diff --git a/eclass/app-alternatives.eclass b/eclass/app-alternatives.eclass
index c6924bfc6d2ae..f670b7d843f9c 100644
--- a/eclass/app-alternatives.eclass
+++ b/eclass/app-alternatives.eclass
@@ -1,4 +1,4 @@
-# Copyright 2022 Gentoo Authors
+# Copyright 2022-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: app-alternatives.eclass
@@ -21,7 +21,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} unsupported."
 esac
 
-if [[ ! ${_APP_ALTERNATIVES_ECLASS} ]]; then
+if [[ -z ${_APP_ALTERNATIVES_ECLASS} ]]; then
 _APP_ALTERNATIVES_ECLASS=1
 
 # @ECLASS_VARIABLE: ALTERNATIVES
diff --git a/eclass/aspell-dict-r1.eclass b/eclass/aspell-dict-r1.eclass
index 1a3bd0b21b248..6d10c9fb302ec 100644
--- a/eclass/aspell-dict-r1.eclass
+++ b/eclass/aspell-dict-r1.eclass
@@ -1,4 +1,4 @@
-# Copyright 1999-2023 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: aspell-dict-r1.eclass
@@ -41,7 +41,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_ASPELL_DICT_R1_ECLASS} ]]; then
+if [[ -z ${_ASPELL_DICT_R1_ECLASS} ]]; then
 _ASPELL_DICT_R1_ECLASS=1
 
 # Most of those aspell packages have an idiosyncratic versioning scheme,
diff --git a/eclass/bash-completion-r1.eclass b/eclass/bash-completion-r1.eclass
index fd16fca665385..643c458dd8a02 100644
--- a/eclass/bash-completion-r1.eclass
+++ b/eclass/bash-completion-r1.eclass
@@ -23,7 +23,7 @@
 # }
 # @CODE
 
-if [[ ! ${_BASH_COMPLETION_R1_ECLASS} ]]; then
+if [[ -z ${_BASH_COMPLETION_R1_ECLASS} ]]; then
 _BASH_COMPLETION_R1_ECLASS=1
 
 inherit toolchain-funcs
diff --git a/eclass/dotnet.eclass b/eclass/dotnet.eclass
index 1fb288dd10942..aeaee2f58e94b 100644
--- a/eclass/dotnet.eclass
+++ b/eclass/dotnet.eclass
@@ -1,4 +1,4 @@
-# Copyright 1999-2023 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: dotnet.eclass
@@ -17,7 +17,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_DOTNET_ECLASS} ]]; then
+if [[ -z ${_DOTNET_ECLASS} ]]; then
 _DOTNET_ECLASS=1
 
 BDEPEND="dev-lang/mono"
diff --git a/eclass/dune.eclass b/eclass/dune.eclass
index ba54e87ceaf94..faf40e47eb1e8 100644
--- a/eclass/dune.eclass
+++ b/eclass/dune.eclass
@@ -19,7 +19,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_DUNE_ECLASS} ]]; then
+if [[ -z ${_DUNE_ECLASS} ]]; then
 _DUNE_ECLASS=1
 
 # @ECLASS_VARIABLE: DUNE_PKG_NAME
diff --git a/eclass/emboss-r3.eclass b/eclass/emboss-r3.eclass
index 84e258e02058b..5db69dd86332c 100644
--- a/eclass/emboss-r3.eclass
+++ b/eclass/emboss-r3.eclass
@@ -1,4 +1,4 @@
-# Copyright 1999-2022 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: emboss-r3.eclass
@@ -38,7 +38,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_EMBOSS_R3_ECLASS} ]]; then
+if [[ -z ${_EMBOSS_R3_ECLASS} ]]; then
 _EMBOSS_R3_ECLASS=1
 
 inherit flag-o-matic
diff --git a/eclass/gkrellm-plugin.eclass b/eclass/gkrellm-plugin.eclass
index 1424fdfe53f96..03b72dffa6e96 100644
--- a/eclass/gkrellm-plugin.eclass
+++ b/eclass/gkrellm-plugin.eclass
@@ -1,4 +1,4 @@
-# Copyright 1999-2023 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: gkrellm-plugin.eclass
@@ -36,7 +36,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_GKRELLM_PLUGIN_ECLASS} ]]; then
+if [[ -z ${_GKRELLM_PLUGIN_ECLASS} ]]; then
 _GKRELLM_PLUGIN_ECLASS=1
 
 inherit multilib
diff --git a/eclass/kernel-build.eclass b/eclass/kernel-build.eclass
index ce87fd72acdd1..dbc41824c4b68 100644
--- a/eclass/kernel-build.eclass
+++ b/eclass/kernel-build.eclass
@@ -33,7 +33,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_KERNEL_BUILD_ECLASS} ]]; then
+if [[ -z ${_KERNEL_BUILD_ECLASS} ]]; then
 _KERNEL_BUILD_ECLASS=1
 
 PYTHON_COMPAT=( python3_{10..13} )
diff --git a/eclass/kernel-install.eclass b/eclass/kernel-install.eclass
index dc337c7862fde..4109810d4d14d 100644
--- a/eclass/kernel-install.eclass
+++ b/eclass/kernel-install.eclass
@@ -49,7 +49,7 @@
 # packages to depend on for building the generic UKI and their licenses.
 # Used in kernel-build.eclass.
 
-if [[ ! ${_KERNEL_INSTALL_ECLASS} ]]; then
+if [[ -z ${_KERNEL_INSTALL_ECLASS} ]]; then
 _KERNEL_INSTALL_ECLASS=1
 
 case ${EAPI} in
diff --git a/eclass/linux-mod-r1.eclass b/eclass/linux-mod-r1.eclass
index 120fb019d74ac..241cfc7886e9d 100644
--- a/eclass/linux-mod-r1.eclass
+++ b/eclass/linux-mod-r1.eclass
@@ -106,7 +106,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_LINUX_MOD_R1_ECLASS} ]]; then
+if [[ -z ${_LINUX_MOD_R1_ECLASS} ]]; then
 _LINUX_MOD_R1_ECLASS=1
 
 inherit dist-kernel-utils edo linux-info multiprocessing toolchain-funcs
diff --git a/eclass/llvm-r1.eclass b/eclass/llvm-r1.eclass
index 0a53b9a3cb5e7..399902181b650 100644
--- a/eclass/llvm-r1.eclass
+++ b/eclass/llvm-r1.eclass
@@ -42,7 +42,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_LLVM_R1_ECLASS} ]]; then
+if [[ -z ${_LLVM_R1_ECLASS} ]]; then
 _LLVM_R1_ECLASS=1
 
 inherit llvm-utils
diff --git a/eclass/llvm-utils.eclass b/eclass/llvm-utils.eclass
index 532e609679b86..d38ba3a988f10 100644
--- a/eclass/llvm-utils.eclass
+++ b/eclass/llvm-utils.eclass
@@ -18,7 +18,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_LLVM_UTILS_ECLASS} ]]; then
+if [[ -z ${_LLVM_UTILS_ECLASS} ]]; then
 _LLVM_UTILS_ECLASS=1
 
 # @FUNCTION: llvm_tuple_to_target
diff --git a/eclass/llvm.eclass b/eclass/llvm.eclass
index bb5ff5d78290a..39ca1123eabb1 100644
--- a/eclass/llvm.eclass
+++ b/eclass/llvm.eclass
@@ -63,7 +63,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_LLVM_ECLASS} ]]; then
+if [[ -z ${_LLVM_ECLASS} ]]; then
 _LLVM_ECLASS=1
 
 inherit llvm-utils
diff --git a/eclass/mozcoreconf-v6.eclass b/eclass/mozcoreconf-v6.eclass
index e7cf85fca24a3..b814663a8f66f 100644
--- a/eclass/mozcoreconf-v6.eclass
+++ b/eclass/mozcoreconf-v6.eclass
@@ -20,7 +20,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_MOZCORECONF_V6_ECLASS} ]]; then
+if [[ -z ${_MOZCORECONF_V6_ECLASS} ]]; then
 _MOZCORECONF_V6_ECLASS=1
 
 inherit toolchain-funcs flag-o-matic python-any-r1
diff --git a/eclass/mozextension.eclass b/eclass/mozextension.eclass
index 52fe26280e6ee..f888491d99190 100644
--- a/eclass/mozextension.eclass
+++ b/eclass/mozextension.eclass
@@ -1,4 +1,4 @@
-# Copyright 1999-2023 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: mozextension.eclass
@@ -12,7 +12,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_MOZEXTENSION_ECLASS} ]]; then
+if [[ -z ${_MOZEXTENSION_ECLASS} ]]; then
 _MOZEXTENSION_ECLASS=1
 
 # @ECLASS_VARIABLE: MOZEXTENSION_TARGET
diff --git a/eclass/mozlinguas-v2.eclass b/eclass/mozlinguas-v2.eclass
index 81e00275a8f6c..c8e4c23939457 100644
--- a/eclass/mozlinguas-v2.eclass
+++ b/eclass/mozlinguas-v2.eclass
@@ -1,4 +1,4 @@
-# Copyright 1999-2023 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: mozlinguas-v2.eclass
@@ -19,7 +19,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_MOZLINGUAS_V2_ECLASS} ]]; then
+if [[ -z ${_MOZLINGUAS_V2_ECLASS} ]]; then
 _MOZLINGUAS_V2_ECLASS=1
 
 inherit mozextension
diff --git a/eclass/postgres-multi.eclass b/eclass/postgres-multi.eclass
index 92299b8cf34c8..5f014389fd895 100644
--- a/eclass/postgres-multi.eclass
+++ b/eclass/postgres-multi.eclass
@@ -19,7 +19,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_POSTGRES_MULTI_ECLASS} ]]; then
+if [[ -z ${_POSTGRES_MULTI_ECLASS} ]]; then
 _POSTGRES_MULTI_ECLASS=1
 
 inherit multibuild postgres
diff --git a/eclass/postgres.eclass b/eclass/postgres.eclass
index 2c44358898e65..6ef6913c2dad7 100644
--- a/eclass/postgres.eclass
+++ b/eclass/postgres.eclass
@@ -19,7 +19,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_POSTGRES_ECLASS} ]]; then
+if [[ -z ${_POSTGRES_ECLASS} ]]; then
 _POSTGRES_ECLASS=1
 
 # @ECLASS_VARIABLE: _POSTGRES_ALL_VERSIONS
diff --git a/eclass/pypi.eclass b/eclass/pypi.eclass
index b80ff9c95d36d..47d40449d13ae 100644
--- a/eclass/pypi.eclass
+++ b/eclass/pypi.eclass
@@ -1,4 +1,4 @@
-# Copyright 2023 Gentoo Authors
+# Copyright 2023-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: pypi.eclass
@@ -40,7 +40,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_PYPI_ECLASS} ]]; then
+if [[ -z ${_PYPI_ECLASS} ]]; then
 _PYPI_ECLASS=1
 
 # @ECLASS_VARIABLE: PYPI_NO_NORMALIZE
diff --git a/eclass/python-any-r1.eclass b/eclass/python-any-r1.eclass
index c1f27cfbac0d8..8796c3eddb680 100644
--- a/eclass/python-any-r1.eclass
+++ b/eclass/python-any-r1.eclass
@@ -43,7 +43,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_PYTHON_ANY_R1_ECLASS} ]]; then
+if [[ -z ${_PYTHON_ANY_R1_ECLASS} ]]; then
 _PYTHON_ANY_R1_ECLASS=1
 
 if [[ ${_PYTHON_R1_ECLASS} ]]; then
diff --git a/eclass/python-single-r1.eclass b/eclass/python-single-r1.eclass
index 481b6cf91ba07..d3cc4024de36a 100644
--- a/eclass/python-single-r1.eclass
+++ b/eclass/python-single-r1.eclass
@@ -42,7 +42,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_PYTHON_SINGLE_R1_ECLASS} ]]; then
+if [[ -z ${_PYTHON_SINGLE_R1_ECLASS} ]]; then
 _PYTHON_SINGLE_R1_ECLASS=1
 
 if [[ ${_PYTHON_R1_ECLASS} ]]; then
diff --git a/eclass/rocm.eclass b/eclass/rocm.eclass
index cf501d5a8861d..acf3a5310a06f 100644
--- a/eclass/rocm.eclass
+++ b/eclass/rocm.eclass
@@ -89,7 +89,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_ROCM_ECLASS} ]]; then
+if [[ -z ${_ROCM_ECLASS} ]]; then
 _ROCM_ECLASS=1
 
 inherit flag-o-matic
diff --git a/eclass/ruby-ng-gnome2.eclass b/eclass/ruby-ng-gnome2.eclass
index c38c5f15904df..ed4fdd91a49f2 100644
--- a/eclass/ruby-ng-gnome2.eclass
+++ b/eclass/ruby-ng-gnome2.eclass
@@ -18,7 +18,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_RUBY_NG_GNOME2_ECLASS} ]]; then
+if [[ -z ${_RUBY_NG_GNOME2_ECLASS} ]]; then
 _RUBY_NG_GNOME2_ECLASS=1
 
 RUBY_FAKEGEM_NAME="${RUBY_FAKEGEM_NAME:-${PN#ruby-}}"
diff --git a/eclass/ruby-single.eclass b/eclass/ruby-single.eclass
index effdbd3e96a0e..10d15c6d19f6b 100644
--- a/eclass/ruby-single.eclass
+++ b/eclass/ruby-single.eclass
@@ -1,4 +1,4 @@
-# Copyright 1999-2023 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: ruby-single.eclass
@@ -28,7 +28,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_RUBY_SINGLE_ECLASS} ]]; then
+if [[ -z ${_RUBY_SINGLE_ECLASS} ]]; then
 _RUBY_SINGLE_ECLASS=1
 
 inherit ruby-utils
diff --git a/eclass/selinux-policy-2.eclass b/eclass/selinux-policy-2.eclass
index e7d6cd9a9e5ff..e8b7ef863ad44 100644
--- a/eclass/selinux-policy-2.eclass
+++ b/eclass/selinux-policy-2.eclass
@@ -23,7 +23,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_SELINUX_POLICY_2_ECLASS} ]]; then
+if [[ -z ${_SELINUX_POLICY_2_ECLASS} ]]; then
 _SELINUX_POLICY_2_ECLASS=1
 
 # @ECLASS_VARIABLE: MODS
diff --git a/eclass/shell-completion.eclass b/eclass/shell-completion.eclass
index d582028847b43..badda02f8d3f0 100644
--- a/eclass/shell-completion.eclass
+++ b/eclass/shell-completion.eclass
@@ -1,4 +1,4 @@
-# Copyright 2023 Gentoo Authors
+# Copyright 2023-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: shell-completion.eclass
@@ -20,7 +20,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported"
 esac
 
-if [[ ! ${_SHELL_COMPLETION_ECLASS} ]]; then
+if [[ -z ${_SHELL_COMPLETION_ECLASS} ]]; then
 _SHELL_COMPLETION_ECLASS=1
 
 # Extend bash-completion-r1
diff --git a/eclass/vcs-snapshot.eclass b/eclass/vcs-snapshot.eclass
index 0279e89ec3aba..14b9a8dd22ffa 100644
--- a/eclass/vcs-snapshot.eclass
+++ b/eclass/vcs-snapshot.eclass
@@ -47,7 +47,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_VCS_SNAPSHOT_ECLASS} ]]; then
+if [[ -z ${_VCS_SNAPSHOT_ECLASS} ]]; then
 _VCS_SNAPSHOT_ECLASS=1
 
 # @FUNCTION: vcs-snapshot_src_unpack
diff --git a/eclass/waf-utils.eclass b/eclass/waf-utils.eclass
index f8d4b0aa94b4f..377b455de7363 100644
--- a/eclass/waf-utils.eclass
+++ b/eclass/waf-utils.eclass
@@ -1,4 +1,4 @@
-# Copyright 1999-2023 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: waf-utils.eclass
@@ -20,7 +20,7 @@ case ${EAPI} in
 	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
 esac
 
-if [[ ! ${_WAF_UTILS_ECLASS} ]]; then
+if [[ -z ${_WAF_UTILS_ECLASS} ]]; then
 _WAF_UTILS_ECLASS=1
 
 inherit multilib toolchain-funcs multiprocessing