Why do I need to run "/bin/bash --login"

Solution 1:

It sounds like the environment necessary for the system to find the installed ruby components is specified in a file that only gets read for login shells. The bash manual page has this to say about the difference between login shells and non-login shells:

INVOCATION
   A  login shell is one whose first character of argument zero is a -, or
   one started with the --login option.

and

   When bash is invoked as an interactive login shell, or as a non-inter‐
   active shell with the --login option, it first reads and executes  com‐
   mands  from  the file /etc/profile, if that file exists. After reading
   that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile,
   in  that order, and reads and executes commands from the first one that
   exists and is readable.  

whereas

   When an interactive shell that is not a login shell  is  started,  bash
   reads  and  executes  commands  from /etc/bash.bashrc and ~/.bashrc, if
   these files exist. 

Hence if the ruby environment variables are in /home/rails/.profile or /etc/profile for example, they will be added to the shell environment

• by explicitly invoking a login shell using su -l rails or su --login rails or the shorthand su - rails
• when user rails logs in via SSH
• by starting a subshell as bash --login after login

If you want the ruby environment to be set regardless of how you switch to user rails, you could move the relevant variable definitions to the user's ~/.bashrc instead.

Solution 2:

I know that this question was asked 2 years ago, but in case if somebody (like me) still facing it: @steeldriver is right -- you are missing something in your bashrc which you do have in one of those 3 files. In my case I just needed to add this line into mine ~/.bashrc:

[[ -s "$HOME/.rvm/scripts/rvm" ]] && source "$HOME/.rvm/scripts/rvm"