diff --git a/Library/Homebrew/cask/cmd/doctor.rb b/Library/Homebrew/cask/cmd/doctor.rb
index ddd033a2e1..0a6a015bba 100644
--- a/Library/Homebrew/cask/cmd/doctor.rb
+++ b/Library/Homebrew/cask/cmd/doctor.rb
@@ -67,7 +67,7 @@ module Cask
 
         if path.exist? && !path.writable?
           add_error "The staging path #{user_tilde(path.to_s)} is not writable by the current user."
-          add_error "To fix, run \'sudo chown -R ${USER}:staff #{user_tilde(path.to_s)}'"
+          add_error "To fix, run \'sudo chown -R $(whoami):staff #{user_tilde(path.to_s)}'"
         end
 
         puts user_tilde(path.to_s)
diff --git a/docs/Gems,-Eggs-and-Perl-Modules.md b/docs/Gems,-Eggs-and-Perl-Modules.md
index 8fafd9655a..9b6bd42308 100644
--- a/docs/Gems,-Eggs-and-Perl-Modules.md
+++ b/docs/Gems,-Eggs-and-Perl-Modules.md
@@ -116,7 +116,7 @@ If you ever did a `sudo gem`, etc. before then a lot of files will have
 been created owned by root. Fix with:
 
 ```sh
-sudo chown -R $USER /Library/Ruby /Library/Perl /Library/Python
+sudo chown -R $(whoami) /Library/Ruby /Library/Perl /Library/Python
 ```
 
 ## Perl CPAN modules without sudo