refactor: use bash string substitution

bin/slaunch

@@ -149,7 +149,8 @@ for no in $nos; do
     fi
 
     # fill in subject ID and split commands
-    subj_command=$(echo "$command" | sed "s/{}/$subject/g" | sed "s/{s}/$subject/g")
+    subj_command=${command/\{\}/$subject}
+    subj_command=${subj_command/\{s\}/$subject}
     echo "$subj_command"
 
     if [[ $test = false ]]; then