diff --git a/solr/CHANGES.txt b/solr/CHANGES.txt
index b594afbfe38..2b6c868bb5c 100644
--- a/solr/CHANGES.txt
+++ b/solr/CHANGES.txt
@@ -118,6 +118,8 @@ Bug Fixes
 
 * SOLR-7729: ConcurrentUpdateSolrClient ignores the collection parameter in some methods.
   (Nicolas Gavalda, Jorge Luis Betancourt Gonzalez via Mark Miller)
+
+* SOLR-8946: bin/post failed to detect stdin usage on Ubuntu; maybe other unixes. (David Smiley)
   
 Optimizations
 ----------------------
diff --git a/solr/bin/post b/solr/bin/post
index 92172ab2429..73e59ed4c41 100755
--- a/solr/bin/post
+++ b/solr/bin/post
@@ -140,7 +140,7 @@ while [ $# -gt 0 ]; do
         shift
         PROPS+=("-Dport=$1")
       elif [[ ("$1" == "-d" || "$1" == "--data" || "$1" == "-") ]]; then
-        if [[ -s /dev/stdin ]]; then
+        if [[ ! -t 0 ]]; then
           MODE="stdin"
         else
           # when no stdin exists and -d specified, the rest of the arguments
@@ -150,6 +150,9 @@ while [ $# -gt 0 ]; do
           if [[ $# -gt 0 ]]; then
             ARGS=("$@")
             shift $#
+          else
+            # SPT needs a valid args string, useful for 'bin/post -c foo -d' to force a commit
+            ARGS+=("<add/>")
           fi
         fi
       else
@@ -209,10 +212,6 @@ if [[ $MODE != "stdin" && $MODE != "args" ]]; then
     PARAMS=("${URLS[@]}")
   fi
 else
-  if [[ ${#ARGS[@]} == 0 ]]; then
-    # SPT needs a valid (to post to Solr) args string, useful for 'bin/post -c foo -d' to force a commit
-    ARGS+=("<add/>")
-  fi
   PARAMS=("${ARGS[@]}")
 fi