Files placed in
cron.monthly need to be executables. If you place a text file with a single line as shown in your question into that directory, it cannot be run at all for the same reason that you could not run such a file as a shell script from the command line, either.
What you mean to say is this:
You could concatenate the second and third lines with a semicolon, but it simply isn't necessary here. It's a full-on shell script, so you don't need to "stack" commands in that way.
Also, be sure to mark the script executable, else it still won't run.
If all of this seems odd to you, based on your knowledge of
crontab entries, realize that executables in these directories are typically run by either
run-parts, not by
cron. Thus, the information from
man 5 crontab doesn't really apply here.