diff --git a/dev.sh b/dev.sh
index e833812..76b2662 100755
--- a/dev.sh
+++ b/dev.sh
@@ -3,17 +3,22 @@
 # to an "agent" pane. CONFIG (JSON settings) and ARGS pass to rexdiff.
 set -e
 BIN=/tmp/rexdiff-dev/rexdiff
-cd "$(dirname "$0")" && go build -o $BIN ./cmd/rexdiff
+cd "$(dirname "$0")"
+S=rexreview-test; R=${REXDIFF_DEMO:-$PWD}
+if ! git -C "$R" rev-parse --show-toplevel >/dev/null 2>&1; then
+  echo "dev.sh: not a Git working tree: $R (set REXDIFF_DEMO to a repository)" >&2
+  exit 1
+fi
+go build -o "$BIN" ./cmd/rexdiff
 export PATH="${REX_BIN_DIR:+$REX_BIN_DIR:}$PATH"
-S=rexreview-test; R=${REXDIFF_DEMO:-/tmp/rexreview-demo}
 has() { rex block inspect "$1" -s $S --json >/dev/null 2>&1; }
-rex ls | grep -q "^$S " || rex new $S --cwd $R -- fish >/dev/null
-has agent || { rex run -s $S --label agent --cwd $R -- fish >/dev/null; sleep 1; }
+rex ls | grep -q "^$S " || rex new $S --cwd "$R" -- "${SHELL:-/bin/sh}" >/dev/null
+has agent || { rex run -s $S --label agent --cwd "$R" -- "${SHELL:-/bin/sh}" >/dev/null; sleep 1; }
 for id in $(rex api call -s $S session.list_blocks '{}' | python3 -c 'import json,sys;[print(b["block_id"]) for b in json.load(sys.stdin)["blocks"] if b.get("label")=="rexdiff"]'); do
   rex api call -s $S block.close "{\"block_id\":\"$id\"}" >/dev/null 2>&1 || true
 done
 sleep 0.5
-rex split -s $S -b agent --label rexdiff --ratio 70 --cwd $R -- $BIN -config "${CONFIG:-}" -log /tmp/rexdiff.log --agent agent $ARGS >/dev/null
+rex split -s $S -b agent --label rexdiff --ratio 70 --cwd "$R" -- "$BIN" -config "${CONFIG:-}" -log /tmp/rexdiff.log --agent agent $ARGS >/dev/null
 sleep 0.5
 rex block call -s $S -b rexdiff resize "{\"columns\":${COLS:-170},\"rows\":${ROWS:-30}}" >/dev/null
 sleep 1.5