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Ubuntu cron cannot schedule several launches inside one minute by itself. Cron examines entries once per minute, so the reliable pattern is to schedule a wrapper every minute and let that wrapper invoke the PHP command repeatedly with controlled sleep delays. Use the PHP command-line interface (CLI), absolute paths, explicit environment settings, logging, and a lock so a slow run cannot collide with the next one.
What cron can and cannot do
A crontab entry has five time fields: minute, hour, day of month, month, and day of week. The minute field accepts values from 0 through 59, and Ubuntu documents that cron(8) examines cron entries every minute. Therefore, cron’s native resolution is one minute; syntax such as a seconds field or a sub-minute interval is not available in a standard Ubuntu crontab.
| Requirement | Suitable approach | What to expect |
|---|---|---|
| Run at a whole-minute cadence | Crontab schedule alone | Reliable minute-based launches, such as every 10 minutes |
| Run several times within each minute | Per-minute cron entry plus a shell wrapper | Approximate spacing controlled by the wrapper and the job’s runtime |
| Strict timing, durable retries, or high throughput | Supervised worker, queue consumer, or service architecture | More lifecycle control, but more deployment and monitoring work |
For example, */10 * * * * runs when the minute is divisible by 10: 00, 10, 20, 30, 40, and 50. It does not mean “wait 10 minutes after the previous launch.” Likewise, Ubuntu warns that 0/35 means minute 0 and minute 35 of every hour, not a continuous 35-minute interval across hour boundaries. Minute-step syntax is useful only for whole-minute schedules.
The recommended design: one cron launch, several PHP invocations
Put the repetition logic in a versioned, executable wrapper. Cron starts that wrapper at the beginning of each minute; the wrapper calls PHP, waits, and calls it again. This keeps the crontab readable and gives you a place to set paths, logging, error handling, and locking.
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A minimal crontab entry is:
* * * * * /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1The schedule means “once each minute.” The wrapper determines how many times the job is attempted during that minute. Four attempts at approximately 15-second intervals are a common example, but the correct count and delay depend on the job’s runtime and acceptable load.
Build a wrapper for a four-times-per-minute job
1. Confirm PHP CLI as the cron user
Run these commands while logged in as the Unix account that will own the crontab:
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php -v
php --iniThe first command shows the executable selected by the interactive shell. php -v confirms that the CLI binary starts, and php --ini displays the configuration files used by CLI PHP. Cron has a smaller environment than an interactive login, so record the absolute PHP path, commonly /usr/bin/php, rather than relying on PATH.
PHP’s CLI SAPI is intended for shell applications. Both forms below execute a script without making an HTTP request:
/usr/bin/php /srv/app/job.php
/usr/bin/php -f /srv/app/job.php2. Create the wrapper at an absolute path
Create /usr/local/bin/run-php-job.sh with content similar to this:
#!/bin/sh
set -u
PHP=/usr/bin/php
JOB=/srv/app/job.php
INTERVAL=15
ATTEMPTS=4
n=1
while [ "$n" -le "$ATTEMPTS" ]; do
"$PHP" "$JOB"
if [ "$n" -lt "$ATTEMPTS" ]; then
sleep "$INTERVAL"
fi
n=$((n + 1))
doneReplace both paths with locations on your server. This example starts immediately, then waits 15 seconds between attempts, producing up to four starts in the minute. The spacing is approximate: if PHP takes five seconds, the next start occurs about 20 seconds after the previous start because the sleep begins after the process exits. If you need a different policy, change INTERVAL, ATTEMPTS, or the loop itself.
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Make the file executable and inspect it:
sudo install -m 0755 /path/to/run-php-job.sh /usr/local/bin/run-php-job.sh
ls -l /usr/local/bin/run-php-job.shKeep the script’s working directory and all application paths explicit. A cron process may start in an unexpected directory, so a relative path that worked in a terminal can fail under cron.
3. Decide what a failure should do
The simple wrapper above continues to the next scheduled attempt even if PHP exits with a nonzero status. That is appropriate when each invocation is independent and failures are logged. If one failure should stop the remaining attempts, add a check:
if ! "$PHP" "$JOB"; then
exit 1
fiDo not silently choose a policy. For a retryable task, you may want to record the error and continue; for a dependent sequence, aborting may prevent harmful follow-on work. Whichever policy you choose, make the exit status visible in the log.
Install and verify the crontab entry
Per-user crontab
Edit the crontab belonging to the account that should execute the job:
crontab -eAdd:
* * * * * /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1Review the installed entries with:
crontab -lA user crontab runs as its owner. Ensure that this account can read the PHP file, write any application data, and append to the selected log. A log under /var/log commonly requires a directory or file permission arrangement prepared by an administrator. If the account cannot write there, choose a writable application log path or create the file with suitable ownership.
System-wide cron files
/etc/crontab and files in /etc/cron.d/ contain an additional username field. The equivalent entry is therefore shaped like:
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* * * * * appuser /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1Do not copy that username column into a per-user crontab; doing so changes the field count and prevents the intended schedule.
Make cron’s environment explicit
Cron commands run through /bin/sh unless the crontab sets another SHELL. They do not automatically inherit everything from your interactive shell: PATH, HOME, the working directory, language settings, credentials, and application variables can differ. Set what the job needs in the wrapper or crontab, and use absolute paths.
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For example, a crontab can define safe defaults before the schedule:
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOME=/home/appuser
APP_ENV=production
* * * * * /usr/local/bin/run-php-job.sh >> /var/log/php-job.log 2>&1For secrets, prefer the application’s existing protected configuration mechanism rather than placing credentials in a world-readable crontab. If the PHP program expects a current directory, have the wrapper change to it explicitly with cd /srv/app || exit 1 before invoking PHP.
Unredirected output may be handed to the local mail system. Redirect both standard output and standard error, as shown above, so diagnostics are retained in a known log and do not depend on local mail configuration.
Prevent overlapping executions
Sub-minute scheduling creates overlap risk. If one invocation takes longer than its interval, the next invocation can begin while the previous one is still modifying the same records or files. Choose an explicit policy: skip a busy run, queue it, or allow concurrency only when the application is designed for it.
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Shell lock with flock
To skip an invocation while another copy holds the lock, wrap the PHP command:
flock -n /run/user/<uid>/php-job.lock -c '/usr/bin/php /srv/app/job.php'Replace <uid> with the numeric user ID and ensure that the directory exists and is writable by that user. In a wrapper, a failed nonblocking lock should be logged as a skipped attempt rather than treated as a mysterious PHP failure. Every competing invocation must use the same lock file and convention.
A wrapper fragment that continues its minute loop while skipping a busy job can look like:
LOCK=/run/user/1000/php-job.lock
if flock -n "$LOCK" -c "$PHP $JOB"; then
:
else
echo "$(date -u '+%Y-%m-%dT%H:%M:%SZ') job already running; skipped" >&2
fiWhen command arguments contain spaces or shell metacharacters, avoid assembling an untrusted command string. Use a dedicated lock descriptor or a small PHP lock wrapper instead.
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PHP’s flock()
PHP can enforce the lock inside the application:
<?php
$lockPath = '/srv/app/job.lock';
$fp = fopen($lockPath, 'c');
if ($fp === false) {
fwrite(STDERR, "Cannot open lock filen");
exit(1);
}
if (!flock($fp, LOCK_EX | LOCK_NB)) {
fwrite(STDERR, "Another invocation is running; skippingn");
fclose($fp);
exit(0);
}
try {
// Job work goes here.
} finally {
flock($fp, LOCK_UN);
fclose($fp);
}
PHP documents flock() as an advisory lock. It works only when every cooperating process honors the same lock. A program that ignores the lock can still modify the same data, so locking is not a substitute for database transactions or application-level idempotency.
Logging, rotation, and observability
The cron line’s >> appends output and 2>&1 combines errors with normal output. Include timestamps and meaningful exit messages in the wrapper or PHP application so you can distinguish a skipped lock, a PHP startup error, and a successful run.
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Because a four-times-per-minute loop can write frequently, arrange log rotation appropriate to the host. A rotation rule should preserve ownership and permissions, signal or restart any process that keeps an open file descriptor when required, and retain enough history to diagnose failures. Do not assume that adding redirection alone solves disk growth.
After installation, check both the system’s cron logging and the application log. On systems using a journal, query the service logs with the host’s normal journal tooling; on systems using a traditional cron log, inspect the configured cron log file. The exact location varies by Ubuntu release and logging configuration, so verify it on the target host rather than hard-coding one path in your procedure.
Test exactly as cron will run it
- Switch to the crontab owner:
sudo -u appuser -H sh(or log in as that account). - Run the absolute command manually:
/usr/local/bin/run-php-job.sh. - Check the PHP exit status with
echo $?and inspect all files the job should create or update. - Confirm that the user can read the script, write its data directories, open the lock file, and append to the log.
- Wait for a scheduled minute, then inspect the cron and application logs for the expected number of attempts.
Manual success from a login shell does not prove cron success. Compare the environments if behavior differs: executable paths, HOME, current directory, PHP configuration, permissions, and required variables are the usual causes.
Common failure modes and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| No execution at all | Malformed crontab, wrong file type, or missing execute permission | Run crontab -l, validate the five fields (plus username only in system-wide files), and use chmod or install -m 0755. |
| “php: not found” | Cron’s PATH differs from the terminal’s |
Use the absolute path from command -v php, such as /usr/bin/php. |
| Script works manually but not under cron | Relative paths, missing variables, different HOME, or permissions |
Set the environment explicitly, use absolute paths, and test as the crontab owner. |
| Multiple processes modify the same data | Job duration exceeds the interval and no shared lock exists | Add a common flock or PHP flock() convention, or redesign for queueing. |
| Only the first attempt runs | Wrapper exits on an error or receives a termination signal | Inspect the log and exit status; choose deliberately between aborting, logging and continuing, and retrying. |
| Log file grows without bound | Frequent output with no rotation | Configure rotation and retain an appropriate history. |
| Lock is always unavailable | Stale path, wrong ownership, or another legitimate long-running process | Verify the directory, permissions, lock path, and process list; do not delete a lock blindly while a job may still be running. |
Choose the right scheduler for the workload
Cron plus a sleep loop
This is the smallest change for a lightweight PHP task on an Ubuntu host that is already running continuously. It is easy to inspect, requires no daemon beyond cron, and works well when approximate sub-minute spacing is acceptable. Its weaknesses are process-start overhead, limited lifecycle supervision, and timing drift when the job duration varies.
A persistent worker or supervised service
A long-running worker can keep PHP initialized, reduce repeated startup overhead, and implement queues, retries, graceful shutdown, and durable state. A service manager can restart it after failure and make startup behavior explicit. This adds deployment, monitoring, and release complexity, so it is better justified when the workload is frequent, latency-sensitive, or mission-critical.
Systemd timer or another queue architecture
A timer can express calendar-based activation and pair with a service unit, while a queue worker can separate task production from task execution. Neither option changes the need to define overlap, retry, and durability behavior. Select them when those operational controls matter more than cron’s simplicity.
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- Use a per-minute cron entry; do not expect cron syntax to provide seconds.
- Invoke the PHP CLI with an absolute executable and script path.
- Set the working directory and required environment explicitly.
- Choose the number of attempts and delay based on actual job runtime and load.
- Decide whether a failed attempt aborts, retries, or is logged and skipped.
- Prevent unsafe overlap with a shared advisory lock or an application design that supports concurrency.
- Redirect standard output and standard error, and rotate the resulting log.
- Install the entry with
crontab -e, verify withcrontab -l, and test as the cron user. - Move to a supervised worker or service when strict timing, durable retries, restart recovery, or high throughput is required.
Final answer
To run a PHP job several times per minute on Ubuntu, schedule a wrapper with * * * * * and let the wrapper call /usr/bin/php repeatedly with controlled sleeps. Cron supplies the once-per-minute trigger; the wrapper supplies sub-minute repetition. Absolute paths, explicit environment variables, complete logging, and a shared lock turn the basic loop into an operation that is predictable and diagnosable.
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