diff --git a/README.md b/README.md
index 241dd8b..255453c 100644
--- a/README.md
+++ b/README.md
@@ -43,6 +43,7 @@ to `slcl`. If required, encryption should be done before uploading e.g.: using
 - [`dynstr`](https://gitea.privatedns.org/xavi92/dynstr)
 (provided as a `git` submodule).
 - `xxd` (for [`usergen`](usergen) only).
+- `jq` (for [`usergen`](usergen) only).
 - CMake (optional).
 
 ### Ubuntu / Debian
diff --git a/usergen b/usergen
index ec63fd3..42c18fe 100755
--- a/usergen
+++ b/usergen
@@ -2,8 +2,29 @@
 
 set -e
 
+usage()
+{
+    echo "$0 <dir>"
+}
+
+if [ $# != 1 ]; then
+    usage >&2
+    exit 1
+fi
+
+DIR=$1
+
 echo Username: >&2
 read -r USER
+
+DB="$DIR/db.json"
+
+if jq '.users[].name' "$DB" | grep -q $USER
+then
+    echo User $USER already in $DB >&2
+    exit 1
+fi
+
 echo Password: >&2
 read -r PWD
 echo "Quota, in MiB (leave empty for unlimited quota):" >&2
@@ -23,12 +44,23 @@ do
 done
 
 echo >&2
-cat <<-EOF
+TMP=$(mktemp)
+
+cleanup()
 {
-    "name": "$USER",
-    "password": "$PWD",
-    "salt": "$SALT",
-    "key": "$KEY",
-    "quota": "$QUOTA"
+    rm -f $TMP
 }
-EOF
+
+trap cleanup EXIT
+
+jq ".users += [
+{
+    \"name\": \"$USER\",
+    \"password\": \"$PWD\",
+    \"salt\": \"$SALT\",
+    \"key\": \"$KEY\",
+    \"quota\": \"$QUOTA\"
+}]" "$DB" > $TMP
+
+mv $TMP "$DB"
+mkdir "$DIR/user/$USER"