A failed cron email is a starting point for diagnosis, not proof that cron itself is broken. Preserve the complete message, identify the scheduler and user that ran the command, reproduce it with that same identity and environment, then confirm the fix with a controlled run and the next scheduled execution.
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Start with the email, not with a restart
Keep the original message intact. Save the complete headers and body before deleting or trimming anything. The first error line is often more useful than the final “job failed” wording.
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- Sender and recipient: record the
FromandToaddresses, including anyMAILTOdestination. - Host and time: note the server name, timestamp, timezone if shown, and whether the time matches the expected schedule.
- Schedule and command: copy the cron expression and the entire command exactly, including redirections, environment assignments and quoted arguments.
- Identity: establish which account owns the crontab or generated service.
- Exit context: retain the exit status, “failed” wording, and every line of standard output and standard error.
Different schedulers format notifications differently. Classic cron generally mails command output to the crontab owner or to the address in MAILTO. The Debian crontab(5) documentation for systemd-cron says failed jobs always generate mail and that the default message contains systemctl status output plus the full failed-run log copied from the journal. Treat either message as evidence about one invocation. A notification does not by itself show that the scheduler daemon is unhealthy.
Identify which scheduler actually ran the job
Before changing the command, find every place where it could be defined. A job can exist in more than one configuration file, or a native systemd timer can take precedence over a cron entry with the same name.
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- List the current user’s entries with
crontab -l. If the job belongs to another account, inspect it as an administrator withsudo crontab -u account -l. - Check system-wide definitions in
/etc/crontaband files under/etc/cron.d/. These files include an explicit user field, so do not assume they run as root. - Inspect anacron configuration if the task is intended to run periodically when the machine is available. Anacron is a separate scheduler and can create an additional invocation.
- List systemd timers with
systemctl list-timers --all. Then inspect a suspected unit usingsystemctl status unit-name.timerand its associated service. - Search for the command, script name and distinctive arguments across these locations. Duplicate definitions should be removed or deliberately separated.
Systemd-cron generates units from cron entries. Its documentation warns that native systemd timers override cron jobs with the same name. It also warns against running systemd-cron together with a cron daemon, because the same scripts may execute more than once. If two notifications have nearly identical timestamps, check ownership and duplicate definitions before debugging application code.
Reproduce the command as the cron user
An interactive shell supplies assumptions that cron does not: a different PATH, working directory, locale, credentials, mounts and configuration files. Reproduction is useful only when those conditions match the scheduled invocation.
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Use the same account and directory
Run the command as the owning account, not as your administrator account. For example, change to the intended directory and invoke the same executable with an explicit environment:
sudo -u cronaccount -- sh -c 'cd /absolute/working/directory && PATH=/usr/local/bin:/usr/bin:/bin /absolute/path/to/script --same-arguments'Use the account’s real home directory and any required service account settings. If the original command uses a relative path, convert it to an absolute path for testing and, usually, for the crontab itself.
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Make the environment deliberate
- PATH: cron may not include the directory where a language runtime or utility was installed. Call the executable by absolute path or set a known
PATHat the top of the crontab. - Configuration: specify configuration files explicitly instead of relying on shell startup files such as
.profileor.bashrc. - Credentials: verify that the cron account can read key files, tokens and password stores. A file readable by you may be inaccessible to the scheduled account.
- Permissions: check execute permission on every directory in the path, read/write permission on output locations, and ownership of generated files.
- Locale and shell: set the required locale and invoke the intended shell when parsing depends on shell behavior or character encoding.
- Mounts and network: confirm that remote filesystems, sockets, databases and network routes are available at the scheduled time.
Capture the exit status during reproduction. A command that prints a warning but exits zero is different from one that terminates nonzero. Conversely, a wrapper can return success while the underlying task failed, so verify the actual side effect as well as the shell status.
Find the authoritative log
When systemd-cron is involved
Use the generated service or timer name shown by systemctl list-timers or in the notification. Run systemctl status unit-name.service for the unit summary and journalctl -u unit-name.service for the complete run history. Generated units record the original cron line in the journal with CRON_JOB=, which helps distinguish similar commands.
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When classic cron is involved
Classic cron normally records daemon activity through the system’s syslog destination. The exact file depends on the operating system and logging configuration, so inspect the cron facility in your syslog or journald configuration. Also inspect any redirection embedded in the job, such as:
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/absolute/path/to/script >> /var/log/task.log 2>&1That redirection is often the most direct record of application output. Keep it while diagnosing; removing it can make a repaired job appear silent while errors disappear into an unavailable mail path.
Check MAILTO and mail delivery
MAILTO controls where cron sends output or failure notifications. In a classic crontab, output is mailed to the crontab owner unless MAILTO names another recipient. In systemd-cron, the Debian manual likewise documents MAILTO as the destination for job mail and says an empty value suppresses mail. Do not set MAILTO="" merely to stop alerts while the command is unverified; that hides evidence rather than fixing the cause.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchIf a healthy run should produce no output, make that decision after validation and document it. If mail is delayed or absent, verify the server’s sendmail-compatible transport and inspect its mail logs. A successful command and a delivered notification are separate outcomes: test both.
Fix the first causal error
Read the log from the top and correct the earliest actionable failure. Later messages are often consequences of the first one.
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Executable or path errors
“Command not found,” an interpreter error, or a missing script usually means the scheduled PATH differs from your login shell or the file is not at the assumed location. Use an absolute executable path, set PATH explicitly, and confirm the interpreter’s execute permission.
Permission and ownership errors
“Permission denied” can apply to the script, a parent directory, an output file, a private key or a mounted filesystem. Test access as the cron account and correct ownership or group membership rather than making sensitive files world-readable.
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Applications often load settings from a login shell, a home directory or a relative path. Pass the configuration path explicitly, provide required environment variables in a controlled way, and ensure secrets are available to the service account without exposing them in the crontab or mail body.
Dependency readiness
A job can start before a network mount, database, container, VPN or remote endpoint is ready. Confirm the dependency at the actual schedule time. Where a transient dependency is expected, use bounded retries with logging rather than an unbounded loop that hides a permanent failure.
Schedule and concurrency mistakes
Check timezone assumptions, daylight-saving transitions and overlapping runs. If a previous invocation can still be running when the next one starts, make the task idempotent and add an appropriate lock or other single-instance control. Remove duplicate cron, anacron and systemd definitions unless multiple executions are intentional.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Verify the repair in two stages
- Controlled run: execute the command once as the cron account, from the intended directory, with the same explicit environment. Confirm a zero exit status and the real side effect: a file changed, a record processed or a report delivered.
- Scheduled run: trigger the scheduler through its normal path or wait for the next occurrence. Compare its status, journal/syslog output, redirection file and notification behavior with the controlled run.
Keep diagnostic logging enabled until the scheduled execution succeeds. Then decide which output should remain permanently: useful application logs, a bounded retention policy and an alert for nonzero exits. Do not remove all capture simply because one run passed.
Prevent the next silent failure
- Use absolute paths and an explicit
PATHin every production entry. - Record the working directory, account, configuration locations and expected side effect alongside the job definition.
- Make reruns safe through idempotent operations, unique work markers or a documented lock.
- Use bounded retries only for dependencies that can recover, and log each attempt with a final failure status.
- Keep local logs and mail enabled for the validation period and test the sendmail-compatible transport.
- For jobs whose failure must wake someone up, add external cron job monitoring or external failure alerts. An outside monitor can still report a missed heartbeat when the server’s local mail service is unavailable.
Classic cron, systemd-cron and native systemd timers
| Aspect | Classic cron | systemd-cron or native systemd |
|---|---|---|
| Where output is found | Daemon syslog destination and any explicit redirection in the job | Journal entries for the generated or native service; systemd-cron records CRON_JOB= |
| Failure mail | Command output is mailed to the crontab owner or the address in MAILTO |
systemd-cron sends mail for failed jobs; its default message includes systemctl status and the full failed-run journal output |
| How schedules are discovered | User crontabs, /etc/crontab and files in /etc/cron.d/ |
Generated units or native timer units, discoverable with systemctl list-timers --all |
| Precedence | A cron entry is handled by the cron implementation that owns it | Native systemd timers override cron jobs with the same name |
| Duplicate-execution risk | Can overlap with another scheduler defining the same command | Running systemd-cron alongside a cron daemon can execute scripts multiple times |
The practical distinction is ownership. Once you know which scheduler created the invocation, use that scheduler’s status and logs, reproduce under its account, and remove competing definitions before declaring the incident resolved.
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