diff --git a/Library/Homebrew/cask/lib/hbc/cli.rb b/Library/Homebrew/cask/lib/hbc/cli.rb
index e2deb6f671..7197303a8f 100644
--- a/Library/Homebrew/cask/lib/hbc/cli.rb
+++ b/Library/Homebrew/cask/lib/hbc/cli.rb
@@ -96,7 +96,7 @@ module Hbc
       if command.respond_to?(:run)
         # usual case: built-in command verb
         command.run(*args)
-      elsif require?(which("brewcask-#{command}.rb"))
+      elsif require?(which("brewcask-#{command}.rb"), ENV["HOMEBREW_PATH"])
         # external command as Ruby library on PATH, Homebrew-style
       elsif command.to_s.include?("/") && require?(command.to_s)
         # external command as Ruby library with literal path, useful
diff --git a/Library/Homebrew/cask/lib/hbc/container/cab.rb b/Library/Homebrew/cask/lib/hbc/container/cab.rb
index 010fccbc40..327aece6ea 100644
--- a/Library/Homebrew/cask/lib/hbc/container/cab.rb
+++ b/Library/Homebrew/cask/lib/hbc/container/cab.rb
@@ -10,7 +10,7 @@ module Hbc
       end
 
       def extract
-        if (cabextract = which("cabextract")).nil?
+        unless cabextract = which("cabextract", PATH.new(ENV["PATH"], HOMEBREW_PREFIX/"bin"))
           raise CaskError, "Expected to find cabextract executable. Cask '#{@cask}' must add: depends_on formula: 'cabextract'"
         end
 
diff --git a/Library/Homebrew/cask/lib/hbc/container/generic_unar.rb b/Library/Homebrew/cask/lib/hbc/container/generic_unar.rb
index 7a465e12e0..63ab917ea2 100644
--- a/Library/Homebrew/cask/lib/hbc/container/generic_unar.rb
+++ b/Library/Homebrew/cask/lib/hbc/container/generic_unar.rb
@@ -6,14 +6,14 @@ module Hbc
   class Container
     class GenericUnar < Base
       def self.me?(criteria)
-        !(lsar = which("lsar")).nil? &&
-          criteria.command.run(lsar,
-                               args:         ["-l", "-t", "--", criteria.path],
-                               print_stderr: false).stdout.chomp.end_with?("passed, 0 failed.")
+        return false unless lsar = which("lsar", PATH.new(ENV["PATH"], HOMEBREW_PREFIX/"bin"))
+        criteria.command.run(lsar,
+                             args:         ["-l", "-t", "--", criteria.path],
+                             print_stderr: false).stdout.chomp.end_with?("passed, 0 failed.")
       end
 
       def extract
-        if (unar = which("unar")).nil?
+        unless unar = which("unar", PATH.new(ENV["PATH"], HOMEBREW_PREFIX/"bin"))
           raise CaskError, "Expected to find unar executable. Cask #{@cask} must add: depends_on formula: 'unar'"
         end
 
diff --git a/Library/Homebrew/cask/lib/hbc/container/gpg.rb b/Library/Homebrew/cask/lib/hbc/container/gpg.rb
index 3f37b5aa66..09c75468ac 100644
--- a/Library/Homebrew/cask/lib/hbc/container/gpg.rb
+++ b/Library/Homebrew/cask/lib/hbc/container/gpg.rb
@@ -24,7 +24,7 @@ module Hbc
       end
 
       def extract
-        if (gpg = which("gpg")).nil?
+        unless gpg = which("gpg", PATH.new(ENV["PATH"], HOMEBREW_PREFIX/"bin"))
           raise CaskError, "Expected to find gpg executable. Cask '#{@cask}' must add: depends_on formula: 'gpg'"
         end
 
diff --git a/Library/Homebrew/cask/lib/hbc/container/lzma.rb b/Library/Homebrew/cask/lib/hbc/container/lzma.rb
index 1221acfbc0..1e165ba2a7 100644
--- a/Library/Homebrew/cask/lib/hbc/container/lzma.rb
+++ b/Library/Homebrew/cask/lib/hbc/container/lzma.rb
@@ -10,7 +10,7 @@ module Hbc
       end
 
       def extract
-        if (unlzma = which("unlzma")).nil?
+        unless unlzma = which("unlzma", PATH.new(ENV["PATH"], HOMEBREW_PREFIX/"bin"))
           raise CaskError, "Expected to find unlzma executable. Cask '#{@cask}' must add: depends_on formula: 'lzma'"
         end
 
diff --git a/Library/Homebrew/cask/lib/hbc/container/xz.rb b/Library/Homebrew/cask/lib/hbc/container/xz.rb
index cd9f9aee2f..4086ae1886 100644
--- a/Library/Homebrew/cask/lib/hbc/container/xz.rb
+++ b/Library/Homebrew/cask/lib/hbc/container/xz.rb
@@ -10,7 +10,7 @@ module Hbc
       end
 
       def extract
-        if (unxz = which("unxz")).nil?
+        unless unxz = which("unxz", PATH.new(ENV["PATH"], HOMEBREW_PREFIX/"bin"))
           raise CaskError, "Expected to find unxz executable. Cask '#{@cask}' must add: depends_on formula: 'xz'"
         end
 
diff --git a/Library/Homebrew/cask/lib/hbc/system_command.rb b/Library/Homebrew/cask/lib/hbc/system_command.rb
index f64d85c075..a890c42e41 100644
--- a/Library/Homebrew/cask/lib/hbc/system_command.rb
+++ b/Library/Homebrew/cask/lib/hbc/system_command.rb
@@ -50,11 +50,7 @@ module Hbc
     end
 
     def command
-      @command ||= if sudo?
-        [*sudo_prefix, executable, *args]
-      else
-        [Shellwords.shellescape(executable), *args]
-      end
+      [*sudo_prefix, executable, *args]
     end
 
     private
@@ -85,8 +81,14 @@ module Hbc
     end
 
     def each_output_line(&b)
+      executable, *args = expanded_command
+
+      unless File.exist?(executable)
+        executable = which(executable, PATH.new(ENV["PATH"], HOMEBREW_PREFIX/"bin"))
+      end
+
       raw_stdin, raw_stdout, raw_stderr, raw_wait_thr =
-        Open3.popen3(*expanded_command, **options)
+        Open3.popen3([executable, executable], *args, **options)
 
       write_input_to(raw_stdin)
       raw_stdin.close_write
diff --git a/Library/Homebrew/compat/hbc.rb b/Library/Homebrew/compat/hbc.rb
index ebf8a9874f..608d46e376 100644
--- a/Library/Homebrew/compat/hbc.rb
+++ b/Library/Homebrew/compat/hbc.rb
@@ -4,7 +4,6 @@ require "compat/hbc/cache"
 require "compat/hbc/caskroom"
 require "compat/hbc/cli"
 require "compat/hbc/dsl"
-require "compat/hbc/system_command"
 
 module Hbc
   class << self
diff --git a/Library/Homebrew/compat/hbc/system_command.rb b/Library/Homebrew/compat/hbc/system_command.rb
deleted file mode 100644
index bb9187db37..0000000000
--- a/Library/Homebrew/compat/hbc/system_command.rb
+++ /dev/null
@@ -1,18 +0,0 @@
-require "shellwords"
-
-module SystemCommandCompatibilityLayer
-  def initialize(executable, args: [], **options)
-    if args.empty? && !File.exist?(executable)
-      odeprecated "`system_command` with a shell string", "`system_command` with the `args` parameter"
-      executable, *args = Shellwords.shellsplit(executable)
-    end
-
-    super(executable, args: args, **options)
-  end
-end
-
-module Hbc
-  class SystemCommand
-    prepend SystemCommandCompatibilityLayer
-  end
-end
diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index 4ac66ffc45..f3b6c1db12 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -1622,7 +1622,7 @@ class Formula
       TEMP: HOMEBREW_TEMP,
       TMP: HOMEBREW_TEMP,
       TERM: "dumb",
-      PATH: PATH.new(ENV["PATH"]).append(HOMEBREW_PREFIX/"bin"),
+      PATH: PATH.new(ENV["PATH"], HOMEBREW_PREFIX/"bin"),
       HOMEBREW_PATH: nil,
       _JAVA_OPTIONS: "#{ENV["_JAVA_OPTIONS"]} -Duser.home=#{HOMEBREW_CACHE}/java_cache",
     }