If a cron job appears not to run, first find out whether the scheduler skipped it or whether the command launched and failed. Check the installed crontab and schedule, confirm the correct daemon is active, then inspect the job’s account, environment, and captured output. A terminal command working interactively does not prove it will work under cron.
1. Confirm the right crontab contains the job
Start by listing the crontab for the account that should run the job. On many systems, crontab -l displays the current user’s table; to inspect another user’s table, use the platform’s documented administrative method. Make sure you edited the same account’s table you are checking.
System-wide entries may instead live in /etc/crontab or /etc/cron.d/. The format differs: system crontabs include a username field after the time fields, while a personal crontab does not. Putting a username in a personal table, or leaving it out of a system entry, can make the fields parse incorrectly. See the Cronie crontab manual and the Debian systemd-cron manual; file locations and details depend on the implementation.
Check when the schedule actually matches
A standard crontab entry has five time and date fields followed by the command. Cronie checks entries every minute, but that does not mean a particular entry runs every minute: its fields must match. Work out the next matching minute, hour, day, and month, then compare it with the host’s clock and effective time zone. The Cronie manual notes that daylight-saving changes can skip a nonexistent local time or repeat a time; behavior can vary across cron implementations.
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2. Check the scheduler and its logs
Identify which scheduler the host uses before checking service status. Cronie commonly uses a daemon named crond; other systems may call it cron or use a compatibility implementation. Check that implementation’s service status and logs using the commands and log locations documented for your operating system. The cron daemon manual describes Cronie behavior, not a universal service name or logging setup.
Look for evidence that the scheduler attempted the entry. No attempt in the relevant logs points you back to the daemon, installed table, schedule, clock, or time zone. An attempt in the logs means the next step is to inspect what the command did.
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3. Verify the execution account and permissions
Confirm which user should run the job. Entries in /etc/crontab and /etc/cron.d/ have a username field; a personal crontab runs as its owner. Then check access for that account along the entire path:
- The account can read the script and traverse its parent directories.
- The interpreter and any programs the script calls are accessible and executable.
- Input, output, configuration, and log paths are reachable and writable where needed.
Cronie also documents restrictions on crontab-file modes: its crontab files must be regular files or symlinks to regular files and generally must not be executable or writable by other users. This applies to the crontab file, not automatically to the script it runs. Do not change permissions broadly as a diagnostic shortcut.
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4. Recreate cron’s environment
Cron does not necessarily inherit the environment of your interactive terminal. POSIX specifies at least HOME, LOGNAME, PATH, and SHELL for a cron command, with sh as the specified shell. Startup files and variables available in a terminal session may be absent. Use absolute paths for scripts and executables, and define only the environment values the job needs. See the POSIX crontab environment requirements.
To expose hidden assumptions, try the command as the scheduled account with a deliberately small environment, rather than relying on your own shell setup. For example, verify that required variables, configuration files, and working-directory assumptions are actually available to that account.
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One implementation-specific trap: Debian’s documented systemd-cron reads environment assignments from top to bottom and does not expand one assignment inside another. In that implementation, PATH=$HOME/bin:$PATH does not behave like the equivalent assignment evaluated by a shell. Check the host’s manual before applying that behavior elsewhere.
5. Check shell syntax and crontab parsing
Cronie runs the command portion with /bin/sh unless SHELL selects another shell. If a command or script depends on Bash-only syntax, invoke Bash explicitly or configure the shell appropriately. A script’s shebang and the shell used to interpret the crontab command are separate details to check.
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6. Capture output and inspect logs
During diagnosis, redirect both standard output and standard error to a persistent, absolute log path that the scheduled account can write. This makes command-level errors visible instead of leaving you to infer what happened from the absence of an expected result.
Without redirection, POSIX says generated output and errors are mailed by an implementation-defined method. Cronie documents mail to the crontab owner or a MAILTO recipient, as well as syslog options. Debian systemd-cron documents different failure-mail behavior, including status and journal output. Mail is not guaranteed to arrive: local mail delivery may not be configured. Check the scheduler’s system log or journal and the output destination you explicitly set.
7. If the command launched, troubleshoot the program
A timestamped output log, journal entry, or mail showing that the command started changes the diagnosis: the scheduler may be working, while the program or one of its dependencies is failing. Follow the actual error and check for:
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- Relative paths or assumptions about the current working directory.
- Missing secrets, configuration, input files, mounts, or network resources.
- Locks or other conditions that prevent the task from proceeding.
- A different runtime version, unavailable dependency, or nonzero application exit status.
Fast triage by symptom
| What you find | Where to look next |
|---|---|
| No scheduler log or other launch evidence | Correct daemon, installed crontab, schedule, host clock, and time zone. |
| Scheduler reports an attempt, but no command output is visible | Explicit stdout and stderr redirection, mail configuration, syslog, or journal. |
| Command reports “not found” | PATH, absolute executable paths, required environment, and shell. |
| “Permission denied” or a missing-file error | Execution identity and access to the script and every path component it needs. |
| Application-specific error appears | The program’s inputs, configuration, runtime, and dependencies. |
When cron itself may not be the right scheduler
Some hosts also offer systemd timers or a cron-to-systemd compatibility layer. The useful comparison depends on the specific implementation: where schedules are expressed, how missed runs after downtime are handled, what logs and failure notifications are available, which time-zone rules apply, and which service owns the job. Debian’s systemd-cron documentation describes its translation and persistence options, but those details do not establish a universal comparison with every timer implementation. Choose based on the behavior the job needs, not simply because one run failed.
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