I would expect Dir.home to change when I change HOME environment variable in Ruby:

$ echo $HOME
/home/efranz
$ pwd
/home/efranz
$ ls -ld ~
drwx------ 2 efranz PZS0714 4096 Oct 31 14:54 /home/efranz
$ HOME=$(mktemp -d)
$ echo $HOME
/tmp/tmp.IkjRJV5IYi
$ pwd
/home/efranz
$ ls -ld ~
drwx------ 2 efranz PZS0714 4096 Oct 31 14:57 /tmp/tmp.IkjRJV5IYi
$ (ls -ld ~)
drwx------ 2 efranz PZS0714 4096 Oct 31 14:57 /tmp/tmp.IkjRJV5IYi
$ ruby -e "puts \`ls -ld ~\`"
drwx------ 2 efranz PZS0714 4096 Oct 31 14:57 /tmp/tmp.IkjRJV5IYi
$ ruby -e "puts Dir.home"
/tmp/tmp.IkjRJV5IYi

It does. But not for Dir.home(USERNAME) or File.expand_path:

$ ruby -e "puts Dir.home('efranz')"
/home/efranz
$ ruby -e "puts File.expand_path '~efranz'"
/home/efranz

What is happening? Lets peek at the implementation of Dir.home.

$ ruby --version
ruby 2.0.0p648 (2015-12-16) [x86_64-linux]

https://ruby-doc.org/core-2.0.0/Dir.html#method-c-home

Returns the home directory of the current user or the named user if given.

This is one function that executes two different code paths based on the argument provided (or omitted).

See https://www.ruby-lang.org/en/news/2015/12/16/ruby-2-0-0-p648-released/ It is actually stored in subversion, but here is a copy in GitHub.

  1. If rb_home_dir is called without the username, it returns the value of getenv("HOME"):
  2. If rb_home_dir is called with a username, it gets the password struct via getpwnam(user)

So this can cause problems if you try to change the home directory for the Ruby process you are executing by modifying the HOME environment variable.

See man 3 getpwnam

The getpwnam() function returns a pointer to a structure containing the broken-out fields of the record in the password database (e.g., the local password file /etc/passwd, NIS, and LDAP) that matches the username name.

Conclusion

Be careful to consider whether or not it is appropriate to use Dir.home or Dir.home(user) and treat them as two separate functions. And assume that File.expand_path (or basically anything that resolves ~ in Ruby) is going to use getpwnam.

rubygems is now careful about this as of v3

See https://github.com/rubygems/rubygems/commit/47c9f378687a6f806561a14dd50eae8eb4652882 which switched from using File.expand_path "~" to Dir.home to find the home directory.