diff --git a/prjxray/util.py b/prjxray/util.py
index 1c5621be..87278062 100644
--- a/prjxray/util.py
+++ b/prjxray/util.py
@@ -163,3 +163,9 @@ def gen_tile_bits(db_root, tilej, strict=False, verbose=False):
                     # 31_06
                     _bit_inv, (bit_addroff, bit_bitoff) = parse_tagbit(bitstr)
                     yield (baseaddr + bit_addroff, bitbase + bit_bitoff, tag)
+
+
+def specn():
+    # ex: build/specimen_001
+    specdir = os.getenv("SPECDIR")
+    return int(re.match(".*specimen_([0-9]*)", specdir).group(1), 10)
diff --git a/utils/genheader.sh b/utils/genheader.sh
index 39094a44..2bb93edd 100644
--- a/utils/genheader.sh
+++ b/utils/genheader.sh
@@ -10,14 +10,14 @@ set -ex
 # for some reason on sourced script set -e doesn't work
 # Scripts may have additional arguments, but first is reserved for build directory
 test $# -ge 1 || exit 1
-test ! -e "$SPECN"
-SPECN=$1
+test ! -e "$SPECDIR"
+export SPECDIR=$1
 
-rm -rf "$SPECN"
-mkdir -p "$SPECN"
-cd "$SPECN"
+rm -rf "$SPECDIR"
+mkdir -p "$SPECDIR"
+cd "$SPECDIR"
 
-export SEED="$(echo $SPECN | md5sum | cut -c1-8)"
+export SEED="$(echo $SPECDIR | md5sum | cut -c1-8)"
 export SEEDN="$(basename $(pwd) |sed s/specimen_0*//)"
 
 function seed_vh () {