new: add leaf_add_shell and leaf_remove_shell

leaf

@@ -607,4 +607,44 @@ leaf_unpack_package() {
   echo -e "${GREEN_COLOR}* Package ${PKG_PREFIX}/${PKG_NAME} has been installed${CLEAR_COLOR}\n"
 }
 
+leaf_add_shell() {
+  # usage: leaf_add_shell <abs_shell_path>
+  local shell="$1" f="/etc/shells" tmp
+
+  # ensure file exists
+  if [ ! -f "$f" ]; then
+    install -m 0644 -o root -g root /dev/null "$f" || return 1
+  fi
+
+  # already present? (exact whole-line match)
+  grep -Fxq -- "$shell" "$f" && return 0
+
+  tmp="$(mktemp "${f}.leaf.XXXXXX")" || return 1
+  cat "$f" > "$tmp" || { rm -f "$tmp"; return 1; }
+
+  # append with newline
+  printf '%s\n' "$shell" >> "$tmp" || { rm -f "$tmp"; return 1; }
+
+  # atomic replace
+  install -m 0644 -o root -g root "$tmp" "$f" || { rm -f "$tmp"; return 1; }
+  rm -f "$tmp"
+}
+
+leaf_remove_shell() {
+  # usage: leaf_remove_shell <abs_shell_path>
+  local shell="$1" f="/etc/shells" tmp
+
+  [ -f "$f" ] || return 0
+  # if not present, nothing to do
+  grep -Fxq -- "$shell" "$f" || return 0
+
+  tmp="$(mktemp "${f}.leaf.XXXXXX")" || return 1
+  # delete exact matching lines only
+  grep -Fvx -- "$shell" "$f" > "$tmp" || true
+
+  install -m 0644 -o root -g root "$tmp" "$f" || { rm -f "$tmp"; return 1; }
+  rm -f "$tmp"
+}
+
+
 main $@