diff --git a/contrib/scripts/nm-code-format.sh b/contrib/scripts/nm-code-format.sh
index f5e0528e89..0b48ed9bbb 100755
--- a/contrib/scripts/nm-code-format.sh
+++ b/contrib/scripts/nm-code-format.sh
@@ -2,74 +2,140 @@
 
 set -e
 
-while (( $# )); do
-    case $1 in
-        -i)
-            FLAGS="-i"
-            shift
-            ;;
-        -h)
-            printf "Usage: %s [OPTION]... [FILE]...\n" $(basename $0)
-            printf "Reformat source files using NetworkManager's code-style.\n\n"
-            printf "If no file is given the script runs on the whole codebase.\n"
-            printf "If no flag is given no file is touch but errors are reported.\n\n"
-            printf "OPTIONS:\n"
-            printf "    -i    Reformat files\n"
-            printf "    -h    Print this help message\n"
-            exit 0
-            ;;
-        *)
-            FILES+=($1)
-            shift
-            ;;
-    esac
-done
+die() {
+    printf '%s\n' "$*" >&2
+    exit 1
+}
 
-NM_ROOT=$(git rev-parse --show-toplevel)
-EXCLUDE="
-         :(exclude)shared/c-list
-         :(exclude)shared/c-list
-         :(exclude)shared/c-list
-         :(exclude)shared/c-rbtree
-         :(exclude)shared/c-siphash
-         :(exclude)shared/c-stdaux
-         :(exclude)shared/n-acd
-         :(exclude)shared/n-dhcp4
-         :(exclude)shared/nm-std-aux/unaligned.h
-         :(exclude)shared/systemd/src
-         :(exclude)src/systemd/src
-"
+EXCLUDE=(
+    ":(exclude)shared/c-list"
+    ":(exclude)shared/c-list"
+    ":(exclude)shared/c-list"
+    ":(exclude)shared/c-rbtree"
+    ":(exclude)shared/c-siphash"
+    ":(exclude)shared/c-stdaux"
+    ":(exclude)shared/n-acd"
+    ":(exclude)shared/n-dhcp4"
+    ":(exclude)shared/nm-std-aux/unaligned.h"
+    ":(exclude)shared/systemd/src"
+    ":(exclude)src/systemd/src"
+)
+
+NM_ROOT="$(git rev-parse --show-toplevel)" || die "not inside a git repository"
+NM_PREFIX="$(git rev-parse --show-prefix)" || die "not inside a git repository"
+
+if [ ! -f "$NM_ROOT/.clang-format" ]; then
+    die "Error: the clang-format file in \"$NM_ROOT\" does not exist"
+fi
 
 if ! command -v clang-format &> /dev/null; then
-    echo -n "Error: clang-format is not installed, "
-    echo "on RHEL/Fedora/CentOS run 'dnf install clang-tools-extra'"
-    exit 1
+    die "Error: clang-format is not installed. On RHEL/Fedora/CentOS run 'dnf install clang-tools-extra'"
 fi
 
-if [ ! -f ${NM_ROOT}/.clang-format ]; then
-    echo "Error: the clang-format file does not exist"
-    exit 1
-fi
-
-
-FLAGS=${FLAGS:-"--Werror -n --ferror-limit=1"}
-
-if [ -z "${FILES[@]}" ]; then
-    cd $NM_ROOT
-    FILES=($(git ls-files --full-name -- '*.[ch]' ${EXCLUDE}))
-fi
-
-for f in "${FILES[@]}"; do
-    if [ -f $f ]; then
-        if ! clang-format $FLAGS $f &> /dev/null; then
-            TMP_FILE=$(mktemp)
-            clang-format $f > $TMP_FILE
-            git --no-pager diff $f $TMP_FILE || true
-            rm $TMP_FILE
-            echo "Error: $(basename $f) code-style is wrong, fix it by running '$0 -i $f)"
-            exit 1
+if test -n "$NM_PREFIX"; then
+    _EXCLUDE=()
+    for e in "${EXCLUDE[@]}"; do
+        REGEX='^:\(exclude\)'"$NM_PREFIX"'([^/].*)$'
+        if [[ "$e" =~ $REGEX ]]; then
+            _EXCLUDE+=(":(exclude)${BASH_REMATCH[1]}")
         fi
-   else
-        echo "Error: $f No such file"
-   fi
+    done
+    EXCLUDE=("${_EXCLUDE[@]}")
+fi
+
+FILES=()
+HAS_EXPLICIT_FILES=0
+SHOW_FILENAMES=0
+TEST_ONLY=1
+
+usage() {
+    printf "Usage: %s [OPTION]... [FILE]...\n" $(basename $0)
+    printf "Reformat source files using NetworkManager's code-style.\n\n"
+    printf "If no file is given the script runs on the whole codebase.\n"
+    printf "If no flag is given no file is touch but errors are reported.\n\n"
+    printf "OPTIONS:\n"
+    printf "    -i                 Reformat files\n"
+    printf "    -n                 Only check the files (this is the default)\n"
+    printf "    -h                 Print this help message\n"
+    printf "    --show-filenames   Only print the filenames that would be checked\n"
+    printf "    --                 Separate options from filenames/directories\n"
+}
+
+HAD_DASHDASH=0
+while (( $# )); do
+    if [ "$HAD_DASHDASH" = 0 ]; then
+        case "$1" in
+            -h)
+                usage
+                exit 0
+                ;;
+            --show-filenames)
+                SHOW_FILENAMES=1
+                shift
+                continue
+                ;;
+            -n)
+                TEST_ONLY=1
+                shift
+                continue
+                ;;
+            -i)
+                TEST_ONLY=0
+                shift
+                continue
+                ;;
+            --)
+                HAD_DASHDASH=1
+                shift
+                continue
+                ;;
+        esac
+    fi
+    if [ -d "$1" ]; then
+        FILES+=( $(git ls-files -- "${1}/*.[hc]" "${EXCLUDE[@]}" ) )
+    elif [ -f "$1" ]; then
+        FILES+=("$1")
+    else
+        usage >&2
+        echo >&2
+        die "Unknown argument \"$1\" which also is neither a file nor a directory."
+    fi
+    shift
+    HAS_EXPLICIT_FILES=1
 done
+
+if [ $HAS_EXPLICIT_FILES = 0 ]; then
+    FILES=( $(git ls-files -- '*.[ch]' "${EXCLUDE[@]}")  )
+fi
+
+if [ $SHOW_FILENAMES = 1 ]; then
+    printf '%s\n' "${FILES[@]}"
+    exit 0
+fi
+
+FLAGS_TEST=( --Werror -n --ferror-limit=1 )
+
+if [ $TEST_ONLY = 1 ]; then
+    # We assume that all formatting is correct. In that mode, passing
+    # all filenames to clang-format is significantly faster.
+    #
+    # Only in case of an error, we iterate over the files one by one
+    # until we find the first invalid file.
+    for f in "${FILES[@]}"; do
+        [ -f $f ] || die "Error: file \"$f\" does not exist (or is not a regular file)"
+    done
+    clang-format "${FLAGS_TEST[@]}" "${FILES[@]}" &>/dev/null && exit 0
+    for f in "${FILES[@]}"; do
+        [ -f $f ] || die "Error: file \"$f\" does not exist (or is not a regular file)"
+        if ! clang-format "${FLAGS_TEST[@]}" "$f" &>/dev/null; then
+            FF="$(mktemp)"
+            trap 'rm -f "$FF"' EXIT
+            clang-format "$f" 2>/dev/null > "$FF"
+            git --no-pager diff "$f" "$FF" || :
+            die "Error: file \"$f\" has code-style is wrong. Fix it by running "'`'"\"$0\" -i \"$f\""'`'
+        fi
+    done
+    die "an unknown error happened."
+fi
+
+clang-format -i "${FILES[@]}"