Cron and Task Scheduling¶

cron handles recurring scheduled tasks; at handles one-time future execution. This entry covers crontab syntax, special shortcuts, one-time scheduling, and system time management.

cron - Recurring Tasks¶

Service Management¶

systemctl status cron       # check status
sudo systemctl start cron   # start
sudo systemctl enable cron  # enable at boot

crontab Commands¶

crontab -l                  # list current user's tasks
crontab -e                  # edit tasks (opens in $EDITOR)
crontab -r                  # delete ALL tasks for current user
crontab -u username -l      # list another user's tasks (needs sudo)

Crontab Syntax¶

* * * * * /path/to/command
| | | | |
| | | | +-- day of week (0-7, 0 and 7 = Sunday)
| | | +---- month (1-12 or jan-dec)
| | +------ day of month (1-31)
| +-------- hour (0-23)
+---------- minute (0-59)

Special values: - * - every unit - */n - every n units (e.g., */5 = every 5 minutes) - 1,15 - specific values - 1-5 - range - jan,jul,oct - named months

Examples¶

15 * * * * /run/script.sh             # every hour at :15
0 22 * * 1-5 /run/script.sh          # 22:00 Mon-Fri
0 0 1,15 * * /run/script.sh          # midnight on 1st and 15th
*/5 * * * * /run/script.sh            # every 5 minutes
0 9 * * * /run/script.sh              # daily at 9:00

Special Shortcuts¶

@reboot    # run once at startup
@hourly    # = 0 * * * *
@daily     # = 0 0 * * *
@weekly    # = 0 0 * * 0
@monthly   # = 0 0 1 * *

Cron Logging¶

grep CRON /var/log/syslog
journalctl -u cron

Reference: https://crontab.guru/ - interactive syntax tester

at - One-Time Tasks¶

at 10pm                    # interactive mode, Ctrl+D to save
at 8:00am tomorrow
at -f script.sh 8:00am    # schedule a script
at now + 2 hours
at 14:30 Jul 31

atq                        # list pending jobs
atrm 3                     # cancel job #3

Install at¶

sudo apt install at
sudo systemctl enable --now atd

batch¶

Like at, but runs only when system load drops below threshold.

System Time¶

date¶

date                            # display current date/time
date +"%Y-%m-%d %H:%M:%S"      # formatted output
sudo date --set="2024-09-06 20:43:40"  # set manually (disable NTP first)

hwclock - Hardware Clock¶

hwclock                     # show hardware clock
hwclock --systohc           # sync hw clock from system time
hwclock --hctosys           # sync system time from hw clock

NTP Sync¶

timedatectl status          # show time, timezone, NTP status
timedatectl set-ntp true    # enable NTP
timedatectl set-timezone Europe/Moscow

Note: active NTP overwrites manual date --set immediately.

Gotchas¶

crontab -r deletes ALL tasks without confirmation - dangerous
Minute 60 and hour 24 are invalid (0-59 and 0-23)
Cron jobs run with a minimal environment - use full paths for commands
@reboot runs once at system boot, not at cron service restart
If both day-of-month and day-of-week are set, cron runs if EITHER matches (OR logic)
cron sends email for job output - redirect to /dev/null or a log file to avoid mail accumulation