not use the system Ruby and install one yourself with RVM or rbenv, but that is a little more work.