How to schedule tasks in Linux using crontab

How to schedule tasks in Linux using crontab

Introduction

Crontab is an utility in Linux that allow us to run jobs or commands on a specific schedule, just like Task Scheduler in Windows.

What is crontab

The crontab (cron derives from chronos, Greek for time; tab stands for table) command, found in Unix and Unix-like operating systems, is used to schedule commands to be executed periodically. To see what crontabs are currently running on your system, you can open a terminal and run:

sudo crontab -l

If you have never used it before, the system may tell you that there’s no crontab for your current user.

To edit the cronjobs, you can run

sudo crontab -e

How crontab works

Each user in Linux system has a file located at

/var/spool/cron/

containing a list of commands that will be executed on a timely basis.

The content of a crontab file looks like this:

# Edit this file to introduce tasks to be run by cron.
#
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h dom mon dow command
0 */3 * * * /home/myname/scripts/my-script.sh

Notice the last line

0 */3 * * * /home/myname/scripts/my-script.sh

That line tells the system to run /home/myname/scripts/my-script.sh every 3 hours.

A crontab file can have multiple entries, each of which represents a cron job.

Note that this file is not intended to be edited directly, but rather by using the crontab command.

Don’t worry, we will go into detail how that works in a couple of minutes.

Structure of a crontab entry

As you can see in the last section, a crontab entry has a syntax like this:

# m h dom mon dow command
0 */3 * * * /home/myname/scripts/my-script.sh

For each entry, the following parameters must be included:

  1. m, a number (or list of numbers, or range of numbers), representing the minute of the hour (0-59);
  2. h, a number (or list of numbers, or range of numbers), representing the hour of the day (0-23);
  3. dom, a number (or list of numbers, or range of numbers), representing the day of the month (1-31);
  4. mon, a number (or list, or range), or name (or list of names), representing the month of the year (1-12);
  5. dow, a number (or list, or range), or name (or list of names), representing the day of the week (0-7, 0 or 7 is Sunday); and
  6. command, which is the command to be run, exactly as it would appear on the command line.

A field may be an asterisk (*), which always stands for “first through last”.

Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive; for example, 8-11 for an “hours” entry specifies execution at hours 8, 9, 10 and 11.

Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: “1,2,5,9“, “0-4,8-12“.

Step values can be used in conjunction with ranges. For example, “0-23/2” can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say “every two hours”, you can use “*/2“.

Names can also be used for the “month” and “day of week” fields. Use the first three letters of the particular day or month (case doesn’t matter). Ranges or lists of names are not allowed.

The “sixth” field (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh or by the shell specified in the SHELL variable of the cronfile. Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters, and all data after the first % will be sent to the command as standard input.

Note that the day of a command’s execution can be specified by two fields: day of month, and day of week. If both fields are restricted (in other words, they aren’t *), the command will be run when either field matches the current time. For example, “30 4 1,15 * 5” would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.

How to use crontab examples

List current crontab

crontab -l

Edit your crontab

crontab -e

This command will open up the crontab in a editor (usually vi or vim). Edit and save your cron jobs as you wanted.

Remove your crontab (un-scheduling all crontab jobs)

crontab -r

Back up your crontab to my-crontab file

crontab -l > my-crontab

Load your crontab from my-crontab file

crontab my-crontab

This does the same syntax checking as crontab -e.

Edit the crontab of the user named “charles”

sudo crontab -u charles -e

Note that the -u option requires administrator privileges, so that command is executed using sudo.

View the crontab of user “jeff”

sudo crontab -u jeff -l

Remove the crontab of user “sandy”

sudo crontab -u sandy -r

Check if your crontab is working

If you want to check if your cron job is executed as expected, or when it was last run, check the crontab log as following.

On a default installation the cron jobs get logged to

/var/log/syslog

You can see just cron jobs in that logfile by running

grep CRON /var/log/syslog

If you haven’t reconfigured anything, the entries will be in there.

Examples of crontab entries

Run the shell script /home/melissa/backup.sh on January 2 at 6:15 A.M.

15 6 2 1 * /home/melissa/backup.sh

Same as the above entry. Zeroes can be added at the beginning of a number for legibility, without changing their value.

15 06 02 Jan * /home/melissa/backup.sh

Run /home/carl/hourly-archive.sh every hour, on the hour, from 9 A.M. through 6 P.M., every day.

0 9-18 * * * /home/carl/hourly-archive.sh

Run /home/wendy/script.sh every Monday, at 9 A.M. and 6 P.M.

0 9,18 * * Mon /home/wendy/script.sh

Run /usr/local/bin/backup at 10:30 P.M., every weekday.

30 22 * * Mon,Tue,Wed,Thu,Fri /usr/local/bin/backup

Run /home/bob/script.sh every 3 hours.

0 */3 * * * /home/bob/script.sh

Run /home/mirnu/script.sh every minute (of every hour, of every day of the month, of every month and every day in the week).

* * * * * /home/mirnu/script.sh

Run /home/mirnu/script.sh 10 minutes past every hour on the 1st of every month.

10 * 1 * * /home/mirnu/script.sh

Special keywords

For the first (minute) field, you can also put in a keyword instead of a number:

@reboot Run once, at startup
@yearly Run once a year "0 0 1 1 *
@annually (same as @yearly)
@monthly Run once a month "0 0 1 * *"
@weekly Run once a week "0 0 * * 0"
@daily Run once a day "0 0 * * *"
@midnight (same as @daily)
@hourly Run once an hour "0 * * * *"

Leaving the rest of the fields empty, this would be valid:

@daily /bin/execute/this/script.sh

Storing the crontab output

By default cron saves the output of /bin/execute/this/script.sh in the user’s mailbox (root in this case). But it’s prettier if the output is saved in a separate logfile. Here’s how:

*/10 * * * * /bin/execute/this/script.sh >> /var/log/script_output.log 2>&1

Explanation

Linux can report on different levels. There’s standard output (STDOUT) and standard errors (STDERR). STDOUT is marked 1, STDERR is marked 2. So the following statement tells Linux to store STDERR in STDOUT as well, creating one datastream for messages & errors:

2>&1

Now that we have 1 output stream, we can pour it into a file. Where > will overwrite the file, >> will append to the file. In this case we’d like to to append:

>> /var/log/script_output.log

Mailing the crontab output

By default cron saves the output in the user’s mailbox (root in this case) on the local system. But you can also configure crontab to forward all output to a real email address by starting your crontab with the following line:

MAILTO="yourname@yourdomain.com"

Mailing the crontab output of just one cronjob

If you’d rather receive only one cronjob’s output in your mail, make sure this package is installed:

$ aptitude install mailx

And change the cronjob like this:

*/10 * * * * /bin/execute/this/script.sh 2>&1 | mail -s "Cronjob ouput" yourname@yourdomain.com

Trashing the crontab output

Now that’s easy:

*/10 * * * * /bin/execute/this/script.sh > /dev/null 2>&1

Just pipe all the output to the null device, also known as the black hole. On Unix-like operating systems, /dev/null is a special file that discards all data written to it.

 Caveats

Many scripts are tested in a Bash environment with the PATH variable set. This way it’s possible your scripts work in your shell, but when run from cron (where the PATH variable is different), the script cannot find referenced executables, and fails.

It’s not the job of the script to set PATH, it’s the responsibility of the caller, so it can help to echo $PATH, and put PATH=<the result> at the top of your cron files (right below MAILTO).

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