Refactor the repo sync script

.local/bin/sync-git-repos

@@ -1,4 +1,7 @@
-#! /usr/bin/sh
+#! /usr/bin/bash
+
+set -euo pipefail
+IFS=$'\n\t'
 
 REPO_ROOT_FILE="$(systemd-path user-state-private)"/repo-roots/repo-roots
 
@@ -16,14 +19,21 @@ sync_repo () {
     echo
 }
 
-if [ "list" = "$1" ]; then
-    while read -r repo_root; do
-        list_repos "${repo_root}"
-    done < "${REPO_ROOT_FILE}"
-else
-    while read -r repo_root; do
-        for repo in $(list_repos "${repo_root}"); do
-            sync_repo "${repo}"
-        done
-    done < "${REPO_ROOT_FILE}"
-fi
+case "${1:-sync}" in
+    "list")
+        while read -r repo_root; do
+            list_repos "${repo_root}"
+        done < "${REPO_ROOT_FILE}"
+        ;;
+    "sync")
+        while read -r repo_root; do
+            for repo in $(list_repos "${repo_root}"); do
+                sync_repo "${repo}"
+            done
+        done < "${REPO_ROOT_FILE}"
+        ;;
+    *)
+        echo Unknown option \""$1"\"
+        exit 1
+        ;;
+esac