style: re-enable shellcheck.

Ensure that `shellcheck` is run by `brew style` again and fix the
various failures.

Library/Homebrew/brew.sh

@@ -5,7 +5,7 @@ then
 fi
 
 # USER isn't always set so provide a fall back for `brew` and subprocesses.
-export USER=${USER:-`id -un`}
+export USER=${USER:-$(id -un)}
 
 # Where we store built products; a Cellar in HOMEBREW_PREFIX (often /usr/local
 # for bottles) unless there's already a Cellar in HOMEBREW_REPOSITORY.

Library/Homebrew/style.rb

@@ -56,7 +56,7 @@ module Homebrew
         File.expand_path(file).start_with? HOMEBREW_LIBRARY_PATH
       end
 
-      if files && !has_non_formula
+      if files.present? && !has_non_formula
         config = if files.first && File.exist?("#{files.first}/spec")
           HOMEBREW_LIBRARY/".rubocop_rspec.yml"
         else
@@ -65,7 +65,7 @@ module Homebrew
         args << "--config" << config
       end
 
-      if files.nil?
+      if files.blank?
         args << HOMEBREW_LIBRARY_PATH
       else
         args += files
@@ -79,7 +79,7 @@ module Homebrew
       when :print
         args << "--debug" if Homebrew.args.debug?
         args << "--display-cop-names" if Homebrew.args.display_cop_names?
-        args << "--format" << "simple" if files
+        args << "--format" << "simple" if files.present?
         system(cache_env, "rubocop", *args)
         rubocop_success = $CHILD_STATUS.success?
       when :json
@@ -100,7 +100,7 @@ module Homebrew
         raise "Invalid output_type for check_style_impl: #{output_type}"
       end
 
-      return rubocop_success if files.present? || !has_non_formula
+      return rubocop_success if files.present?
 
       shellcheck   = which("shellcheck")
       shellcheck ||= which("shellcheck", ENV["HOMEBREW_PATH"])

Library/Homebrew/utils/ruby.sh

@@ -44,7 +44,7 @@ If there's no Homebrew Portable Ruby available for your processor:
   then
     HOMEBREW_RUBY_PATH="$vendor_ruby_path"
     [[ -z "$HOMEBREW_MACOS" ]] && TERMINFO_DIRS="$vendor_ruby_terminfo"
-    if [[ $vendor_ruby_current_version != $vendor_ruby_latest_version ]]
+    if [[ $vendor_ruby_current_version != "$vendor_ruby_latest_version" ]]
     then
       if ! brew vendor-install ruby
       then

bin/brew

@@ -23,8 +23,8 @@ symlink_target_directory() {
 builtin enable compgen unset
 for cmd in $(builtin compgen -A builtin)
 do
-  builtin unset -f $cmd
-  builtin enable $cmd
+  builtin unset -f "$cmd"
+  builtin enable "$cmd"
 done
 unset cmd