diff --git a/cd.sh b/cd.sh
index 470a666..9496890 100644
--- a/cd.sh
+++ b/cd.sh
@@ -1,11 +1,11 @@
 #! /usr/bin/env sh
 
 # defaults
-[ "${CD_DNS}" ] || CD_DNS="\
+[ -n "${CD_DNS}" ] || CD_DNS="\
 9.9.9.9 \
 "
-[ "${CD_GIT_CHILD}" ] || CD_GIT_CHILD="cd"
-[ "${CD_GIT_PARENT}" ] || CD_GIT_PARENT="rwx"
+[ -n "${CD_GIT_CHILD}" ] || CD_GIT_CHILD="cd"
+[ -n "${CD_GIT_PARENT}" ] || CD_GIT_PARENT="rwx"
 
 # main
 cd_main () {
@@ -260,28 +260,28 @@ Acquire::https::Verify-Peer False;
     CD_URL_CHOSEN="${CD_URL_DEFAULT}"
     case "${CD_OS_ID}" in
         "${CD_OS_ALMA}")
-            [ "${CD_URL_ALMA}" ] && CD_URL_CHOSEN="${CD_URL_ALMA}"
+            [ -n "${CD_URL_ALMA}" ] && CD_URL_CHOSEN="${CD_URL_ALMA}"
         ;;
         "${CD_OS_ALPINE}")
-            [ "${CD_URL_ALPINE}" ] && CD_URL_CHOSEN="${CD_URL_ALPINE}"
+            [ -n "${CD_URL_ALPINE}" ] && CD_URL_CHOSEN="${CD_URL_ALPINE}"
         ;;
         "${CD_OS_ARCH}")
-            [ "${CD_URL_ARCH}" ] && CD_URL_CHOSEN="${CD_URL_ARCH}"
+            [ -n "${CD_URL_ARCH}" ] && CD_URL_CHOSEN="${CD_URL_ARCH}"
         ;;
         "${CD_OS_DEBIAN}")
-            [ "${CD_URL_DEBIAN}" ] && CD_URL_CHOSEN="${CD_URL_DEBIAN}" \
+            [ -n "${CD_URL_DEBIAN}" ] && CD_URL_CHOSEN="${CD_URL_DEBIAN}" \
             || CD_URL_CHOSEN="https://deb.debian.org/debian"
         ;;
         "${CD_OS_FEDORA}")
-            [ "${CD_URL_FEDORA}" ] && CD_URL_CHOSEN="${CD_URL_FEDORA}" \
+            [ -n "${CD_URL_FEDORA}" ] && CD_URL_CHOSEN="${CD_URL_FEDORA}" \
             || CD_URL_CHOSEN="https://rpmfind.net/linux/fedora/linux/releases"
         ;;
         "${CD_OS_ROCKY}")
-            [ "${CD_URL_ROCKY}" ] && CD_URL_CHOSEN="${CD_URL_ROCKY}" \
+            [ -n "${CD_URL_ROCKY}" ] && CD_URL_CHOSEN="${CD_URL_ROCKY}" \
             || CD_URL_CHOSEN="https://dl.rockylinux.org/\$contentdir"
         ;;
         "${CD_OS_UBUNTU}")
-            [ "${CD_URL_UBUNTU}" ] && CD_URL_CHOSEN="${CD_URL_UBUNTU}" \
+            [ -n "${CD_URL_UBUNTU}" ] && CD_URL_CHOSEN="${CD_URL_UBUNTU}" \
             || CD_URL_CHOSEN="https://ubuntu.mirrors.ovh.net/ubuntu"
         ;;
     esac
@@ -335,15 +335,15 @@ Acquire::https::Verify-Peer False;
     cd_split
     cd_echo "CD_PYTHON_COMMAND" "CD_PYTHON_PACKAGE" "CD_PYTHON_PACKAGES"
     # variables
-    [ "${CD_CA_1}" ] && CD_CA=true
+    [ -n "${CD_CA_1}" ] && CD_CA=true
     # continuous integration platform
-    if [ "${GITHUB_ACTIONS}" ] ; then
+    if [ -n "${GITHUB_ACTIONS}" ] ; then
         # github → gitea → forgejo
         CD_SERVER_URL="${GITHUB_SERVER_URL}"
         CD_PROJECTS_GROUP="$(dirname "${GITHUB_REPOSITORY}")"
         CD_PROJECT_NAME="$(basename "${GITHUB_REPOSITORY}")"
         CD_PROJECT_BRANCH="${GITHUB_REF_NAME}"
-    elif [ "${GITLAB_CI}" ] ; then
+    elif [ -n "${GITLAB_CI}" ] ; then
         # gitlab
         CD_SERVER_URL="${CI_SERVER_URL}"
         CD_PROJECTS_GROUP="$(dirname "${CI_PROJECT_PATH}")"
@@ -353,10 +353,10 @@ Acquire::https::Verify-Peer False;
         # unsupported
         cd_error_ci "ø"
     fi
-    [ "${CD_SERVER_URL}" ] || cd_error_ci "CD_SERVER_URL"
-    [ "${CD_PROJECTS_GROUP}" ] || cd_error_ci "CD_PROJECTS_GROUP"
-    [ "${CD_PROJECT_NAME}" ] || cd_error_ci "CD_PROJECT_NAME"
-    [ "${CD_PROJECT_BRANCH}" ] || cd_error_ci "CD_PROJECT_BRANCH"
+    [ -n "${CD_SERVER_URL}" ] || cd_error_ci "CD_SERVER_URL"
+    [ -n "${CD_PROJECTS_GROUP}" ] || cd_error_ci "CD_PROJECTS_GROUP"
+    [ -n "${CD_PROJECT_NAME}" ] || cd_error_ci "CD_PROJECT_NAME"
+    [ -n "${CD_PROJECT_BRANCH}" ] || cd_error_ci "CD_PROJECT_BRANCH"
     #
     CD_PROJECTS_URL="${CD_SERVER_URL}/${CD_PROJECTS_GROUP}"
     #
@@ -448,7 +448,7 @@ cd_list_working_directory () {
 }
 
 cd_set_https_verification_off () {
-    if [ "${CD_CA}" ] || [ "${CD_PM}" = "${CD_PM_APT}" ] ; then
+    if [ -n "${CD_CA}" ] || [ "${CD_PM}" = "${CD_PM_APT}" ] ; then
         cd_step "Set HTTPS verification off"
         cd_mkdir "$(dirname "${CD_PM_HTTPS_PATH}")"
         cd_write "${CD_PM_HTTPS_PATH}" "${CD_PM_HTTPS_TEXT}"
@@ -486,7 +486,7 @@ cd_write_ca_certificates () {
     cd_write_ca_certificates__index=1
     eval "cd_write_ca_certificates__text=\
 \"\${CD_CA_${cd_write_ca_certificates__index}}\""
-    while [ "${cd_write_ca_certificates__text}" ] ; do
+    while [ -n "${cd_write_ca_certificates__text}" ] ; do
         cd_write_ca_certificates__path="\
 ${CD_CA_ROOT}/${cd_write_ca_certificates__index}.crt"
         cd_split
@@ -605,7 +605,7 @@ ${CD_PYTHON_PACKAGES}/${CD_GIT_CHILD}"
 
 cd_cat () {
     cd_cat__file="${1}"
-    if [ "${cd_cat__file}" ] ; then
+    if [ -n "${cd_cat__file}" ] ; then
         cd_open "${cd_cat__file}"
         cat "${cd_cat__file}" || exit
         cd_shut "${cd_cat__file}"
@@ -613,7 +613,7 @@ cd_cat () {
 }
 
 cd_echo () {
-    if [ "${1}" ] ; then
+    if [ -n "${1}" ] ; then
         for cd_echo__name in "${@}" ; do
             eval "echo ${cd_echo__name} = \\\"\${${cd_echo__name}}\\\""
         done
@@ -634,7 +634,7 @@ cd_error_os () {
 
 cd_grep_os () {
     cd_grep_os__variable="${1}"
-    if [ "${cd_grep_os__variable}" ] ; then
+    if [ -n "${cd_grep_os__variable}" ] ; then
         grep "^${cd_grep_os__variable}=" "/etc/os-release" \
         | sed "s|^${cd_grep_os__variable}=||" \
         | sed "s|^\"\(.*\)\"$|\1|"
@@ -643,14 +643,14 @@ cd_grep_os () {
 
 cd_install_package () {
     cd_install_package__name="${1}"
-    if [ "${cd_install_package__name}" ] ; then
+    if [ -n "${cd_install_package__name}" ] ; then
         ${CD_PM_INSTALL} "${cd_install_package__name}" || exit
     fi
 }
 
 cd_ln_python () {
     cd_ln_python__command="${1}"
-    if [ "${cd_ln_python__command}" ] ; then
+    if [ -n "${cd_ln_python__command}" ] ; then
         echo "→ ${CD_PYTHON_ALIAS} → ${cd_ln_python__command}"
         ln -f -s "${cd_ln_python__command}" "/usr/bin/${CD_PYTHON_ALIAS}" \
         || exit
@@ -659,7 +659,7 @@ cd_ln_python () {
 
 cd_ls () {
     cd_ls__path="${1}"
-    if [ "${cd_ls__path}" ] ; then
+    if [ -n "${cd_ls__path}" ] ; then
         cd_open "${cd_ls__path}"
         ls -a -l "${cd_ls__path}" || exit
         cd_shut "${cd_ls__path}"
@@ -668,7 +668,7 @@ cd_ls () {
 
 cd_mkdir () {
     cd_mkdir__path="${1}"
-    if [ "${cd_mkdir__path}" ] ; then
+    if [ -n "${cd_mkdir__path}" ] ; then
         echo "→ ${cd_mkdir__path}"
         mkdir --parents "${cd_mkdir__path}" || exit
     fi
@@ -728,7 +728,7 @@ ${CD___UP}"
 cd_write () {
     cd_write__file="${1}"
     cd_write__text="${2}"
-    if [ "${cd_write__file}" ] ; then
+    if [ -n "${cd_write__file}" ] ; then
         [ -f "${cd_write__file}" ] && cd_cat "${cd_write__file}"
         echo "→ ${cd_write__file}"
         printf "%s" "${cd_write__text}" > "${cd_write__file}" || exit