Cron & systemd Timers: Scheduling Jobs in Linux

cronis a daemon that wakes once a minute, reads every user's crontab plus /etc/crontab and /etc/cron.d/*, and executes any job whose five-field schedule matches the current minute. The fields are, in order: minute, hour, day-of-month, month, day-of-week — each accepting a number, a range (1-5), a step (*/15), a list (1,15,30), or a wildcard (*).

Special strings

Why a cron job that runs manually can fail on schedule

cron executes jobs with a minimal environment — no login shell profile, a bare PATH (often just /usr/bin:/bin), and no working directory guarantee. A script that relies on a tool installed outside that PATH, or a relative file path, works fine when you run it interactively (your shell has a full environment) and fails silently under cron.

systemd timers as a modern alternative

A systemd timer unit (.timer) is paired with a service unit of the same base name (.service) that defines the actual work. Timers support calendar expressions (OnCalendar=Mon..Fri 09:00) and monotonic expressions (OnBootSec=15min), and — unlike cron — every run is logged to the journal, missed runs can be caught up with Persistent=true, and a timer can depend on other units being ready first.

Part 1 — a cron job that backs up a directory nightly:

crontab -e
# Add this line — runs at 02:30 every day, full paths only:
30 2 * * * /usr/bin/tar -czf /backups/home-$(date +\%Y\%m\%d).tar.gz /home/deploy 2>> /var/log/backup-cron.log

crontab -l                    # verify it saved
grep CRON /var/log/syslog     # confirm cron actually triggered it (Debian/Ubuntu)

Part 2 — the same job as a systemd timer:

sudo tee /etc/systemd/system/home-backup.service <<'EOF'
[Unit]
Description=Nightly home directory backup

[Service]
Type=oneshot
# ExecStart isn't run through a shell, so command substitution needs an
# explicit shell — and a literal "%" must be escaped as "%%" since systemd
# treats a single "%" as the start of its own specifier syntax (e.g. %n, %i).
ExecStart=/bin/bash -c 'tar -czf /backups/home-$(date +%%Y%%m%%d).tar.gz /home/deploy'
EOF

sudo tee /etc/systemd/system/home-backup.timer <<'EOF'
[Unit]
Description=Run home-backup.service nightly at 02:30

[Timer]
OnCalendar=*-*-* 02:30:00
Persistent=true

[Install]
WantedBy=timers.target
EOF

sudo systemctl daemon-reload
sudo systemctl enable --now home-backup.timer
systemctl list-timers --all           # confirm next scheduled run
journalctl -u home-backup.service     # see every past run's output

Cron job doesn't run at all: confirm the cron daemon is active (systemctl status cron or crond), check /var/log/syslog or journalctl -u cronfor a line showing the job was picked up, and verify the crontab's owner has permission to run under /etc/cron.allow/cron.deny if configured.

systemd timer shows as active but the service never fires: run systemctl list-timers and compare the NEXT column against the current time — a common cause is an OnCalendarexpression that's syntactically valid but not what you intended; validate it directly with systemd-analyze calendar "Mon..Fri 09:00".

cron polls a five-field schedule every minute against a minimal execution environment — always use full paths and redirect output. systemd timers pair a .timer with a .service, support calendar and monotonic schedules, log every run to the journal, and can catch up missed runs — the better default on any systemd-based host.