diff --git a/cd.sh b/cd.sh
index 3466b53..ee3241f 100644
--- a/cd.sh
+++ b/cd.sh
@@ -38,7 +38,7 @@ cd_main () {
 
 cd_set_environment_variables () {
     cd_step "Set environment variables"
-    # operating system
+    # set operating system id
     CD_OS_ID="$(cd_grep_os ID)"
     case "${CD_OS_ID}" in
         "almalinux") CD_OS_ID="${CD_OS_ALMA}" ;;
@@ -50,6 +50,7 @@ cd_set_environment_variables () {
         "ubuntu") CD_OS_ID="${CD_OS_UBUNTU}" ;;
         *) cd_error_os "CD_OS_ID" ;;
     esac
+    # set operating system version
     case "${CD_OS_ID}" in
         "${CD_OS_ALMA}"|"${CD_OS_ARCH}"|"${CD_OS_FEDORA}"|"${CD_OS_ROCKY}")
             CD_OS_VERSION=$(cd_grep_os VERSION_ID \
@@ -63,6 +64,7 @@ cd_set_environment_variables () {
             CD_OS_VERSION="$(cd_grep_os VERSION_CODENAME)"
         ;;
     esac
+    # check operating system version
     case "${CD_OS_ID}" in
         "${CD_OS_ALMA}"|"${CD_OS_ROCKY}")
             case "${CD_OS_VERSION}" in
@@ -106,9 +108,10 @@ cd_set_environment_variables () {
     CD_DNS_FILE="/etc/resolv.conf"
     CD_PKG_CA="ca-certificates"
     CD_PKG_GIT="git"
+    CD_PYTHON_ALIAS="python3"
     cd_split
-    cd_echo "CD_DNS_FILE" "CD_PKG_CA" "CD_PKG_GIT"
-    # shared
+    cd_echo "CD_DNS_FILE" "CD_PKG_CA" "CD_PKG_GIT" "CD_PYTHON_ALIAS"
+    # set ca command & root
     case "${CD_OS_ID}" in
         "${CD_OS_ALMA}"|"${CD_OS_FEDORA}"|"${CD_OS_ROCKY}")
             CD_CA_ROOT="/etc/pki/ca-trust/source/anchors"
@@ -129,7 +132,7 @@ cd_set_environment_variables () {
     esac
     cd_split
     cd_echo "CD_CA_ROOT" "CD_CMD_CA"
-    # common
+    # set package manager
     case "${CD_OS_ID}" in
         "${CD_OS_ALMA}"|"${CD_OS_FEDORA}"|"${CD_OS_ROCKY}")
             CD_PM="${CD_PM_DNF}"
@@ -253,7 +256,7 @@ Dir::Etc::SourceParts \"\";
     esac
     cd_split
     cd_echo "CD_URL_DEFAULT" "CD_URL_CHOSEN"
-    # python
+    # set python command & package
     case "${CD_OS_ID}" in
         "${CD_OS_ALMA}"|"${CD_OS_ROCKY}")
             CD_PYTHON_COMMAND="python3.11"
@@ -286,6 +289,7 @@ Dir::Etc::SourceParts \"\";
             CD_PYTHON_PACKAGE="python3"
         ;;
     esac
+    # set python packages
     case "${CD_OS_ID}" in
         "${CD_OS_ALMA}"|"${CD_OS_FEDORA}"|"${CD_OS_ROCKY}")
             CD_PYTHON_PACKAGES="/usr/lib64/${CD_PYTHON_COMMAND}/site-packages"
@@ -294,7 +298,7 @@ Dir::Etc::SourceParts \"\";
             CD_PYTHON_PACKAGES="/usr/lib/${CD_PYTHON_COMMAND}/site-packages"
         ;;
         "${CD_OS_DEBIAN}"|"${CD_OS_UBUNTU}")
-            CD_PYTHON_PACKAGES="/usr/lib/python3/dist-packages"
+            CD_PYTHON_PACKAGES="/usr/lib/${CD_PYTHON_ALIAS}/dist-packages"
         ;;
     esac
     cd_split
@@ -532,7 +536,7 @@ ${self}
 ↓
 ${1}
 "
-    "${CD_PYTHON_COMMAND}" -m "${1}" "${CD_STEP}" "${self}"
+    "${CD_PYTHON_ALIAS}" -m "${1}" "${CD_STEP}" "${self}"
 }
 
 # functions
@@ -581,9 +585,10 @@ cd_install_package () {
 }
 
 cd_ln_python () {
-    if [ "${1}" ] ; then
-        echo "python3 → ${1}"
-        ln -f -s "${1}" "/usr/bin/python3" || exit
+local command="${1}"
+    if [ "${command}" ] ; then
+        echo "${CD_PYTHON_ALIAS} → ${command}"
+        ln -f -s "${command}" "/usr/bin/${CD_PYTHON_ALIAS}" || exit
     fi
 }
 
@@ -603,46 +608,46 @@ cd_mkdir () {
 
 cd_openssl () {
 local file="${1}"
-    if [ "${file}" ] ; then
+    if [ -f "${file}" ] ; then
         openssl x509 -noout -text -in "${file}" || exit
     fi
 }
 
 cd_rm () {
-    if [ -e "${1}" ] ; then
-        echo "← ${1}"
-        rm -r "${1}" || exit
+local path="${1}"
+    if [ -e "${path}" ] ; then
+        echo "← ${path}"
+        rm -r "${path}" || exit
     fi
 }
 
 cd_sed () {
 local expression
-local file
-    if [ -f "${1}" ] ; then
-        file="${1}"
-        shift
-        for expression in "${@}" ; do
-            sed --in-place "s${expression}g" "${file}" || exit
-        done
+local file="${1}"
+shift
+    if [ -f "${file}" ] ; then
         cd_cat "${file}"
+        for expression in "${@}" ; do
+            sed --in-place "s${expression}g" "${file}" \
+            && cd_cat "${file}" \
+            || exit
+        done
     fi
 }
 
 cd_split () {
     echo -n "\
-╶─╌╌┄┄┈┈
+╶${CD_SPLIT}
 "
 }
 
 cd_step () {
-    if [ "${1}" ] ; then
-        CD_STEP=$((CD_STEP+1))
-        echo -n "\
-╭─╌╌┄┄┈┈
-│ ${CD_STEP} ${1}
-╰─╌╌┄┄┈┈
+    CD_STEP=$((CD_STEP+1))
+    echo -n "\
+╭${CD_SPLIT}
+│ ${CD_STEP} ${@}
+╰${CD_SPLIT}
 "
-    fi
 }
 
 cd_write () {
@@ -674,5 +679,7 @@ CD_PM_APT="apt"
 CD_PM_DNF="dnf"
 CD_PM_PACMAN="pacman"
 
+CD_SPLIT="─╌╌┄┄┈┈"
+
 # run
 cd_main