One-off and recurring jobs with at and cron

Do these on your own system. They get harder. Check each result, then tick it off.

1. Warm-upDone

   Run something in two minutes

   Make sure atd is running. Schedule a job for two minutes from now that appends the date to ~/lab/at.log. Show the queue, wait, and show the file.

   Check your result

   atq lists one job before it runs. After two minutes ~/lab/at.log contains a date line and atq is empty.

   Hint

   sudo dnf install -y at; sudo systemctl enable --now atd. Feed the command on standard input: echo 'date >> ~/lab/at.log' | at now + 2 minutes.

   Solution

   sudo dnf install -y at
   sudo systemctl enable --now atd
   echo 'date >> ~/lab/at.log' | at now + 2 minutes
   atq

2. PracticeDone

   Inspect and cancel a job

   Schedule a job for tomorrow at 09:00 that runs echo hi. Display exactly what the job will run, then cancel it.

   Check your result

   at -c JOBNUMBER prints the environment followed by echo hi at the end. After atrm JOBNUMBER, atq no longer lists it.

   Hint

   at -c N shows the job script. atrm N removes it.

   Solution

   echo 'echo hi' | at 09:00 tomorrow
   atq
   at -c $(atq | awk '{print $1}' | tail -1) | tail -1
   atrm $(atq | awk '{print $1}' | tail -1)
   atq

3. PracticeDone

   A daily job for one user

   As the user alex, create a cron job that runs every day at 14:23 and writes EX200 in progress to the system log with logger. Show it with crontab -l, and explain each of the five time fields in your entry.

   Check your result

   crontab -l prints 23 14 * * * logger "EX200 in progress". The fields are minute 23, hour 14, any day of month, any month, any weekday.

   Hint

   crontab -e opens your own table. The order is: minute, hour, day of month, month, day of week.

   Solution

   crontab -e
   # 23 14 * * * logger "EX200 in progress"
   crontab -l

4. PracticeDone

   A system job in /etc/cron.d

   As root, schedule /usr/local/bin/cleanup.sh (create it, make it executable, and have it write the line cleanup ran to the log) to run on weekdays at 02:30, as user root, using a file in /etc/cron.d. Check the syntax rule that differs from a user crontab.

   Check your result

   /etc/cron.d/cleanup contains 30 2 * * 1-5 root /usr/local/bin/cleanup.sh. The extra field is the user name.

   Hint

   Files in /etc/cron.d have a sixth field: the user to run as. 1-5 is Monday to Friday.

   Solution

   printf '#!/bin/bash\nlogger "cleanup ran"\n' | sudo tee /usr/local/bin/cleanup.sh
   sudo chmod 755 /usr/local/bin/cleanup.sh
   echo '30 2 * * 1-5 root /usr/local/bin/cleanup.sh' | sudo tee /etc/cron.d/cleanup
   cat /etc/cron.d/cleanup

5. ChallengeDone

   Find out why a job did not run

   A user reports that their cron job * * * * * date > out.txt "does nothing". Create it for alex, wait a minute, and find the evidence. Identify two common causes: where the file is written, and the minimal PATH. Then fix the job so it writes to an absolute path and calls programs by full path.

   Check your result

   journalctl -u crond (or /var/log/cron) shows the job was started for alex. out.txt appears in alex's home, not in the directory they were in. After the fix ~/lab/cron-out.txt updates every minute.

   Hint

   Cron starts jobs in the user's home with a short PATH (/usr/bin:/bin). Use absolute paths for files and commands.

   Solution

   (crontab -l 2>/dev/null; echo '* * * * * date > out.txt') | crontab -
   sleep 70
   sudo journalctl -u crond --no-pager | tail -3
   ls ~/out.txt
   (echo '* * * * * /usr/bin/date > /home/alex/lab/cron-out.txt') | crontab -