diff --git a/lib/ossbuild-common.bash b/lib/ossbuild-common.bash
old mode 100755
new mode 100644
index bac6906..eb1f66d
--- a/lib/ossbuild-common.bash
+++ b/lib/ossbuild-common.bash
@@ -38,13 +38,20 @@ ossbuild-pushd() {
         bsl_logfe "failed to change directory to '$(realpath -Lm "${d}")'"
         return 2
     }
-    bsl_logi "directory: '${PWD}'"
+    bsl_logi "enter directory: '${PWD}'"
 }
 
 #D# Logged popd.
 ossbuild-popd() {
-    bsl_run_cmd_quiet popd || return 0
-    bsl_logi "directory: '${PWD}'"
+    local dirstack_size
+    dirstack_size="$(dirs -p | wc -l)"
+    [ "${dirstack_size}" -gt 1 ] || {
+        # directory stack is empty (the current directory is always present)
+        return 0
+    }
+    bsl_logi "leave directory: '${PWD}'"
+    bsl_run_cmd_quiet popd
+    [ "${dirstack_size}" -le 2 ] || bsl_logi "directory: '${PWD}'"
 }
 
 #D# Run a shell command.
@@ -194,8 +201,8 @@ ossbuild-gh-version-latest() {
 # The first of the following steps which has a non-empty result is used as
 # version string:
 #
-#    1. get value of project variable VERSION
-#    2. get value of project variable DEFAULT_VERSION
+#    1. get value of project variable <PRJ>_VERSION
+#    2. get value of project variable <PRJ>_DEFAULT_VERSION
 #    3. value returned by ``ossbuild-gh-version-latest`` #d#
 # shellcheck disable=SC2120
 ossbuild-project-version() {
@@ -375,19 +382,20 @@ ossbuild-run-tests-p() {
     [ "$(ossbuild-project-var-get BUILD_TESTS "${BUILD_TESTS:-0}")" -gt 0 ]
 }
 
-#D# Write name of project build step function to stdoutif (it exists).
+#D# Write name of project build step function to stdout (if it exists).
 ossbuild-project-build-step-fn() {
-    local step_fn="ossbuild-${OSSBUILD_PROJECT}-${1}"
-    bsl_run_cmd_nostdout declare -F "${step_fn}" || return 1
+    local step="${1}"
+    local step_fn="ossbuild-${step}-${OSSBUILD_PROJECT}"
+    bsl_run_cmd_nostdout declare -F "${step_fn}" || return 2
     printf '%s' "${step_fn}"
 }
 
 #D# Run project build step function (if it exists).
 ossbuild-project-build-step-run() {
-    local step_fn
-    step_fn="$(ossbuild-project-build-step-fn "${@}")" || return 1
+    local step="${1}" step_fn
+    step_fn="$(ossbuild-project-build-step-fn "${step}")" || return "${?}"
     shift
-    bsl_logi "${OSSBUILD_STEP} '${OSSBUILD_PROJECT}'"
+    bsl_logi "${step} '${OSSBUILD_PROJECT}'"
     "${step_fn}" "${@}"
 }
 
@@ -402,6 +410,7 @@ ossbuild-project-init() {
     trap ossbuild-report-error ERR
 
     [ -v OSSBUILD_PROJECT ] || {
+        # use global var for error reporting
         declare -g OSSBUILD_STEP='init'
         OSSBUILD_PROJECT="$(basename "$(dirname "${BASH_SOURCE[-1]}")")"
     }
@@ -431,7 +440,7 @@ ossbuild-project-build-in-tree-p() {
           = "${ossbuild-project-var-get BUILD_TREE}" ]
 }
 
-ossbuild-archive-generic-prepare() {
+ossbuild-archive-prepare() {
     local archive_url="${1}" src_tree="${2:-}" archive="${3:-}"
 
     [ -n "${src_tree}" ] || \
@@ -459,7 +468,7 @@ ossbuild-archive-gnu-prepare() {
     src_tree="$(ossbuild-src-base)/$(basename "${archive}" .tar.gz)"
     archive_url="${archive_base_url}/${archive}"
 
-    ossbuild-archive-generic-prepare \
+    ossbuild-archive-prepare \
         "${archive_url}" \
         "${src_tree}" \
         "${archive}"
@@ -467,15 +476,20 @@ ossbuild-archive-gnu-prepare() {
 
 #D# ...
 ossbuild-autotools-configure-ok-p() {
-    local build_tree="${1}"
+    local build_tree="${1}" \
+          configlog="${2:-config.log}" \
+          regexp="${3:-^configure: exit 0$}"
     [ -n "${build_tree}" ] || ossbuild-project-var-get -v build_tree BUILD_TREE
-    local config_log="${build_tree}/config.log"
+    configlog="${build_tree}/${configlog}"
 
-    [ -e "${config_log}" ] && grep -qE '^configure: exit 0$' "${config_log}"
+    [ -e "${configlog}" ] || return 2
+    grep -qE "${regexp}" "${configlog}" || return 3
+    bsl_logi "${OSSBUILD_PROJECT}: reuse previous configure results" \
+             "(found 'configure: exit 0' in '${configlog}')"
 }
 
 #D# ...
-ossbuild-autotools-generic-build() {
+ossbuild-autotools-build() {
     local configure_extra_opts=(${@})
 
     local src_tree build_tree
@@ -488,9 +502,10 @@ ossbuild-autotools-generic-build() {
     [ -d "${build_tree}" ] || mkdir -p "${build_tree}"
     ossbuild-pushd "${build_tree}"
 
-    if ossbuild-autotools-configure-ok-p "${build_tree}"; then
-        bsl_logi "skip configure (found config.status)"
-    else
+    local configure_ok_p_fn
+    configure_ok_p_fn="$(ossbuild-project-build-step-fn "configure-ok-p")" \
+        || configure_ok_p_fn='ossbuild-autotools-configure-ok-p'
+    if ! "${configure_ok_p_fn}" "${build_tree}"; then
         bsl_logi "run configure ..."
         local configure=(
             "${src_tree}/configure"
@@ -505,7 +520,7 @@ ossbuild-autotools-generic-build() {
 }
 
 #D# ...
-ossbuild-autotools-generic-install() {
+ossbuild-autotools-install() {
     local build_tree
     ossbuild-project-var-get -v build_tree BUILD_tree
 
@@ -530,6 +545,10 @@ ossbuild-project-build-steps-all() {
 
     declare -g OSSBUILD_STEP
     for OSSBUILD_STEP in prepare build install; do
+        [[ ! "${OSSBUILD_SKIP:=}" =~ (^|,)${OSSBUILD_STEP}(,|$) ]] || {
+            bsl_logi "${OSSBUILD_PROJECT}: skip ${OSSBUILD_SKIP}"
+            continue
+        }
         ossbuild-project-build-step-run "${OSSBUILD_STEP}" "${@}"
     done
 }
diff --git a/projects/abseil-cpp/ossbuild-abseil-cpp.bash b/projects/abseil-cpp/ossbuild-abseil-cpp.bash
index db40872..5ea2d5f 100755
--- a/projects/abseil-cpp/ossbuild-abseil-cpp.bash
+++ b/projects/abseil-cpp/ossbuild-abseil-cpp.bash
@@ -10,7 +10,7 @@ source "${LIB_DIR}/ossbuild-common.bash"
 ABSEIL_CPP_DEFAULT_VERSION='20230125.3'
 #https://github.com/abseil/abseil-cpp/archive/refs/tags/20230125.3.tar.gz
 
-ossbuild-abseil-cpp-prepare() {
+ossbuild-prepare-abseil-cpp() {
     local version gh_url archive archive_url src_tree
     ossbuild-project-var-get -v version VERSION
     ossbuild-project-var-get -v gh_url GH_URL
@@ -31,7 +31,7 @@ ossbuild-abseil-cpp-prepare() {
     }
 }
 
-ossbuild-abseil-cpp-build() {
+ossbuild-build-abseil-cpp() {
     local src_tree build_tree
     ossbuild-project-var-get -v src_tree SRC_TREE
     build_tree="${src_tree}/ossbuild"
@@ -65,9 +65,10 @@ ossbuild-abseil-cpp-build() {
 
     ossbuild-cmd cmake "${generate_opts[@]}" "${src_tree}"
     ossbuild-cmd cmake --build "${build_tree}"
+    return 1
 }
 
-ossbuild-abseil-cpp-install() {
+ossbuild-install-abseil-cpp() {
     local build_tree
     ossbuild-project-var-get -v build_tree BUILD_TREE
 
@@ -80,9 +81,12 @@ ossbuild-abseil-cpp-install() {
     ossbuild-cmd "${cmd[@]}"
 }
 
-_main_() {
+ossbuild-abseil-cpp() {
     ossbuild-project-gh-init 'abseil' 'abseil-cpp'
     ossbuild-project-build-steps-all "${@}"
 }
 
-_main_ "${@}"
+#
+# run script, skip if sourced (while avoiding shellcheck issues)
+#
+(return 0 2>/dev/null) || ossbuild-abseil-cpp "${@}"
diff --git a/projects/llvm/ossbuild-llvm.bash b/projects/llvm/ossbuild-llvm.bash
index 406c53b..e984ab1 100755
--- a/projects/llvm/ossbuild-llvm.bash
+++ b/projects/llvm/ossbuild-llvm.bash
@@ -3,7 +3,7 @@
 LIB_DIR="$(dirname "$(realpath "${BASH_SOURCE[0]}")")/../../lib"
 source "${LIB_DIR}/ossbuild-common.bash"
 
-llvm-source-workon() {
+ossbuild-prepare-llvm() {
     echo "[INF] ${FUNCNAME[0]} ..."
     local version="${1}"
 
@@ -22,7 +22,7 @@ llvm-source-workon() {
     pushd "${src_dir}" &> /dev/null
 }
 
-llvm-cmake-generate() {
+ossbuild-generate-llvm() {
     echo "[INF] ${FUNCNAME[0]} ..."
     local build_type="${1:-Release}"
     local projects="${2:-}"
@@ -190,7 +190,7 @@ llvm-cmake-generate() {
 }
 
 # shellcheck disable=SC2120
-llvm-cmake-build() {
+ossbuild-build-llvm() {
     echo "[INF] ${FUNCNAME[0]} ..."
     local build_type="${1:-Release}"
     local build_args="${2:-}"
@@ -222,7 +222,7 @@ llvm-cmake-build() {
     "${cmd[@]}"
 }
 
-llvm-cmake-install() {
+ossbuild-install-llvm() {
     echo "[INF] ${FUNCNAME[0]} ..."
     local build_type="${1:-Release}"
     local build_dir="${2:-build-${build_type,,}}"
@@ -235,7 +235,7 @@ llvm-cmake-install() {
     "${cmd[@]}"
 }
 
-llvm-cmake-generate-fallback() {
+ossbuild-generate-llvm-fallback() {
     echo "[INF] ${FUNCNAME[0]} ..."
     local build_type="${1:-Release}"
     local build_dir="${2:-build-${build_type,,}}"
@@ -247,14 +247,14 @@ llvm-cmake-generate-fallback() {
         lld
     )
     local IFS=';'
-    llvm-cmake-generate "${build_type}" "${llvm_projects[*]}"
+    ossbuild-generate-llvm "${build_type}" "${llvm_projects[*]}"
 
     # Fallback 2: Generate build systems for individual projects
     IFS=';' read -a LLVM_PROJECTS \
        <<< $(grep 'set(LLVM_ALL_PROJECTS ' llvm/CMakeLists.txt | cut -d\" -f2)
     local project
     for project in "${llvm_projects[@]}"; do
-        llvm-cmake-generate "${build_type}" "${project}"
+        ossbuild-generate-llvm "${build_type}" "${project}"
     done
 }
 
@@ -262,15 +262,15 @@ ossbuild-llvm() {
     local version="${1:-15.0.5}"
     local build_type="${2:-Release}" # or 'RelWithDebInfo', but beware of the huge binaries
 
-    llvm-source-workon "${version}"
-    llvm-cmake-generate "${build_type}"
+    ossbuild-prepare-llvm "${version}"
+    ossbuild-generate-llvm "${build_type}"
 
     # Ideally, llcm-cmake-build should build the most LLVM projects. However, if
     # it doesn't, it might make sense to build projects individually (see
-    # `llvm-cmake-generate-fallback` above).
-    llvm-cmake-build "${build_type}"
+    # `ossbuild-generate-llvm-fallback` above).
+    ossbuild-build-llvm "${build_type}"
     # Install the toolchain
-    llvm-cmake-install "${build_type}"
+    ossbuild-install-llvm "${build_type}"
 
     return $?
 }
diff --git a/projects/protobuf/ossbuild-protobuf.bash b/projects/protobuf/ossbuild-protobuf.bash
index f1b9784..87f547a 100755
--- a/projects/protobuf/ossbuild-protobuf.bash
+++ b/projects/protobuf/ossbuild-protobuf.bash
@@ -12,7 +12,7 @@ source "${LIB_DIR}/ossbuild-common.bash"
 #PROTOBUF_DEFAULT_VERSION='23.3'
 #https://github.com/protocolbuffers/protobuf/releases/download/v23.3/protobuf-23.3.tar.gz
 
-ossbuild-protobuf-prepare() {
+ossbuild-prepare-protobuf() {
     local version gh_url archive archive_url src_tree
     ossbuild-project-var-get -v version VERSION
     ossbuild-project-var-get -v gh_url GH_URL
@@ -33,7 +33,7 @@ ossbuild-protobuf-prepare() {
     }
 }
 
-ossbuild-protobuf-build() {
+ossbuild-build-protobuf() {
     local src_tree build_tree
     ossbuild-project-var-get -v src_tree SRC_TREE
     build_tree="${src_tree}/ossbuild"
@@ -62,7 +62,7 @@ ossbuild-protobuf-build() {
     ossbuild-cmd cmake --build "${build_tree}"
 }
 
-ossbuild-protobuf-install() {
+ossbuild-install-protobuf() {
     local build_tree
     ossbuild-project-var-get -v build_tree BUILD_TREE
 
@@ -75,9 +75,12 @@ ossbuild-protobuf-install() {
     ossbuild-cmd "${cmd[@]}"
 }
 
-_main_() {
+ossbuild-protobuf() {
     ossbuild-project-gh-init 'protocolbuffers' 'protobuf'
     ossbuild-project-build-steps-all "${@}"
 }
 
-_main_ "${@}"
+#
+# run script, skip if sourced (while avoiding shellcheck issues)
+#
+(return 0 2>/dev/null) || ossbuild-protobuf "${@}"
diff --git a/projects/quilt/ossbuild-quilt.bash b/projects/quilt/ossbuild-quilt.bash
index 602face..fdfa546 100755
--- a/projects/quilt/ossbuild-quilt.bash
+++ b/projects/quilt/ossbuild-quilt.bash
@@ -10,25 +10,28 @@ source "${LIB_DIR}/ossbuild-common.bash"
 # shellcheck disable=SC2034
 QUILT_DEFAULT_VERSION='0.67'
 
-ossbuild-quilt-prepare() {
+ossbuild-prepare-quilt() {
     # download URL example:
     # http://download.savannah.gnu.org/releases/quilt/quilt-0.67.tar.gz
     ossbuild-archive-gnu-prepare \
         "http://download.savannah.gnu.org/releases/quilt"
 }
 
-ossbuild-quilt-build() {
+ossbuild-build-quilt() {
     # quilt does not support a separate build tree
     ossbuild-project-var-set BUILD_TREE "$(ossbuild-project-var-get SRC_TREE)"
-    ossbuild-autotools-generic-build
+    ossbuild-autotools-build
 }
 
-ossbuild-quilt-install() {
-    ossbuild-autotools-generic-install
+ossbuild-install-quilt() {
+    ossbuild-autotools-install
 }
 
-_main_() {
+ossbuild-quilt() {
     ossbuild-project-build-steps-all "${@}"
 }
 
-_main_ "${@}"
+#
+# run script, skip if sourced (while avoiding shellcheck issues)
+#
+(return 0 2>/dev/null) || ossbuild-quilt "${@}"