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A failed-cron email is evidence that a scheduled command exited with an error or produced output—not proof that the scheduler itself is broken. Preserve the complete message, identify the exact command, user, host, schedule and timestamp, then reproduce that command under the same account and environment. The workflow below separates command failures, logging problems, mail-delivery issues and duplicate schedulers so you can fix the first causal error and verify the next run.
1. Preserve the email and classify the failure
Do not delete or trim the notification. Save the complete message, including headers and every line of standard output and standard error. The first error line is often the most useful clue; later lines may only be consequences.
- From and To: These show which local mail system sent the message and which address was selected.
- Host: Confirm the machine that actually ran the job. A similarly named staging or production host can make a correct diagnosis look wrong.
- Timestamp and time zone: Compare the time with the server’s clock and the schedule. A clock or time-zone mismatch can make a healthy run appear to have been missed.
- Schedule: Record the minute, hour, day-of-month, month and day-of-week fields, or the timer expression shown in the message.
- Command and user: Copy the exact job line and identify the account that owns the crontab or service.
- Exit wording and output: Note an exit status, “command not found,” permission error, timeout, missing file, authentication failure or other first symptom.
A classic cron daemon normally mails output from a command to the crontab owner or to the address in MAILTO. Systemd-cron sends a failure notification containing status information and journal output; the Debian crontab(5) documentation says the default message includes systemctl status and the full failed-run log copied from the journal. Treat those formats differently when choosing where to investigate.
2. Find which scheduler owns the job
Before editing anything, establish whether the command is defined once or several times. A job can exist in a user’s crontab, a system-wide file, anacron, a native systemd timer or a generated systemd-cron unit.
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Inspect cron definitions
crontab -l
sudo crontab -l -u USER
sudo cat /etc/crontab
sudo ls -la /etc/cron.d
sudo grep -R "distinct-command-fragment" /etc/cron.d /etc/crontab 2>/dev/nullReplace USER with the account shown in the email and use a distinctive part of the command when searching. Check anacron configuration as well if the machine uses it; anacron jobs are another source of scheduled execution outside a user’s personal crontab.
Inspect systemd timers and generated units
systemctl list-timers --all
systemctl status NAME.timer
systemctl cat NAME.timer
systemctl cat NAME.serviceUse the actual timer and service names discovered on the host. Systemd-cron generates units from cron entries, while native systemd timers have their own .timer and .service files. Native systemd timers take precedence over cron jobs with the same name according to the systemd-cron documentation. Do not assume that seeing a line in crontab -l means that line is the only owner.
Look for duplicate daemons
Running a cron daemon together with systemd-cron can execute the same scripts more than once. The systemd-cron documentation explicitly warns against operating it alongside a cron daemon. If a side effect occurred twice, compare timestamps in the logs and inspect enabled services before changing the command itself.
3. Reproduce the command as the cron user
The most common reason a job works in an interactive shell but fails on the server is that cron starts it with a different environment. A login shell may supply a long PATH, a home directory, locale settings, mounted credentials and a working directory that cron does not.
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Use the owning identity
sudo -u USER -H /bin/sh -c 'cd /absolute/working/directory && /absolute/path/to/command arg1 arg2'Use the account from the job definition, not your own account. The -H option gives that user the expected home directory; the explicit cd removes ambiguity about relative paths. If the job runs as root, reproduce it only with the minimum necessary privileges and take the usual care with destructive commands.
Make the environment deliberate
Use absolute paths for executables, scripts, configuration files and output files. Set a known PATH in the crontab or wrapper script instead of relying on an interactive shell:
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MAILTO=admin@example.net
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- Working directory: A relative input or output path may resolve under a different directory, or fail because the directory does not exist.
- Permissions and ownership: Verify read, write and execute permission on the script, parent directories, sockets and destination storage.
- Credentials: Cron may not load an interactive key agent, cloud credential helper, password store or shell profile. Point the command at an explicit, restricted configuration file where appropriate.
- Mounts and network: A remote filesystem, database or API may not be ready when the schedule fires. Check reachability and mount state from the cron identity.
- Locale and encoding: Parsing can change when
LANG,LC_ALLor character encoding differs from your terminal. - Configuration files: A script that works after sourcing a profile may fail because cron never reads that profile. Make required settings explicit rather than sourcing a broad, interactive configuration.
Capture the environment during a controlled test if the difference is unclear:
sudo -u USER -H /usr/bin/env | sortCompare that output with the environment from the successful interactive run, then add only the variables the job actually requires.
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4. Read the authoritative log
Systemd-cron jobs
Start with the generated unit named by the notification or discovered with systemctl list-units:
systemctl status GENERATED.service
journalctl -u GENERATED.service --since "2026-10-15 00:00" --until "2026-10-15 23:59"
journalctl -u GENERATED.service -n 200 --no-pagerGenerated units record the original job line in the journal with CRON_JOB=. Filter for that field when several jobs write to the same service or when the status output is abbreviated:
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Classic cron jobs
Classic cron usually records start and execution messages through the system’s syslog destination. Depending on the distribution, inspect the configured syslog file or the journal for the cron daemon:
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journalctl -u cron --since "1 hour ago"
journalctl -u crond --since "1 hour ago"
Use the daemon name that exists on the host. If the crontab redirects output, inspect that file directly. A typical diagnostic line is:
15 2 * * * /usr/local/sbin/backup-job >>/var/log/backup-job.log 2>&1Keep this redirection while validating the repair. Check file ownership, rotation and available disk space; an unwritable log can hide the original command output.
5. Understand and test cron mail controls
MAILTO controls who receives cron mail. In classic cron, command output is mailed to the crontab owner or the address in MAILTO. In systemd-cron, the manual states that mail is always sent for failed jobs, with MAILTO selecting the recipient. An empty assignment such as MAILTO="" suppresses mail.
Check the effective setting in the relevant crontab and any system-wide file:
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crontab -l | sed -n '1,20p'
sudo grep -R '^MAILTO' /etc/crontab /etc/cron.d 2>/dev/nullDo not silence a noisy job by setting an empty MAILTO before it is healthy. You may hide a genuine failure and lose the only evidence of a regression.
When the job fails but no email arrives
- Confirm the job actually ran by checking the cron or systemd log.
- Verify that
MAILTOis not empty and contains the intended address. - Check that a local sendmail-compatible transport is installed and accepting messages.
- Inspect mail-system logs for queueing, rejection, DNS, authentication or delivery-delay errors.
- Compare the envelope recipient with the visible
Toheader; aliases can redirect mail.
Mail delivery is a separate failure path from command execution. A successful local handoff does not prove that the message reached an external mailbox, so keep log-based monitoring for jobs whose failure must page someone.
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6. Fix the first causal error
Work from the earliest concrete failure, not the final cascade. For example, correct a missing executable before changing a later “file not found” symptom, and repair a permission problem before increasing retries.
- “command not found”: Replace the bare command with its absolute path or set an explicit
PATH; confirm the file is executable. - “No such file or directory”: Check the working directory, spelling, mount availability and interpreter path in the script’s shebang.
- Permission denied: Verify account ownership and directory traversal permissions. Do not make a file world-writable as a shortcut.
- Authentication or secret errors: Provide the intended credential source to the cron identity and restrict its permissions.
- Network or service-not-ready errors: Confirm dependency readiness at the scheduled time. Add bounded retries only when repeating the operation is safe.
- Unexpected duplicate effects: Find and disable the extra cron, anacron, native timer or cron daemon before rerunning a non-idempotent command.
- Incorrect schedule: Correct the expression after checking server time zone and daylight-saving behavior; document the intended time zone where the implementation supports it.
Keep output capture enabled until the command succeeds in the real scheduler context. Remove sensitive values from logs, but do not discard diagnostics that are needed to prove the repair.
7. Verify the repair in two stages
- Run once manually as the cron identity. Use the exact command, directory and environment that the scheduler will use. Record the exit status:
sudo -u USER -H /bin/sh -c 'cd /absolute/working/directory && /absolute/path/to/command'; echo "exit=$?"- Check the intended side effect. Confirm the backup artifact, report, database change, queue message or other expected result—not merely a zero exit code.
- Trigger or wait for one scheduled run. For a systemd timer, use the unit’s normal test procedure or wait for its next elapse; for classic cron, wait for the next matching minute. Avoid creating an ad-hoc second schedule that could be mistaken for the production job.
- Compare logs and notification behavior. Confirm one execution, the expected output, a successful status and the absence of an unexpected failure email. For systemd-cron, inspect the generated service journal; for classic cron, inspect syslog and the explicit log file.
8. Prevent the next failure
Once the job is healthy, make its assumptions visible and its failure observable.
- Use a wrapper script with strict, reviewed paths and a deliberate environment.
- Write structured, timestamped logs to a location with rotation and sufficient disk space.
- Make operations idempotent where possible so a retry or accidental duplicate does not corrupt data.
- Use bounded retries for transient dependencies, with delays that fit the job’s deadline.
- Document the owning user, schedule, time zone, input paths, credentials and expected side effect.
- Keep local mail enabled during validation and test the sendmail-compatible transport separately.
- Add external cron job monitoring or external failure alerts when a missed run must be detected even if the server, scheduler or local mail system is unavailable.
Classic cron and systemd-cron: what changes during diagnosis?
| Diagnostic aspect | Classic cron | systemd-cron |
|---|---|---|
| Where output is logged | Daemon syslog destination, plus any redirection in the job line | Journal for the generated service; output is associated with the generated unit |
| Failure-mail format | Command output is mailed to the crontab owner or MAILTO recipient |
Failed-job mail includes systemctl status and the full failed-run journal output by default |
| How to identify a run | Crontab entry, daemon log and the command’s explicit log file | Generated service or timer; journal entries include CRON_JOB= with the job line |
| Timer precedence | Runs according to the cron daemon’s installed crontab | Native systemd timers override cron jobs with the same name |
| Duplicate-execution risk | Can occur if another scheduler defines the same command | Running systemd-cron alongside a cron daemon can execute scripts multiple times |
The table describes the documented behavior of these implementations. Other cron variants can differ, so use the commands and unit names present on your server as the authority.
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