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Use the PHP command-line interface with absolute paths, redirect both output streams to a log, and add a lock so a slow minute cannot overlap the next one. The steps below implement six starts approximately 10 seconds apart, then show how to adapt the design for other intervals and workloads.
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What cron can and cannot schedule
A traditional cron expression has five scheduling fields: minute, hour, day of month, month, and day of week. Ubuntu’s cron documentation states that cron(8) examines entries every minute. The minute field therefore describes which minute marks should trigger a command; it does not provide a seconds field or a sub-minute repeat operator.
| Expression | What it does | What it does not do |
|---|---|---|
* * * * * |
Starts the command once during every minute. | It does not start the command every few seconds. |
*/10 * * * * |
Starts at minutes 0, 10, 20, 30, 40, and 50 of each hour. | It does not create six starts inside each minute. |
0/35 * * * * |
Starts at minute 0 and minute 35 of every hour. | It does not mean “every 35 minutes” across hour boundaries. |
The */10 form is useful when a whole-minute cadence is enough. Ubuntu specifically cautions that 0/35 means two executions per hour, not a continuously advancing 35-minute interval. For sub-minute work, the cron entry should be the once-per-minute trigger and another process should control the repeats.
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Choose the scheduling architecture
There are three practical designs. The right one depends on timing tolerance, failure handling, and how long the PHP process remains alive.
| Design | Cadence | Advantages | Costs and risks |
|---|---|---|---|
| Cron only | Whole-minute marks | Minimal setup and very low operational overhead. | Cannot express several launches per minute. |
| Once-per-minute cron plus shell wrapper | Approximate sub-minute spacing, controlled by sleep |
Simple, transparent, and works with the PHP CLI. | Each launch pays PHP startup cost; timing drifts when the job is slow; a lock is needed to prevent overlapping wrappers. |
| Persistent worker or supervised service | Can maintain its own loop and state | Better lifecycle control, graceful shutdown, retries, and lower interpreter startup overhead. | More deployment and monitoring work than cron. |
A wrapper is a good fit for lightweight work such as polling a game-service API, expiring records, or processing a small queue. If the task requires strict sub-minute timing, durable state through restarts, high throughput, or coordinated retries, use a long-running worker or another supervised service instead of stretching cron beyond its resolution.
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Run these checks as the same Unix user that will own the cron entry. An interactive shell may use a different PHP binary, configuration file, home directory, or environment than cron.
-
Find the executable:
command -v php -
Check its version and confirm that it starts:
php -v -
Inspect the configuration files loaded by the CLI, if your application depends on extensions or a specific
php.ini:php --ini
Record the absolute path returned by command -v php. The examples use /usr/bin/php; replace it if your Ubuntu installation places PHP elsewhere. The PHP manual describes the CLI SAPI as the interface intended for shell applications and demonstrates both php my_script.php and php -f my_script.php. A cron job should call that CLI binary directly, not a web URL or an HTTP request to the application.
Build a wrapper for repeated starts
Create a stable executable such as /usr/local/bin/run-php-job.sh. This example starts the job six times with a 10-second sleep between starts. Six is a workload choice: it is the number of nominal 10-second slots in a minute, not a value supplied by cron.
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PHP='/usr/bin/php'
SCRIPT='/srv/app/job.php'
INTERVAL=10
RUNS=6
cd '/srv/app' || exit 1
i=1
while [ "$i" -le "$RUNS" ]; do
"$PHP" "$SCRIPT"
status=$?
if [ "$status" -ne 0 ]; then
printf '%s job failed with exit %sn' "$(date -Is)" "$status" >&2
fi
i=$((i + 1))
if [ "$i" -le "$RUNS" ]; then
sleep "$INTERVAL"
fi
done
exit 0Install it with an owner and mode that allow the cron user to execute it:
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sudo install -o appuser -g appuser -m 0755 /tmp/run-php-job.sh /usr/local/bin/run-php-job.shReplace appuser, the temporary source path, and the application path with your values. Use absolute paths for the PHP binary, script, and any files the script opens. The cd is intentional: cron does not promise the working directory you get in an interactive shell.
How the interval behaves
The wrapper starts the first process immediately, waits 10 seconds after it exits, and then starts the next one. Consequently, the interval between starts is the PHP runtime plus the sleep, not always exactly 10 seconds. If each run takes two seconds, starts occur roughly 12 seconds apart. If a run takes longer than the chosen interval, the wrapper cannot create a true fixed-rate schedule without allowing processes to overlap.
For a different approximate cadence, choose a run count and sleep value explicitly. For example, three starts with a 20-second sleep are represented by RUNS=3 and INTERVAL=20. Decide whether the final invocation may extend into the next minute; a fixed-count loop does not impose a one-minute deadline.
Stop the loop at a minute window when that matters
If the wrapper must stop launching new work near the end of its minute, replace the fixed run count with a deadline-based loop. This prevents a new start after the selected window, although a PHP process that is already running will continue unless you add an application-level timeout.
#!/bin/sh
PHP='/usr/bin/php'
SCRIPT='/srv/app/job.php'
INTERVAL=10
END=$(( $(date +%s) + 55 ))
cd '/srv/app' || exit 1
while [ "$(date +%s)" -lt "$END" ]; do
"$PHP" "$SCRIPT"
status=$?
if [ "$status" -ne 0 ]; then
printf '%s job failed with exit %sn' "$(date -Is)" "$status" >&2
fi
if [ "$(date +%s)" -ge "$END" ]; then
break
fi
sleep "$INTERVAL"
doneThe 55-second window leaves a small margin before the next cron trigger. It is not a precision timer: process startup, PHP runtime, scheduler delay, and system load all affect the actual times.
Install the once-per-minute cron entry
Edit the crontab belonging to the Unix account that should run the job:
crontab -eAdd one line:
* * * * * /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1The five asterisks mean every minute, every hour, every day of the month, every month, and every day of the week. Cron starts the wrapper once; the wrapper is responsible for all additional launches.
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Review the installed entry with:
crontab -lA user crontab runs as its owner. If you instead place the job in /etc/crontab or a file under /etc/cron.d/, add the username field between the schedule and command:
* * * * * appuser /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1Ensure that the log path is writable by that account, or create the file and grant suitable ownership before enabling the schedule. Unredirected output can be mailed by cron, which is rarely the notification method wanted for a frequently repeating task. Redirect both standard output and standard error as shown.
Prevent overlapping minutes with a lock
If six launches take longer than a minute, the next cron trigger can start a second wrapper while the first is still working. That can duplicate updates, process the same queue item twice, or contend for a database row. Decide explicitly whether overlap should be skipped, queued, or allowed. A nonblocking lock implements the skip policy.
Wrap the command with the system flock utility:
* * * * * /usr/bin/flock -n /run/user/1000/php-job.lock /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1Replace 1000 with the numeric UID of the cron user. The lock directory must exist and be writable by that user; a service account may need an administrator-created directory such as a properly permissioned location under /run or /var/lock. With -n, a second wrapper exits immediately when the first one owns the lock. Without a lock, two wrappers can both invoke PHP even though cron itself is behaving normally.
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The lock protects the wrapper as a whole. If you need protection inside the application as well, PHP’s advisory flock() can guard a dedicated file:
<?php
$fp = fopen(__DIR__ . '/job.lock', 'c');
if ($fp === false || !flock($fp, LOCK_EX | LOCK_NB)) {
exit(0);
}
// Perform one unit of work here.
flock($fp, LOCK_UN);
fclose($fp);PHP documents flock() as an advisory lock. Every cooperating invocation must open the same lock file and honor the same convention; an unrelated process that ignores it can still modify the data. Keep the lock file in a directory with permissions appropriate to the application and do not place it in a location that is routinely deleted between runs.
Make cron’s environment explicit
Cron commands normally run through /bin/sh, not your interactive shell. They may have a minimal PATH, a different HOME, no shell profile, and no exported application variables. Add required settings near the top of a user crontab:
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SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
APP_ENV=production
* * * * * /usr/bin/flock -n /run/user/1000/php-job.lock /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1Prefer passing configuration through a controlled environment file or your application’s supported configuration mechanism rather than relying on secrets in a crontab. If the PHP program needs a working directory, set it in the wrapper. If it needs a particular CLI configuration, call the verified binary and confirm the loaded files with php --ini as the target user.
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Choose failure behavior deliberately
The sample wrapper logs a nonzero exit status and continues to the next slot. That is only one policy. Select the behavior that matches the job:
- Continue and log: useful when each invocation is independent and a later slot can make progress.
- Abort the wrapper: use an explicit
exit "$status"after a failure when later runs would be unsafe. - Retry with a limit: add a bounded retry count and backoff; never create an unbounded retry loop inside a minute.
- Queue for later: record failed work durably so a worker can process it after the cron wrapper exits.
Make the PHP script return meaningful exit codes and write enough context to identify the item being processed. A cron lock prevents concurrent wrappers, but it does not make a failed operation transactional; database transactions, idempotency keys, or checkpoint records belong in the job itself.
Log, rotate, and observe the job
The redirection >> /var/log/php-job.log 2>&1 appends standard output and standard error to one file. Use a dedicated log-rotation policy appropriate to the host so six or more messages per minute cannot fill the disk. Include timestamps in the wrapper or application output, as the example does for failures.
After installation, inspect the host’s cron service log or system journal to confirm that cron launched the wrapper, then inspect the application log to confirm that PHP ran. Log locations and service names can differ by Ubuntu release and local configuration, so use the logging facility enabled on that machine. A cron launch record with no application output usually points to an executable, permission, path, or environment problem.
Test in the same account before relying on cron
-
Run the wrapper manually as the cron user:
sudo -u appuser /usr/local/bin/run-php-job.sh -
Run the exact PHP command from the wrapper:
sudo -u appuser /usr/bin/php /srv/app/job.php -
Check the exit status immediately with
echo $?and inspect the log for PHP warnings, missing extensions, or permission errors. -
Confirm the crontab entry with
crontab -lor, for a system crontab, inspect the installed file and username field.Best Value
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-
Wait through at least one minute boundary, then verify the expected number of PHP starts in the application log. Compare timestamps rather than assuming that a successful cron launch means the PHP work succeeded.
-
Test the overlap case by temporarily making the job run longer than a minute. The lock should cause the next wrapper to skip instead of launching a concurrent copy.
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Troubleshoot the common failure modes
It runs once per minute instead of several times
Confirm that the cron line is * * * * * and that the repeat loop is inside the wrapper. A cron expression such as */10 changes the minute marks, not the number of starts within each minute. Check that the installed wrapper is the file referenced by the crontab and that it is executable.
The shell reports “php: not found”
Use the absolute PHP path found with command -v php. Do not assume the interactive PATH is present. Re-run php --ini as the cron user if the script now finds PHP but fails because an extension or configuration value is missing.
The script works manually but not from cron
Compare users, current directory, environment variables, file permissions, and the PHP configuration. Add an explicit cd, set required variables in the crontab or application configuration, and use absolute paths for every file. Capture stderr in the log; otherwise the useful error may be mailed or discarded by an incomplete redirection.
Two copies run at once
Check that the lock path is writable and stable for the cron user, that the crontab invokes flock, and that every competing entry uses the same lock file. If only the PHP code uses flock(), verify that every cooperating process opens that exact file; advisory locks do nothing to programs that ignore them.
Runs drift or fewer starts occur
Measure the PHP runtime. A sleep is added after each completed invocation, so a slow job naturally reduces the number of starts in a minute. Decide whether to shorten the work, increase the interval, permit controlled concurrency, or move to a persistent worker. Do not add more cron entries to compensate; that creates independent wrappers and makes overlap more likely.
When to replace the wrapper
Cron is deliberately simple and ubiquitous, but it is not a high-frequency process supervisor. Move to a supervised service, queue worker, or systemd-based worker architecture when the job must maintain state across restarts, guarantee durable retries, coordinate multiple consumers, shut down gracefully, or sustain a rate that makes repeated PHP startup expensive. A persistent worker can keep the PHP process alive and implement its own timing and backoff, while a supervisor can restart it and record lifecycle events.
For a lightweight task that tolerates approximate spacing, the once-per-minute cron trigger plus a locked wrapper remains easy to deploy and audit. Treat the sleep value, run count, timeout policy, and overlap rule as application decisions rather than properties of cron syntax.
Quick Recap
Deployment checklist
- Verify
command -v php,php -v, andphp --inias the target user. - Use absolute paths and an executable wrapper.
- Set the working directory and required environment explicitly.
- Schedule the wrapper with
* * * * *, not a sub-minute cron expression. - Redirect both output streams and configure log rotation.
- Install one shared nonblocking lock for the wrapper or an equivalent application lock.
- Choose whether failures continue, abort, retry, or enter a durable queue.
- Run the exact command manually, then verify cron and application logs after a minute boundary.
- Reconsider cron if timing precision, restart durability, throughput, or retry guarantees are strict requirements.
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