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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Ubuntu cron cannot schedule a command several times inside one minute: it evaluates crontab entries once per minute, and the minute field only represents values 0 through 59. The reliable pattern is to have cron launch a wrapper once each minute. That wrapper runs the PHP command, waits for a chosen interval, and repeats within the minute. Use the PHP command-line interface, absolute paths, explicit environment settings, logging, and a lock so slow runs do not overlap.
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What cron can and cannot schedule
A standard crontab expression has five time fields: minute, hour, day of month, month, and day of week. Ubuntu’s cron implementation examines entries every minute. The minute field accepts 0–59, so cron itself has no seconds field and cannot express a five-minute, 30-second, or other sub-minute interval.
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*/10 * * * * /path/to/commandThis runs at minutes divisible by 10: 00, 10, 20, 30, 40, and 50 of each hour. It does not create a continuously rolling ten-minute timer. Ubuntu also documents a common trap: 0/35 in the minute field means minute 0 and minute 35 of every hour, not one execution every 35 minutes across hour boundaries.
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For several launches per minute, cron should be treated as the once-per-minute trigger. A shell wrapper, a persistent worker, or a supervised service must provide the shorter interval.
The practical cron-plus-wrapper design
The simplest arrangement is one crontab entry that starts a wrapper at the beginning of every minute:
* * * * * /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1The wrapper invokes the PHP CLI, sleeps for the selected delay, and repeats. If the job should start about every 20 seconds, the wrapper might attempt three starts per minute. If it should start about every 15 seconds, it might attempt four. Those counts are workload decisions; cron does not calculate them for you.
“Every 20 seconds” is approximate. PHP startup time, the job’s own runtime, scheduler delays, and system load all affect the actual start times. A wrapper that sleeps after each invocation spaces starts by approximately runtime plus sleep duration, not by the sleep duration alone. To target a fixed wall-clock cadence, the wrapper must account for elapsed time, as shown below.
A straightforward interval wrapper
Create the wrapper at a stable absolute path:
sudo install -m 0755 /dev/stdin /usr/local/bin/run-php-job.sh <<'EOF'
#!/bin/sh
PHP=/usr/bin/php
SCRIPT=/srv/app/job.php
INTERVAL=20
RUNS=3
n=1
while [ "$n" -le "$RUNS" ]; do
"$PHP" "$SCRIPT"
status=$?
if [ "$status" -ne 0 ]; then
printf '%s PHP job exited with status %sn' "$(date -Is)" "$status" >&2
fi
n=$((n + 1))
[ "$n" -le "$RUNS" ] && sleep "$INTERVAL"
done
EOFReplace /srv/app/job.php with the script’s real absolute path. This version makes three attempts and waits 20 seconds between attempts. A long-running PHP process delays the next attempt, so it is suitable only when that behavior is acceptable.
A wrapper that targets minute boundaries
If you want starts near 00, 20, and 40 seconds of each minute, calculate a deadline rather than blindly sleeping after work. This keeps the wrapper inside the minute window when the job finishes quickly:
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PHP=/usr/bin/php
SCRIPT=/srv/app/job.php
INTERVAL=20
END=$(( $(date +%s) + 60 ))
NEXT=$(date +%s)
while [ "$NEXT" -lt "$END" ]; do
"$PHP" "$SCRIPT"
status=$?
if [ "$status" -ne 0 ]; then
printf '%s PHP job exited with status %sn' "$(date -Is)" "$status" >&2
fi
NEXT=$((NEXT + INTERVAL))
NOW=$(date +%s)
WAIT=$((NEXT - NOW))
[ "$WAIT" -gt 0 ] && sleep "$WAIT"
doneThis is still best-effort scheduling, not a real-time guarantee. If one invocation takes longer than the next target, the wrapper does not create a backlog of missed starts; it proceeds to the next loop iteration. Decide explicitly whether that skip behavior is right for the job.
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Use PHP’s CLI binary, not a web request
PHP’s CLI SAPI is intended for shell applications. A cron job should execute the PHP file directly rather than making an HTTP request to a web endpoint. Direct CLI execution avoids web-server timeouts, authentication layers, and differences between web and command-line PHP configuration.
Check the binary as the Unix account that will own the cron entry:
command -v php
php -v
php --inicommand -v php shows which executable an interactive shell would select; php -v confirms the version; and php --ini shows the configuration files loaded by the CLI SAPI. Cron has a smaller environment than an interactive shell, so use the discovered absolute path, commonly /usr/bin/php, in the wrapper.
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PHP supports both forms below:
/usr/bin/php /srv/app/job.php
/usr/bin/php -f /srv/app/job.phpUse the form that your script and team standardize on. Ensure the cron user can read the PHP file and every required application file, and can write any output, cache, or temporary directory the job uses.
Install the wrapper and cron entry safely
- Choose the Unix account. Run the job as the least-privileged account that can access its files. A user’s crontab runs as that user; it does not inherit your login account’s permissions.
- Create the script. Store the wrapper outside a home directory if several administrators need to maintain it, and keep absolute paths in the script.
- Make it executable. Use
chmod 0755 /usr/local/bin/run-php-job.shor install it with the mode shown above. - Run it manually as the cron user. Test the exact command, not a shortened interactive equivalent:
sudo -u appuser /usr/local/bin/run-php-job.sh- Edit that user’s crontab. Open it with
crontab -e. Add:
* * * * * /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1- Review the installed entry.
crontab -l- Observe the logs. Wait for a minute boundary, then inspect the wrapper log and the application’s own log. A cron line can be syntactically valid while the PHP process fails because of permissions, a missing environment variable, or a wrong working directory.
If you use /etc/crontab or a file in /etc/cron.d instead, include the username field:
* * * * * appuser /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1Do not copy that six-field form into a personal crontab; crontab -e entries do not contain a username column.
Make the cron environment explicit
Cron runs commands through /bin/sh unless the crontab sets a different SHELL. It also does not provide the aliases, shell startup files, working directory, or exported variables that you may rely on interactively. Set every required value explicitly.
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For simple settings, define variables at the top of the crontab:
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
APP_ENV=production
* * * * * /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1For secrets and larger configuration, prefer the application’s protected environment file or a dedicated configuration mechanism rather than placing credentials directly in a world-readable crontab. The wrapper can change to the application’s directory before running PHP if relative paths are unavoidable, but absolute paths are safer:
cd /srv/app || exit 1
exec /usr/bin/php /srv/app/job.phpAlways redirect both standard output and standard error. Unredirected output may be mailed by cron, and mail delivery is often not configured or monitored on servers.
Prevent overlapping executions
A new minute can begin while the previous wrapper is still running. Without a guard, two PHP processes may update the same rows, send duplicate messages, or compete for a file. Choose an overlap policy before enabling the schedule: skip a launch, queue work, or allow concurrent processing with application-level coordination.
Shell lock with flock
On Ubuntu, a shell wrapper can use the flock utility to implement the skip policy:
#!/bin/sh
LOCK=/run/user/1001/php-job.lock
PHP=/usr/bin/php
SCRIPT=/srv/app/job.php
exec 9>"$LOCK"
if ! flock -n 9; then
printf '%s another wrapper is still runningn' "$(date -Is)" >&2
exit 0
fi
n=1
while [ "$n" -le 3 ]; do
"$PHP" "$SCRIPT"
status=$?
[ "$status" -eq 0 ] || printf '%s PHP job exited with status %sn' "$(date -Is)" "$status" >&2
n=$((n + 1))
[ "$n" -le 3 ] && sleep 20
doneUse a lock location writable by the cron user and appropriate to the host’s account setup. A lock file alone is not a lock; the process must hold an open file descriptor with flock for the duration of the work.
Lock inside PHP
If several entry points can start the same job, put the guard in PHP as well. PHP’s flock() provides an advisory lock, meaning every cooperating invocation must open the same lock file and honor the convention:
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<?php
$lockPath = '/var/lock/my-app-php-job.lock';
$handle = fopen($lockPath, 'c');
if ($handle === false) {
fwrite(STDERR, "Cannot open lock filen");
exit(1);
}
if (!flock($handle, LOCK_EX | LOCK_NB)) {
fwrite(STDOUT, "Another invocation is activen");
exit(0);
}
try {
// Job work goes here.
} finally {
flock($handle, LOCK_UN);
fclose($handle);
}
The account must be able to create or open the lock file. A PHP lock protects only processes that use that same advisory lock; it cannot stop an unrelated process from modifying the data.
Decide what a failure means
A wrapper needs an explicit failure policy. The basic loop above logs a non-zero exit status and continues to later attempts. That is useful when each invocation is independent, but it can be dangerous when a failed first run makes later runs invalid.
- Abort the minute: exit immediately after a non-zero status when later attempts should not run.
- Continue: record the status and keep the cadence when each attempt handles separate work.
- Retry: perform a bounded retry with backoff, then record a final failure. Avoid an unbounded retry loop inside a once-per-minute cron launch.
- Queue: write durable work items to a database or queue and let a worker process them. This is preferable when missed work must not disappear.
Return meaningful exit codes from the PHP script and include timestamps, identifiers, and error details in the application log. Cron’s redirection captures output, but it does not provide durable retry or alerting by itself.
Logging, permissions, and rotation
The redirection target must be writable by the cron user. A root-owned file in /var/log may cause the wrapper to fail before PHP starts. Create the file and directory with ownership and permissions that match the account, or log to an application directory managed by the service.
Repeated launches can grow a log quickly. Configure the host’s normal log-rotation mechanism, or direct structured application logs to the platform’s logging service. Rotation must preserve write access after a new file is created. Include enough context to distinguish wrapper messages from PHP output, especially when multiple jobs share a host.
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How this approach compares with alternatives
| Approach | Cadence | Lifecycle and failure handling | Best fit |
|---|---|---|---|
| Cron entry alone | One-minute granularity | Simple launch; no sub-minute loop | Whole-minute schedules and lightweight maintenance |
| Cron plus shell wrapper | Approximate sub-minute spacing using sleep |
Easy to deploy; you must define locking, retries, and logging | Small jobs that can tolerate best-effort timing |
| Persistent worker or supervised service | Can maintain a tighter cadence without starting PHP each time | Stronger restart, health, and failure control; more deployment work | High-frequency or mission-critical processing |
| Queue-backed worker | Driven by queued work rather than a fixed launch interval | Durable retries and explicit concurrency controls | Work that must survive restarts or never be silently skipped |
Repeated CLI launches incur PHP process startup overhead. A persistent worker can reduce that overhead, but it introduces deployment, memory-leak, restart, and monitoring concerns. Choose the simplest architecture that meets the job’s timing and durability requirements.
Troubleshoot the common failure modes
Nothing runs
- Run
crontab -las the intended user and verify the line was installed in that user’s crontab. - Run the wrapper manually with
sudo -u appuserand capture its exit status. - Check that the wrapper has an executable shebang and mode 0755.
- Use absolute paths for PHP, the script, configuration files, and log destinations.
- Inspect the host’s cron service logs as well as the redirected application log.
PHP works interactively but fails in cron
- Compare
php --inifor the interactive account with the CLI configuration used by cron. - Set
PATH, application environment variables, and working directory explicitly. - Check permissions on the script, dependencies, lock file, log, and any data directories.
- Remember that cron invokes
/bin/sh; bash-only syntax requires a bash shebang and an installed bash interpreter.
Runs overlap or duplicate work appears
- Confirm every launch path uses the same shell or PHP lock file.
- Check whether the wrapper itself lasts longer than a minute.
- Decide whether skipping, queuing, or concurrent processing is the intended policy; a lock implements skipping, not queuing.
The cadence drifts
- Measure the PHP runtime. A sleep-after-run loop spaces starts by runtime plus sleep.
- Use the elapsed-time wrapper when starts should align with approximate positions within each minute.
- For strict timing, use a supervised worker or another scheduler designed for that lifecycle; cron is not a real-time scheduler.
When cron is the wrong tool
Cron plus a wrapper is appropriate for lightweight periodic work on a machine that remains available. It is not a durable scheduler: a host shutdown, reboot, process termination, or PHP failure can leave an invocation unfinished, and cron does not automatically create a durable record of missed work.
Use a supervised service, queue worker, or systemd-based worker architecture when the job requires strict sub-minute timing, restart supervision, durable state across host restarts, coordinated retries, or sustained throughput. Those systems add configuration and operational responsibility, but they make lifecycle and failure behavior explicit instead of hiding it inside a shell loop.
A production-readiness checklist
- The job is executed with the PHP CLI and an absolute binary path.
- The wrapper and PHP script use absolute paths and an explicit working directory where needed.
- The cron account, file permissions, environment variables, and CLI configuration have been tested together.
- The once-per-minute crontab entry redirects both output streams to a writable, rotated log.
- A lock prevents unsafe overlap, and every cooperating process honors the same advisory lock.
- The wrapper’s interval, number of attempts, deadline behavior, and failure policy are documented.
- Manual and cron-triggered runs have been observed at a minute boundary.
- The architecture has been reconsidered if work must be durable, strictly timed, or high-throughput.
Frequently Asked Questions
Can I write a cron expression for every 30 seconds?
No. Ubuntu cron examines entries once per minute and has no seconds field. Start a once-per-minute wrapper and perform the sub-minute repetition there, or use a worker or service designed for that cadence.
Does */10 mean every ten minutes continuously?
It runs at minute values divisible by 10 within each hour: 00, 10, 20, 30, 40, and 50. Minute-field steps do not create a rolling interval across hour boundaries.
Should the cron line call a PHP web URL?
Use the PHP CLI directly, such as /usr/bin/php /srv/app/job.php. The CLI SAPI is intended for shell applications and avoids web-server request limits.
Why does the next run start later than the sleep interval?
If the wrapper sleeps after PHP exits, the spacing includes the PHP runtime plus the sleep duration. Use elapsed-time scheduling for approximate minute positions, or a persistent worker for tighter control.
The Bottom Line
Use cron as a once-per-minute trigger, not as a sub-minute scheduler. A carefully written wrapper can run the PHP CLI several times during that minute, but absolute paths, explicit environment settings, logging, locking, and a clear failure policy determine whether the arrangement is safe. Move to a supervised worker or queue when timing, durability, or throughput matters more than cron’s simplicity.
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