Exporting an Environment Variable in Ruby

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You can’t export environment variables to the shell the ruby script runs in, but
you could write a ruby script that creates a source-able bash file.

For example 

% echo set_var.rb
#!/usr/bin/env ruby
varname = ARGV[0]
puts "#{varname}=#{STDIN.gets.chomp}"
% set_var.rb FOO
1
FOO=1
% set_var.rb BAR > temp.sh ; . temp.sh
2
% echo $BAR
2
%

Another alternative is that using ENV[]= does set environment variables for subshells opened from within the ruby process. For example:

outer-bash% echo pass_var.rb
#!/usr/bin/env ruby
varname = ARGV[0]
ENV[varname] = STDIN.gets.chomp
exec '/usr/bin/env bash'
outer-bash% pass_var.rb BAZ
quux
inner-bash% echo $BAZ
quux

This can be quite potent if you combine it with the shell’s exec command, which will replace the outer-shell with the ruby process (so that when you exit the inner shell, the outer shell auto-exits as well, preventing any “I thought I set that variable in this shell” confusion).

# open terminal
% exec pass_var.rb BAZ
3
% echo $BAZ
3
% exit
# terminal closes

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