General Information

Automating tasks with cron (the daemon that executes scheduled commands).

Checklist

• Distro(s): Any

Individual users can create/edit their own cron jobs.

List your user's crontab jobs

crontab -l

Edit your crontab

crontab -e

User crontabs are stored at:

• /var/spool/cron/<username>

• /etc/cron.allow
• /etc/cron.deny

About these files:

• One username per line
• Root can always use crontab, regardless of file existence below

For any below, asterisk means all values.

# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  *  command to be executed

Executes script.sh at 0600,0630,1200,and 1230 every day of the month, every month, and everyday of the week.

0,30 6,12 * * * /home/user/script.sh

Executes script.sh every minute.

* * * * * /home/user/script.sh

Executes script.sh every 5 minutes.

*/5 * * * * /home/user/script.sh

Executes script.sh on system startup.

@reboot /home/user/script.sh

To run jobs in the system directories, look to /etc.

• /etc/cron.d/ = system executed jobs (format like crontab)
• /etc/cron.daily/ = daily executed scripts (format bash script)
• /etc/cron.hourly/ = hourly executed scripts (format bash script)
• /etc/cron.monthly/ = monthly executed scripts (format bash script)
• /etc/cron.weekly/ = weekly executed scripts (format bash script)
• /etc/crontab = system wide crontab

It is recommended to drop scripts into one of the /etc/cron.* directories instead of editing the system wide crontab file.

All formats are bash scripts, except for the /etc/cron.d/ directory, which accepts crontab formatted files, with the addition of specifying a username.

For example, /etc/cron.d/crashplan-mirror:

# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed
 
# Rsync the crashplan backups to another usb drive for redundancy
0 2,14 * * * root /root/scripts/rsync_cp_mirror.sh

The above executes at 0200 and 1400, runs as the user root, the script specified.

Keywords that can be used in place of a specific time/date schedule.

• @reboot - Run once at start up.
• @yearly - Run once a year.
• @annually - Same as yearly.
• @monthly - Run once a month.
• @weekly - Run once a week.
• @daily - Run once a day.
• @midnight - Same as daily.
• @hourly - Run once an hour.

Cron is great at being specific about minutes, hours, dates, months, and days of the week, but is not able to handle relative days.

• Relative days could come in handy when you need a job to execute on something such as the 1st, 2nd and 3rd Tuesday of every month.
• In order to accomplish this, a combination of cron and a date checker script is used.

In this example, we want a script to execute on the 1st, 2nd, and 3rd Tuesday of every month, with different arguments depending upon which Tuesday it is.

• Cron entry: Execute a date checker script every Tuesday
  • Create the system wide cron entry (/etc/cron.d/automated_jobs)

    cron-automated-jobs

    # .---------------- minute (0 - 59)
    # |  .------------- hour (0 - 23)
    # |  |  .---------- day of month (1 - 31)
    # |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
    # |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
    # |  |  |  |  |
    # *  *  *  *  *  user-name  command to be executed
     
    #Every Tuesday at 0800: Determine if an automated job should be run
    #-Target: 1st,2nd,3rd Tues of month
    00 08 * * tue  root  /automation/automated-job-check.sh

• Date checker script: Determine if the actual automated program should run and what arguments to send it.
  • Create the date checker script (/automation/automated-job-check.sh)

    automated-job-check.sh

    #!/bin/bash
    # Title: automated-job-check.sh
    # Description: Determine if an automated job should be run
     
    # Script to execute: See 'if statement' for dynamically selecting an environment
    script="/automation/scripts/automated-job.py"
     
    # Log file
    log_file="/var/log/automation/automated-job-check_$(date +%Y%m).log"
     
    echo "==== Log Started: $(date) ====" >> ${log_file}
     
    # If today is Tuesday AND today is >=1 and <=21(1st,2nd,3rd Tue)
    if [[ "$(date '+%a')" == "Tue" ]] && [[ $(date +%-d) -ge 1 ]]; then
     
      # First Tue: Development Environment
      if [[ $(date +%-d) -le 7 ]]; then
        echo ">> OK: Today is the first Tue of the Month; execute script(${script} --group all_dev)." >> ${log_file}
        ${script} --group all_dev 2>&1 >> ${log_file}
     
      # Second Tue: Test Environment
      elif [[ $(date +%-d) -le 14 ]]; then
        echo ">> OK: Today is the second Tue of the Month; execute script(${script} --group all_test)." >> ${log_file}
        ${script} --group all_test 2>&1 >> ${log_file}
     
      # Third Tue: Production Environment
      elif [[ $(date +%-d) -le 21 ]]; then
        echo ">> OK: Today is the third Tue of the Month; execute script(${script} --group all_prod)." >> ${log_file}
        ${script} --group all_prod 2>&1 >> ${log_file}
     
      else
        echo ">> NO-EXEC: Today is NOT the first,second, or third Tue of the Month. Will NOT execute script(${script})." >> ${log_file}
      fi
    else
      echo ">> NO-EXEC: Today is NOT the first,second, or third Tue of the Month. Will NOT execute script(${script})." >> ${log_file}
    fi
     
    echo -e "==== Log Ended: $(date) ====\n" >> ${log_file}

A few other examples of date checker scripts with relative days before/after.

7 Days before the first Tuesday of the month

#!/bin/bash
# Title: automated-job-check.sh
# Description: Determine if an automated job should be run
 
# Script to execute:
script="/automation/scripts/automated-job.py"
 
# Log file
log_file="/var/log/automation/automated-job-check_$(date +%Y%m).log"
 
echo "==== Log Started: $(date) ====" >> ${log_file}
 
# If today is Tuesday AND 7 days from now is >=1 and <=7(first Tue)
if [[ "$(date '+%a')" == "Tue" ]] && [[ $(date +%-d -d "+7 days") -ge 1 ]]; then
  if [[ $(date +%-d -d "+7 days") -le 7 ]]; then
    echo ">> OK: Seven days from now is the first Tue of the Month; execute script(${script})." >> ${log_file}
    ${script} 2>&1 >> ${log_file}
  else
    echo ">> NO-EXEC: Seven days from now is NOT the first Tue of the Month. Will NOT execute script(${script})." >> ${log_file}
  fi
else
  echo ">> NO-EXEC: Seven days from now is NOT the first Tue of the Month. Will NOT execute script(${script})." >> ${log_file}
fi
 
echo -e "==== Log Ended: $(date) ====\n" >> ${log_file}

2 Days after the third Tuesday of the month

#!/bin/bash
# Title: automated-job-check.sh
# Description: Determine if an automated job should be run
 
# Script to execute
script="/automation/scripts/automated-job.py"
 
# Log file
log_file="/var/log/automation/automated-job-check_$(date +%Y%m).log"
 
echo "==== Log Started: $(date) ====" >> ${log_file}
 
# If today is Thursday AND two days ago was >=15 and <=21(third Tue)
if [[ "$(date '+%a')" == "Thu" ]] && [[ $(date +%-d -d "-2 days") -ge 15 ]]; then
  if [[ $(date +%-d -d "-2 days") -le 21 ]]; then
    echo ">> OK: Two days ago was the third Tue of the Month; execute script(${script})." >> ${log_file}
    ${script} 2>&1 >> ${log_file}
  else
    echo ">> NO-EXEC: Two days ago was NOT the third Tue of the Month. Will NOT execute script(${script})." >> ${log_file}
  fi
else
  echo ">> NO-EXEC: Two days ago was NOT the third Tue of the Month. Will NOT execute script(${script})." >> ${log_file}
fi
 
echo -e "==== Log Ended: $(date) ====\n" >> ${log_file}