systemd-stable/tools/find-build-dir.sh

#!/bin/sh
set -e

# Try to guess the build directory:
# we look for subdirectories of the parent directory that look like ninja build dirs.

if [ -n "$BUILD_DIR" ]; then
        echo "$(realpath "$BUILD_DIR")"
        exit 0
fi

root="$(dirname "$(realpath "$0")")"

found=
for i in "$root"/../*/build.ninja; do
        c="$(dirname $i)"
        [ -d "$c" ] || continue
        [ "$(basename "$c")" != mkosi.builddir ] || continue

        if [ -n "$found" ]; then
                echo 'Found multiple candidates, specify build directory with $BUILD_DIR' >&2
                exit 2
        fi
        found="$c"
done

if [ -z "$found" ]; then
        echo 'Specify build directory with $BUILD_DIR' >&2
        exit 1
fi

echo "$(realpath $found)"