crontab Syntax: Common Pitfalls and Debugging

Cron jobs run manually but not when scheduled, or output is silently swallowed, making debugging difficult.

Understand crontab syntax, the limited cron environment, and redirect output for debugging.

# Edit user crontab
crontab -e

# List crontab
crontab -l

# Cron field order: minute hour day month weekday
# 0 2 * * *     = daily at 2:00 AM
# */15 * * * *  = every 15 minutes
# 0 9 * * 1-5  = weekdays at 9:00 AM
# 0 0 1 * *    = first of each month at midnight

# Redirect all output to a log file
0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

# Set PATH in crontab (cron has minimal environment)
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# Test the environment cron will use
* * * * * env > /tmp/cron-env.txt

# Use full paths for all commands in scripts

Cron runs in a minimal shell environment — PATH contains only /usr/bin:/bin. Commands that work interactively fail in cron because they are not found or because HOME/USER are not set.