Never Run Twice at Once

/home/student/bin/sync.sh runs from cron every minute, but on slow days a run takes longer than a minute and two copies overwrite each other's data.

Task

Change sync.sh so that only one copy can run at a time:

• while one copy is running, a second one prints sync: already running to stderr and exits 1 straight away, without doing any work
• the first copy carries on normally
• once a copy has finished, the next run works again
• if a copy is killed with kill -9, the next run must still work (no stale lock blocks it)