Check qpdf is available before starting

optipdf

@@ -105,6 +105,22 @@ if [[ ${#FILES[@]} -eq 0 ]]; then
     exit 2
 fi
 
+check_required_binaries() {
+    local BIN MISSING=()
+    for BIN; do
+        if ! type -t "$BIN" &>/dev/null; then
+            MISSING+=("$BIN")
+        fi
+    done
+    if [[ ${#MISSING[@]} -gt 0 ]]; then
+        printf 'Error: You are missing required programs:\n' >&2
+        for BIN in "${MISSING[@]}"; do
+            printf -- '- %s\n' "$BIN" >&2
+        done
+        exit 2
+    fi
+}
+
 file_size() {
     stat \
         --format='%s' \
@@ -200,6 +216,9 @@ use_smaller() {
 
 }
 
+check_required_binaries \
+    qpdf
+
 QPDF_ARGS=(
     --compression-level=9
     --deterministic-id