'rails server'命令返回错误：`mkdir'：权限被拒绝@ dir_s_mkdir

Question

I am trying to run a rails server sample that I downloaded. But, I am getting a permissions error:

I run:

rails server

I get:

=> Booting Puma
=> Rails 4.2.4 application starting in development on http://localhost:3000
=> Run `rails server -h` for more startup options
=> Ctrl-C to shutdown server
Exiting
/usr/share/rvm/rubies/ruby-2.1.7/lib/ruby/2.1.0/fileutils.rb:250:in `mkdir': Permission denied @ dir_s_mkdir - /home/username/web-app/tmp (Errno::EACCES)
    from /usr/share/rvm/rubies/ruby-2.1.7/lib/ruby/2.1.0/fileutils.rb:250:in `fu_mkdir'
    from /usr/share/rvm/rubies/ruby-2.1.7/lib/ruby/2.1.0/fileutils.rb:224:in `block (2 levels) in mkdir_p'
    from /usr/share/rvm/rubies/ruby-2.1.7/lib/ruby/2.1.0/fileutils.rb:222:in `reverse_each'
    from /usr/share/rvm/rubies/ruby-2.1.7/lib/ruby/2.1.0/fileutils.rb:222:in `block in mkdir_p'
    from /usr/share/rvm/rubies/ruby-2.1.7/lib/ruby/2.1.0/fileutils.rb:208:in `each'
    from /usr/share/rvm/rubies/ruby-2.1.7/lib/ruby/2.1.0/fileutils.rb:208:in `mkdir_p'
    from /home/username/.rvm/gems/ruby-2.1.7/gems/railties-4.2.4/lib/rails/commands/server.rb:134:in `block in create_tmp_directories'
    from /home/username/.rvm/gems/ruby-2.1.7/gems/railties-4.2.4/lib/rails/commands/server.rb:133:in `each'
    from /home/username/.rvm/gems/ruby-2.1.7/gems/railties-4.2.4/lib/rails/commands/server.rb:133:in `create_tmp_directories'
    from /home/username/.rvm/gems/ruby-2.1.7/gems/railties-4.2.4/lib/rails/commands/server.rb:77:in `start'
    from /home/username/.rvm/gems/ruby-2.1.7/gems/railties-4.2.4/lib/rails/commands/commands_tasks.rb:80:in `block in server'
    from /home/username/.rvm/gems/ruby-2.1.7/gems/railties-4.2.4/lib/rails/commands/commands_tasks.rb:75:in `tap'
    from /home/username/.rvm/gems/ruby-2.1.7/gems/railties-4.2.4/lib/rails/commands/commands_tasks.rb:75:in `server'
    from /home/username/.rvm/gems/ruby-2.1.7/gems/railties-4.2.4/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
    from /home/username/.rvm/gems/ruby-2.1.7/gems/railties-4.2.4/lib/rails/commands.rb:17:in `<top (required)>'
    from bin/rails:4:in `require'
    from bin/rails:4:in `<main>'

Seeing that it was a permissions error I tried:

sudo rails server

But I only received this error

Your Ruby version is 1.9.3, but your Gemfile specified 2.1.7

How do I overcome the permissions problem when the root user seems to be using a different version of ruby?

EDIT 1:

ruby --version

returns:

ruby 2.1.7p400 (2015-08-18 revision 51632) [i686-linux]

EDIT 2:

sudo ruby --version

returns:

ruby 1.9.3p484 (2013-11-22 revision 43786) [i686-linux]

EDIT 3:

There is no tmp file in my web-app directory. When I type ls -la I get:

drwxr-xr-x 12 root    root     4096 Nov 15 10:32 .
drwxr-xr-x 31 username username 4096 Nov 17 04:23 ..
drwxr-xr-x 10 root    root     4096 Nov 15 10:32 app
drwxr-xr-x  2 root    root     4096 Nov 15 10:32 bin
-rw-r--r--  1 root    root      126 Nov 15 10:32 browserslist
-rw-r--r--  1 root    root      363 Nov 15 10:32 Capfile
drwxr-xr-x  6 root    root     4096 Nov 15 10:32 config
-rw-r--r--  1 root    root      153 Nov 15 10:32 config.ru
drwxr-xr-x  3 root    root     4096 Nov 15 10:32 db
-rw-r--r--  1 root    root     1301 Nov 15 10:32 Gemfile
-rw-r--r--  1 root    root    10953 Nov 15 10:32 Gemfile.lock
drwxr-xr-x  8 root    root     4096 Nov 15 10:32 .git
-rw-r--r--  1 root    root      470 Nov 15 10:32 .gitignore
drwxr-xr-x  4 root    root     4096 Nov 15 10:32 lib
drwxr-xr-x  2 root    root     4096 Nov 15 10:32 log
drwxr-xr-x  2 root    root     4096 Nov 15 10:32 public
-rw-r--r--  1 root    root      249 Nov 15 10:32 Rakefile
-rw-r--r--  1 root    root      478 Nov 15 10:32 README.rdoc
-rw-r--r--  1 root    root        8 Nov 15 10:32 .rspec
drwxr-xr-x  3 root    root     4096 Nov 15 10:32 spec
drwxr-xr-x  3 root    root     4096 Nov 15 10:32 vendor

Answer 1

First one must create the tmp directory, then give it the necessary permissions. This can be done with the following code:

sudo mkdir tmp
sudo chmod 777 tmp

Answer 2

Because every time you run rails server it create new file called server.pid in your tmp/pids folder. In your case when you run the server , it tries to create pid but you don't have the permission to create any file under this folder . you need to provide write permission to tmp directory.