diff --git a/libexec/rbenv-exec b/libexec/rbenv-exec
index e05ce5110..2de3274b7 100755
--- a/libexec/rbenv-exec
+++ b/libexec/rbenv-exec
@@ -1,17 +1,21 @@
 #!/usr/bin/env bash
 #
-# Summary: Run an executable with the selected Ruby version
+# Summary: Run an executable with the current Ruby version
 #
-# Usage: rbenv exec <command> [arg1 arg2...]
+# Usage: rbenv exec <command> [<args>...]
 #
-# Runs an executable by first preparing PATH so that the selected Ruby
-# version's `bin' directory is at the front.
+# Runs an executable by first prepending the current Ruby version's
+# `bin' directory to PATH.
 #
-# For example, if the currently selected Ruby version is 1.9.3-p327:
-#   rbenv exec bundle install
+# For example, this invocation:
 #
-# is equivalent to:
-#   PATH="$RBENV_ROOT/versions/1.9.3-p327/bin:$PATH" bundle install
+#   RBENV_VERSION=3.1.2 rbenv exec bundle install
+#
+# roughly translates to:
+#
+#   PATH="$RBENV_ROOT/versions/3.1.2/bin:$PATH" bundle install
+#
+# See `rbenv help which' for information about the fallback mechanism.
 
 set -e
 [ -n "$RBENV_DEBUG" ] && set -x
@@ -41,7 +45,7 @@ for script in "${scripts[@]}"; do
 done
 
 shift 1
-if [ "$RBENV_VERSION" != "system" ]; then
+if [ "$RBENV_BIN_PATH" = "${RBENV_ROOT}/versions/${RBENV_VERSION}/bin" ]; then
   export PATH="${RBENV_BIN_PATH}:${PATH}"
 fi
 exec -a "$RBENV_COMMAND" "$RBENV_COMMAND_PATH" "$@"
diff --git a/libexec/rbenv-which b/libexec/rbenv-which
index 80f0df87e..c917c3b35 100755
--- a/libexec/rbenv-which
+++ b/libexec/rbenv-which
@@ -6,6 +6,17 @@
 #
 # Displays the full path to the executable that rbenv will invoke when
 # you run the given command.
+#
+# If GEM_HOME is set, the result will be `$GEM_HOME/bin/<command>' if
+# it exists.
+#
+# If the current Ruby version is "system", the command will be looked up
+# in PATH.
+#
+# For other Ruby versions, the command is first looked up in
+# `$RBENV_ROOT/versions/$RBENV_VERSION/bin'. If it does not exist and if
+# the command is not one of the core Ruby executables (e.g. "ruby",
+# "gem", "bundle"), it will be looked up in PATH.
 
 set -e
 [ -n "$RBENV_DEBUG" ] && set -x
@@ -36,15 +47,28 @@ fi
 
 RBENV_VERSION="${RBENV_VERSION:-$(rbenv-version-name)}"
 
-if [ "$RBENV_VERSION" = "system" ]; then
-  PATH="$(remove_from_path "${RBENV_ROOT}/shims")" \
-    RBENV_COMMAND_PATH="$(command -v "$RBENV_COMMAND" || true)"
+if [[ -n "$GEM_HOME" && -x "${GEM_HOME}/bin/${RBENV_COMMAND}" ]]; then
+  RBENV_COMMAND_PATH="${GEM_HOME}/bin/${RBENV_COMMAND}"
+elif [ "$RBENV_VERSION" = "system" ]; then
+  PATH="$(remove_from_path "${RBENV_ROOT}/shims")" RBENV_COMMAND_PATH="$(type -P "$RBENV_COMMAND" || true)"
 else
   RBENV_COMMAND_PATH="${RBENV_ROOT}/versions/${RBENV_VERSION}/bin/${RBENV_COMMAND}"
-fi
-
-if [[ ! -x "$RBENV_COMMAND_PATH" && -n "$GEM_HOME" && -x "${GEM_HOME}/bin/${RBENV_COMMAND}" ]]; then
-  RBENV_COMMAND_PATH="${GEM_HOME}/bin/${RBENV_COMMAND}"
+  if [ ! -x "$RBENV_COMMAND_PATH" ]; then
+    # Look up the command in PATH as fallback.
+    case "$RBENV_COMMAND" in
+    ruby | rake | gem | bundle | bundler | irb | rdoc | ri )
+      # Do not allow fallback to PATH for core Ruby commands. If any of these are missing
+      # in the current Ruby version, error out instead of allowing a potentially unrelated
+      # Ruby version to activate and potentially muck up the current project.
+      ;;
+    * )
+      if PATH="$(remove_from_path "${RBENV_ROOT}/shims")" RBENV_COMMAND_FALLBACK="$(type -P "$RBENV_COMMAND")"; then
+        RBENV_COMMAND_PATH="$RBENV_COMMAND_FALLBACK"
+      fi
+      unset RBENV_COMMAND_FALLBACK
+      ;;
+    esac
+  fi
 fi
 
 OLDIFS="$IFS"
diff --git a/test/exec.bats b/test/exec.bats
index 1ee743578..7b33fd8a2 100644
--- a/test/exec.bats
+++ b/test/exec.bats
@@ -15,8 +15,7 @@ create_executable() {
 }
 
 @test "fails with invalid version" {
-  export RBENV_VERSION="2.0"
-  run rbenv-exec ruby -v
+  RBENV_VERSION="2.0" run rbenv-exec ruby -v
   assert_failure "rbenv: version \`2.0' is not installed (set by RBENV_VERSION environment variable)"
 }
 
@@ -29,6 +28,7 @@ create_executable() {
 }
 
 @test "completes with names of executables" {
+  # shellcheck disable=SC2030,SC2031
   export RBENV_VERSION="2.0"
   create_executable "ruby" "#!/bin/sh"
   create_executable "rake" "#!/bin/sh"
@@ -49,13 +49,13 @@ hellos=(\$(printf "hello\\tugly world\\nagain"))
 echo HELLO="\$(printf ":%s" "\${hellos[@]}")"
 SH
 
-  export RBENV_VERSION=system
-  IFS=$' \t\n' run rbenv-exec env
+  RBENV_VERSION=system IFS=$' \t\n' run rbenv-exec env
   assert_success
   assert_line "HELLO=:hello:ugly:world:again"
 }
 
 @test "forwards all arguments" {
+  # shellcheck disable=SC2030,SC2031
   export RBENV_VERSION="2.0"
   create_executable "ruby" <<SH
 #!$BASH
@@ -78,7 +78,31 @@ ${RBENV_ROOT}/versions/2.0/bin/ruby
 OUT
 }
 
+@test "prepends Ruby to PATH" {
+  # shellcheck disable=SC2030,SC2031
+  export RBENV_VERSION="2.0"
+  create_executable "ruby" <<SH
+#!$BASH
+echo \$PATH
+SH
+
+  run rbenv-exec ruby
+  assert_success
+  [[ $output == "${RBENV_ROOT}/versions/${RBENV_VERSION}/bin:"* ]]
+}
+
+@test "does not modify PATH for fallback" {
+  # shellcheck disable=SC2030,SC2031
+  export RBENV_VERSION="2.0"
+  create_executable "ruby" "#!/bin/sh"
+
+  # shellcheck disable=SC2016
+  run rbenv-exec bash -c 'echo $PATH'
+  assert_success "$PATH"
+}
+
 @test "supports ruby -S <cmd>" {
+  # shellcheck disable=SC2030,SC2031
   export RBENV_VERSION="2.0"
 
   # emulate `ruby -S' behavior