Crontab best practices

The software utility Cron is a time-based job scheduler in Unix-like computer operating systems

Environment

First of all, crontab environment is not the same as your shell. Only few variables are valued, for example $HOME, $PATH and $MAILTO. A common pit fall is to schedule a script and it does not work cause it needs environment varibles. The best trick I found is to use cron SHELL power! Create a .cron.bash file in your home dir, containing something like the following

#!/bin/bash

source .bashrc

# Restore SHELL env var for cron
SHELL=/bin/bash

# execute the cron command in an actual shell
exec /bin/bash --norc "$@"

Then on top of your crontab

# Let scheduled scripts run with the same environment as bash
# shell interactive session.
SHELL=.cron.bash

Error handling

Make sure your scripts follow Rule of silence but do not send standard error stream on /dev/null. The best practice is to write scripts that run commands in quiet mode with no feedback unless a verbose flag, or whatever, is passed as argument. In this way, when a script outputs an error, crontab will send you an email if you configure the MAILTO variable, for example

# Send scripts output by email.
MAILTO=alarms@example.com

Scheduled jobs

Organize your schedules by frequency (hourly, daily, monthly, etc) and write then in cronological order.

Pad left single digits with a zero, to get rows alligned. Please, indent and group where it makes sense

# Bad

0 5 * * * script1.sh param1
5 5 * * * script2.sh
15 5 * * * script3.sh param1 param2
5 10 * * * script4.sh

15 10 * * * script5.sh param1
5,35 15 * * * script6.sh
15 15 * * * script7.sh

# Good

00 05 * * * script1.sh param1
05 05 * * * script2.sh
15 05 * * * script3.sh param1 param2

05 10 * * * script4.sh
15 10 * * * script5.sh param1

5,35 15 * * * script6.sh
15   15 * * * script7.sh

Use comments to organize and inform, in particular when you deschedule a task temporarily with a comment, put your nick or email and a date.

Just to have an idea of what I mean:

# Countinous integration deploy.
*/10 * * * * continuous_deploy.sh

# Every hour
# ----------

10 * * * * rotate_logs.sh

20 * * * * load_data.sh

# Every day
# ---------

# jsmith 20160405: paused cause waiting for bla bla bla
#10 04 * * * script2.sh

00 05 * * * script3.sh param1
05 05 * * * script4.sh
15 05 * * * script5.sh param1 param2

10 06 * * * script7.sh

00 19 * * 0 script8.sh

00 20 * * 0 script9.sh

# Every sunday
# ------------

00 20 * * 0 rotate_table.sh campaign_dd 60
05 20 * * 0 rotate_table.sh request 90
10 20 * * 0 rotate_table.sh sales_dd 7

Other tips

Since % is a special char you need to escape it, for example when you schedule a date command with formatting, for example

0 5 * * * script.sh >> /my/log_`date +\%y\%m\%d`.log

See the original article: Using the date command in your crontab.