How to set RVM GEM_HOME and GEM_PATH properly for BASH + TMUX

.fix_rvm

# be sure to change the values below correctly for your system!
export GEM_HOME="$HOME/.rvm/gems/ruby-2.4.1"
export GEM_PATH="$HOME/.rvm/gems/ruby-2.4.1:$HOME/.rvm/gems/ruby-2.4.1@global"

add to startup script

[ -f "$HOME/.fix_rvm" ] && source "$HOME/.fix_rvm" # read in the necessary GEM_HOME and GEM_PATH values for our default ruby
# place the above line just before RVM's:
# [[ -s "$HOME/.rvm/scripts/rvm" ]] && source "$HOME/.rvm/scripts/rvm" # Load RVM into a shell session *as a function* 

howto fix rvm for tmux when using bash shell

# The short version:
# 1) Run 'rvm use default' once in a shell.
# 2) echo $GEM_HOME; echo $GEM_PATH
# 3) Export these values to the shell in a startup file.  
# 4) Update the values if you update your default rvm ruby

# The very verbose version:
# Manually run once: ("$ " at start of a line indicates a command prompt, it is not part of the input to type)
$ rvm use default

# Next,
$ echo $GEM_HOME
# example output: /home/user/.rvm/gems/ruby-2.4.1
$ echo $GEM_PATH
# example output: /home/user/.rvm/gems/ruby-2.4.1:/home/user/.rvm/gems/ruby-2.4.1@global

# RVM should have set these variables appropriately for your default ruby, so we can now steal them to add to our startup scripts
# the caveat is we will have to update our scripts if we update our default ruby
# in my case, I created a specific file in my home directory (example: /home/user/.fix_rvm) with these lines that export the variables
# with the values we got above:
# [begin file .fix_rvm]

export GEM_HOME="$HOME/.rvm/gems/ruby-2.4.1"
export GEM_PATH="$HOME/.rvm/gems/ruby-2.4.1:$HOME/.rvm/gems/ruby-2.4.1@global"

# [end file .fix_rvm]

# NOTE: $HOME is a variable that equals the '/home/user' part of the examples, the benefit is regardless of your username, it will be
# correct where "/home/user" would almost certainly not (except if your username is, in fact, "user", and your system puts your home
# directory under /home)

# Finally, I add the file to my startup scripts so that the commands in it are executed.  I placed them just before RVM does its own
# command loading
# [snip of my .profile where the command is added]

# Add RVM to PATH for scripting. Make sure this is the last PATH variable change.
export PATH="$HOME/.rvm/bin:$PATH"
[ -f "$HOME/.fix_rvm" ] && source "$HOME/.fix_rvm" # read in the necessary GEM_HOME and GEM_PATH values for our default ruby
[[ -s "$HOME/.rvm/scripts/rvm" ]] && source "$HOME/.rvm/scripts/rvm" # Load RVM into a shell session *as a function* 

# [end snip]

For bonus nerd points, short of hacking on rvm itself, you could make a shell function wrapper that updates the file along with running the necessary rvm command(s) to switch the default. I do not feel the need to earn bonus nerd points.