A failed-cron email is evidence that one scheduled run produced an error, not proof that the scheduler itself is broken. Preserve the complete message, identify the exact command, user, host, schedule, timestamp and exit information, then reproduce that command as the cron user. Most failures come from differences between an interactive shell and cron’s sparse environment: paths, working directories, permissions, credentials, locale, configuration files, mounts or network readiness.
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The workflow below separates four questions: what failed, where the authoritative output is logged, which scheduler owns the job, and whether your repair worked on the next scheduled run.
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1. Preserve the email and classify the failure
Do not delete or trim the notification. Save the complete raw message, including headers and every line of command output. Before changing the job, record:
- the
FromandToaddresses; - the server hostname and, if shown, the container or virtual-machine name;
- the timestamp and timezone;
- the schedule expression and the account that owns the job;
- the exact command, including redirections and wrappers;
- the exit status or failure wording; and
- all standard output and standard error, especially the first error line.
The first error is often the causal one. Later lines may only describe cleanup failures or a non-zero exit propagated from an earlier command. Preserve the message even if it contains sensitive paths; redact passwords, tokens and private data before forwarding it to anyone else.
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Separate a job failure from a mail-delivery problem
A message arriving proves that some mail path worked. It does not prove that every run generated a message, nor that the job’s own output reached the intended mailbox. If a run failed without an email, investigate MAILTO, the local sendmail-compatible transport and mail logs separately from the command itself.
2. Find every place the job could be scheduled
Duplicate definitions are a common explanation for apparently contradictory emails, overlapping runs or a command that seems to execute twice. Check all scheduler sources before editing one entry.
- List the owning user’s crontab with
crontab -l. For another account, inspect it with appropriate administrative privileges, for examplesudo crontab -u alice -l. - Read the system crontab,
/etc/crontab, and enumerate files in/etc/cron.d. These files can run the same command under a different user. - Check anacron configuration if the machine uses it. Anacron entries can launch scripts that look unrelated to the line you first found.
- List native systemd timers with
systemctl list-timers --all. Inspect enabled and inactive timers, not only timers that are currently running. - Search the cron files and unit definitions for the script name, destination path or distinctive arguments. Compare schedules, users and wrappers, not just the visible command.
Native systemd timers take precedence over cron jobs with the same name. Also, running a cron daemon alongside systemd-cron can execute a script more than once. Establish which mechanism owns the job before you make a repair.
3. Reproduce the command as the cron user
Running a command successfully in your login shell is not a valid cron test. Cron normally starts with a limited environment, a different current directory and no interactive shell profile. Reproduce those conditions deliberately.
Use the same identity and working directory
First run the command as the account shown in the crontab, from the directory the script expects:
sudo -u alice sh -c 'cd /srv/app && exec /usr/local/bin/nightly-job'If the crontab calls a shell script with arguments, copy the complete command exactly. Keep wrappers such as flock, nice, timeout or a shell pipeline; removing one can hide the actual failure.
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Simulate cron’s sparse environment
Make the executable search path explicit instead of relying on an interactive profile:
sudo -u alice env -i HOME=/home/alice PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin /bin/sh -c 'cd /srv/app && exec /usr/local/bin/nightly-job'Add only the variables the job intentionally requires. If the program needs an API endpoint, credentials supplied by a protected file, a locale or a configuration path, set or reference those explicitly. Never solve a missing variable by placing a secret directly in a crontab or an email-producing command line.
Check the differences that make shell runs misleading
- PATH and executable names: use absolute paths for interpreters and utilities, or set a deliberate
PATH. - Current directory: replace relative file names with absolute paths, or add an explicit
cdbefore execution. - Permissions: verify that the cron account can read configuration and input files, write the destination, traverse every parent directory and execute the interpreter.
- Credentials: confirm that the account can access its key, certificate, secret store or database socket without an interactive prompt.
- Locale: scripts that parse dates, decimal separators or command output may behave differently when locale variables are absent.
- Mounts and network readiness: a scheduled run can start before a remote mount, DNS service, VPN or dependent application is ready.
- Configuration loading: login-shell files are not automatically read. Point the program at the intended configuration file instead of assuming
.profileor.bashrcwas sourced.
When the reproduction fails, fix the first concrete error and repeat it under the same identity. Do not switch to root merely to make the symptom disappear; that changes permissions and can create a job that will fail again under its real account.
4. Read the authoritative log for the scheduler you are using
The email is a useful starting point, but the scheduler’s own log normally has the complete execution context. The commands differ for systemd-cron and classic cron.
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Systemd-cron creates generated service and timer units. Use the generated unit name shown by your system’s status output:
systemctl status <generated-service-or-timer>
journalctl -u <generated-service> --since '2026-10-15 00:00:00'The generated unit records the job line in the journal with CRON_JOB=. The failure email is designed to include the equivalent of systemctl status plus the full log for the failed run copied from the journal. Failed jobs generate mail by default.
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Use the timestamp from the email to narrow the journal query. If several runs overlap, compare each run’s start and end times and inspect the unit’s exit status rather than relying on a subject line.
Classic cron
Classic cron mails command output to the crontab owner or to the address in MAILTO. The daemon’s start, stop and execution records go to the system’s configured syslog destination; depending on the distribution, that may be a cron log file or the system journal. Check the logging configuration for the host instead of assuming a single path.
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15 * * * * /usr/local/bin/nightly-job >>/var/log/nightly-job.log 2>&1puts both streams in the specified file, so the mail may contain little or nothing. Read that file with the same timestamp window and verify that its owner and permissions allow the cron account to write. Keep explicit redirection while diagnosing, because it gives you a durable record for the next controlled run.
5. Verify mail controls before muting notifications
Understand MAILTO
MAILTO selects the recipient for cron mail. In a classic crontab, command output is mailed to the job owner unless MAILTO names another address. In systemd-cron, the setting directs failure mail in the generated unit. An explicitly empty value, MAILTO="", suppresses mail.
Check the variable in the relevant crontab, system crontab or generated configuration. A user crontab can differ from /etc/crontab, and a line placed after the job may not affect earlier entries in implementations that process variables sequentially.
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Test the delivery path without hiding errors
If the job fails and no message arrives, inspect the local sendmail-compatible transport and its mail logs. Look for queueing, rejection, DNS errors or an invalid recipient. A delayed message is a delivery issue; it does not change the command’s exit status.
Do not set MAILTO="" simply to stop noisy alerts. Keep output capture and notifications enabled until the command has passed a controlled run and one scheduled run. Suppression can make a recurring failure invisible.
6. Compare classic cron, systemd-cron and native timers
The scheduler determines where you look for evidence and how duplicate ownership can occur.
| Area | Classic cron | Systemd-cron | Native systemd timer |
|---|---|---|---|
| Where output is logged | The daemon’s syslog destination, plus any file explicitly named with redirection. | The journal for the generated service; entries identify the job line with CRON_JOB=. |
The journal for the native service unit. |
| Failure mail format | Command output is mailed to the crontab owner or the address in MAILTO. |
Failure mail is sent by default and includes status information and the full failed-run journal output. | Mail behavior depends on the timer’s service and notification configuration. |
| How schedules are discovered | User crontabs, /etc/crontab, /etc/cron.d and anacron configuration. |
Generated service and timer units derived from cron entries; inspect them with systemctl and journalctl. | Timer unit files listed by systemctl list-timers --all. |
| Timer precedence | A cron entry with the same name can be overridden by a native systemd timer. | Do not assume a generated unit is the only owner; check for a native timer with the same name. | Native timers take precedence over cron jobs with the same name. |
| Duplicate-execution risk | Running the cron daemon while systemd-cron also handles the entries can run scripts multiple times. | Documentation warns against running it together with a cron daemon for this reason. | A second scheduler or duplicate timer can independently start the same service. |
The table describes ownership and evidence paths, not a universal missed-run policy. Persistence after downtime depends on the specific timer implementation and its settings; verify that behavior in the unit you actually run.
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7. Fix the first causal error
Apply the smallest change that addresses the earliest reproducible failure:
- replace an unresolved executable with an absolute path or a deliberate
PATH; - add the required
cdand convert fragile relative paths to absolute ones; - grant the cron account only the file, directory or device permissions it needs;
- make configuration and credential files readable by that account without exposing secrets in command output;
- wait for a dependency that may not be ready at boot, or move the work to a scheduler that can express the dependency;
- correct the schedule or remove the duplicate definition after confirming which scheduler owns it; and
- retain logging and error output while validating the change.
Avoid changing several unrelated variables at once. If you alter the command, user, schedule and mail settings together, a later success will not tell you which change solved the problem.
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8. Verify the repair in two stages
Run once under the cron identity
Execute the corrected command manually with the same user, directory and environment used during diagnosis. Confirm a zero exit status and inspect the intended side effect: a file created, a backup updated, a report delivered or a record changed. A zero exit code without the expected side effect is not a successful repair.
Observe one scheduled execution
Trigger or wait for the next scheduled run. Record its start time, unit or daemon log entry, exit status, output file and notification behavior. For systemd-cron, compare systemctl status and journalctl output with the manual run. For classic cron, check the daemon’s syslog destination and any explicit redirection.
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9. Prevent the next failure from becoming a mystery
Make execution observable
Keep a stable log destination or journal query and include timestamps in the job’s own messages. Capture standard error during rollout instead of discarding it. Make the command line self-contained enough that an operator can identify the account, input and destination from the scheduler record.
Make retries safe
Use bounded retries only when the operation can be repeated safely. Design the job to be idempotent: rerunning it should not duplicate records, send the same notification repeatedly or corrupt a partially written file. A retry cannot compensate for an incorrect path or missing permission.
Monitor failures outside the server
For jobs whose failure must page someone, add external cron job monitoring or external failure alerts after local logging and mail are working. An outside monitor can detect a missing heartbeat when the host is down or local mail is unavailable. Keep the local journal and notification path anyway; external monitoring should add an independent signal, not replace the evidence needed to repair the command.
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