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Cron Job Not Running

A scheduled job produces nothing, although its script works when run by hand. The interviewer checks whether the candidate separates "cron did not start it" from "cron started it and it failed", and knows how the cron environment differs from a login shell.

Symptom

"We moved the nightly sales report to a new server. The script works when I run it, but no report files appear. What do you check?"

Clarifying Questions

  • How was it scheduled? A user crontab, /etc/cron.d, a cron.daily script or a systemd timer each fail differently.
  • How was it tested by hand? From which directory, as which user, and with which shell?
  • Which distribution? RHEL logs job output in /var/log/cron; Ubuntu discards it without a mail server.
  • Is there any trace of a run? No trace points to the scheduler; a trace with errors points to the script.
  • Did anything else change? Time zone, user, paths, permissions and installed tools all move with a server migration.

Diagnostic Path

The job belongs to the user reports on Rocky Linux 10.2. /opt/reports/run.sh reads a config file next to it and calls report-gen from /usr/local/bin:

#!/bin/bash
# nightly sales report
source ./report.conf
report-gen > "$REPORT_DIR/sales-$(date +%F).csv"

As the owner runs it, from the script's directory, it works:

su - reports -c "cd /opt/reports && bash run.sh; cat /srv/reports/*"

Output:

orders,2026-09-17,42

1. Check the Scheduler

rm -f /srv/reports/*; systemctl is-active crond; systemctl is-enabled crond
crontab -l -u reports; ls -l /var/spool/cron/
grep reports /var/log/cron | tail -3

Output:

inactive
enabled
*/1 * * * * /opt/reports/run.sh
total 4
-rw------- 1 root root 32 Sep 17 06:25 reports
Sep 17 06:25:29 rocky-01 crontab[2987]: (root) REPLACE (reports)

The job is installed, but crond is not running, so nothing ran: the log shows only the crontab install. Enabled but inactive means it was stopped after boot, or it failed.

systemctl start crond; systemctl status crond --no-pager | sed -n "1,3p"

Output:

● crond.service - Command Scheduler
     Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; preset: enabled)
     Active: active (running) since Thu 2026-09-17 06:25:47 UTC; 25ms ago

The test runs every minute (*/1) so each fix shows up quickly; the real schedule is nightly.

2. Read What the Run Printed

cronie on a host without sendmail logs each output line as CMDOUT.

grep reports /var/log/cron | tail -3; ls -l /opt/reports/run.sh

Output:

Sep 17 06:26:01 rocky-01 CROND[3218]: (reports) CMD (/opt/reports/run.sh)
Sep 17 06:26:01 rocky-01 CROND[3202]: (reports) CMDOUT (/bin/sh: line 1: /opt/reports/run.sh: Permission denied)
Sep 17 06:26:01 rocky-01 CROND[3202]: (reports) CMDEND (/opt/reports/run.sh)
-rw-r--r-- 1 root root 105 Sep 17 06:25 /opt/reports/run.sh

The manual test used bash run.sh, which needs only read permission; cron executes the file, which needs the execute bit.

3. Reproduce the Cron Environment

Waiting a minute per attempt is slow. Running the script as the job's user, from its home directory, with an empty environment and cron's PATH, reproduces the conditions at once. As root:

chmod 755 /opt/reports/run.sh
cd /home/reports && su reports -s /bin/sh -c 'env -i HOME=/home/reports LOGNAME=reports SHELL=/bin/sh PATH=/usr/bin:/bin /opt/reports/run.sh'; echo rc=$?

Output:

/opt/reports/run.sh: line 3: ./report.conf: No such file or directory
/opt/reports/run.sh: line 4: /sales-2026-09-17.csv: Permission denied
rc=1

The next cron run logged the same two lines:

grep reports /var/log/cron | tail -4

Output:

Sep 17 06:27:01 rocky-01 CROND[3428]: (reports) CMD (/opt/reports/run.sh)
Sep 17 06:27:01 rocky-01 CROND[3410]: (reports) CMDOUT (/opt/reports/run.sh: line 3: ./report.conf: No such file or directory)
Sep 17 06:27:01 rocky-01 CROND[3410]: (reports) CMDOUT (/opt/reports/run.sh: line 4: /sales-2026-09-17.csv: Permission denied)
Sep 17 06:27:01 rocky-01 CROND[3410]: (reports) CMDEND (/opt/reports/run.sh)
Line Cause
./report.conf: No such file or directory cron starts in the user's home directory, not in /opt/reports
/sales-2026-09-17.csv: Permission denied REPORT_DIR was never set, so the path became /sales-...; by hand it also came from .bash_profile, which cron does not read

4. Fix and Confirm

sed -i 's|^source ./report.conf|source /opt/reports/report.conf|' /opt/reports/run.sh
printf 'PATH=/usr/local/bin:/usr/bin:/bin\n*/1 * * * * /opt/reports/run.sh >> /var/tmp/reports-cron.log 2>&1\n' > /tmp/reports.cron; crontab -u reports /tmp/reports.cron; crontab -l -u reports
cd /home/reports && su reports -s /bin/sh -c 'env -i HOME=/home/reports LOGNAME=reports SHELL=/bin/sh PATH=/usr/bin:/bin /opt/reports/run.sh'; echo rc=$?
touch /var/tmp/reports-cron.log; chown reports: /var/tmp/reports-cron.log
rm -f /srv/reports/*; sleep 50; grep reports /var/log/cron | tail -2; ls -l /srv/reports; cat /srv/reports/*; cat /var/tmp/reports-cron.log

Output:

Backup of reports's previous crontab saved to /root/.cache/crontab/crontab.reports.bak
PATH=/usr/local/bin:/usr/bin:/bin
*/1 * * * * /opt/reports/run.sh >> /var/tmp/reports-cron.log 2>&1
/opt/reports/run.sh: line 4: report-gen: command not found
rc=127
Sep 17 06:28:01 rocky-01 CROND[3556]: (reports) CMD (/opt/reports/run.sh >> /var/tmp/reports-cron.log 2>&1)
Sep 17 06:28:01 rocky-01 CROND[3540]: (reports) CMDEND (/opt/reports/run.sh >> /var/tmp/reports-cron.log 2>&1)
total 4
-rw-r--r-- 1 reports reports 21 Sep 17 06:28 sales-2026-09-17.csv
orders,2026-09-17,42

With the config path fixed, the manual test exposed the third problem: report-gen lives in /usr/local/bin, outside cron's PATH. The PATH= line in the crontab fixes it for the real run, which produced the report and left the log file empty.

5. Know the Ubuntu Differences

On Ubuntu 24.04, cron discards output when no mail server exists, and a cron.d file without a user field is rejected when cron loads it. As root:

echo '* * * * * echo noisy output' > /etc/cron.d/nouser; echo '* * * * * laborant echo noisy output' > /etc/cron.d/noisy; date; sleep 58; journalctl -u cron --since -70s -o cat --no-pager | grep -v pam_unix

Output:

Thu Sep 17 06:01:54 UTC 2026
Error: bad username; while reading /etc/cron.d/nouser
(*system*nouser) ERROR (Syntax error, this crontab file will be ignored)
(laborant) CMD (echo noisy output)
(CRON) info (No MTA installed, discarding output)

Debian's run-parts and cron also skip any file in cron.daily or cron.d whose name contains a dot, without a log line; see cron and at.

Root Causes

Branch Evidence Fix
Daemon not running systemctl is-active crond (or cron) is inactive; no CMD lines systemctl enable --now crond
Job not installed where expected crontab -l -u <user> empty; job in another user's crontab Install it for the right user
Syntax error bad minute on install; bad command or bad username for cron.d Correct the line; add the user field in cron.d
Ignored file Dot in the name on Debian; not executable in cron.daily Rename; chmod +x; run-parts --test
Not executable CMDOUT (... Permission denied) chmod 755, or call the interpreter explicitly
Relative paths No such file or directory for ./file Absolute paths, or cd "$(dirname "$0")"
Missing variables Empty values from .bashrc or .bash_profile Set them in the script, the crontab or an env file
PATH command not found, exit 127 Full paths, or PATH= in the crontab
% in the command unexpected EOF while looking for matching \%
Access denied You (user) are not allowed to use this program (crontab) cron.allow / cron.deny
Output lost Nothing in logs on Ubuntu Redirect to a file inside the job
Wrong time Server time zone differs from the expected one timedatectl; CRON_TZ= or convert the schedule

Fix

Correct the proven causes only: here the daemon, the execute bit, the config path and PATH. Confirm with the next run's log and the output file, not with another manual run.

Prevention

  • Write cron scripts as if the environment were empty: absolute paths, set -euo pipefail, variables defined in the script.
  • Redirect every job's output to a log file, or use a systemd timer so output lands in the journal.
  • Test with env -i as the job's user before scheduling.
  • Monitor the job's result with a heartbeat or a file age check; a running daemon proves nothing about the job.
  • Keep job names in /etc/cron.d and cron.daily free of dots.

Captured on Rocky Linux 10.2 and Ubuntu 24.04.4 LTS (iximiuz Labs microVMs, kernel 6.1.167), 2026-09.