Understanding and Using crontab for Task Scheduling on Linux

The crontab command is used to schedule periodic commands by storing them in a crontab file, which the system reads and executes at defined times, making it ideal for offline tasks like daily data updates.

Most Linux distributions include the crontab service; if missing, install it with:

# vixie-cron is the main cron program
yum -y install vixie-cron
yum -y install crontabs

Control the service with the following commands:

service crond start      # start service
service crond stop       # stop service
service crond restart    # restart service
service crond reload     # reload configuration
service crond status     # check status
# enable at boot
chkconfig --level 345 crond on

Global cron configuration files are located in directories such as cron.d/ , cron.daily/ , cron.hourly/ , cron.weekly/ , cron.monthly/ , and the user’s crontab file.

Each user can edit their personal crontab with crontab -e ; the file is saved under /var/spool/cron/ named after the user. The cron daemon checks /var/spool/cron , /etc/crontab , and /etc/cron.d for jobs.

A crontab line consists of six fields: minute, hour, day, month, week, and command. Special symbols can be used:

* – wildcard (all values)

, – list separator (e.g., 1,2,5)

- – range (e.g., 2-6)

/ – step interval (e.g., */10 for every ten minutes)

Common scheduling examples:

00 05 * * * – every day at 05:00

20 12 1,10,20 * * – on the 1st, 10th, and 20th of each month at 12:20

0 */2 * * * – every two hours

To run a script at 23:58 daily and log its output, add to the crontab:

58 23 * * * sh /home/work/update.sh >> /home/work/log/update.log 2>&1

Redirecting both stdout and stderr ensures errors are captured in the same log file. Standard file descriptors are:

Name

Type

FD

Operator

stdin

standard input

0

<, <<

stdout

standard output

1

>, >>

stderr

standard error

2

2>, 2>>

Using nohup before the command ignores input, and appending & runs the job in the background:

58 23 * * * nohup sh /home/work/update.sh >> /home/work/log/update.log 2>&1 &

For daily log files, embed the date in the filename using the date command, remembering to escape the percent sign inside crontab:

58 23 * * * nohup sh /home/work/update.sh >> /home/work/log/`date +\%Y\%m\%d`.log 2>&1 &

Running the same command directly in a shell works without escaping, but the escaped version will fail inside crontab.

For further reading, see the “Linux Knowledge Encyclopedia”.