What causes "Input cannot be set while the process is running." in symfony/process?

Call setInput() before start()/run().

symfony/process · error · LogicException

Input cannot be set while the process is running.

Error message

Input cannot be set while the process is running.

What it means

setInput() throws this LogicException when called on a process that is already running. The child's stdin is established at start time, so input can only be provided while the process is in a not-yet-started or terminated state.

Solutions

Call setInput() before start()/run().
Stop() or wait() for the process to finish before setting new input, then restart if needed.
Provide all input up front at construction: new Process($cmd, null, null, $input).
For interactive mid-run input, use a TTY or communicate()/InputStream from the symfony/process stream API.

Example fix

// before
$process->start();
$process->setInput('foo'); // LogicException

// after
$process->setInput('foo');
$process->start();

Defensive patterns

Strategy: validation

Validate before calling

if (!$process->isRunning()) {
    $process->setInput($input);
}

Try / catch

try {
    $process->setInput($input);
} catch (\LogicException $e) {
    // running: stop() or wait() first, or feed via InputStream
}

Prevention

Pass input at construction: new Process($cmd, $cwd, $env, $input).
For interactive stdin, use symfony/process InputStream with start() instead of setInput().

When it happens

Trigger: Calling setInput() after start()/run() while isRunning() is true. Also reachable via constructors that call setInput() with input on a reused/running instance.

Common situations: Streaming data to a subprocess interactively and trying to change stdin mid-run; retry loops reconfiguring a still-running process; fluent builders invoked after start.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of symfony/process@99b85026db (2026-09-14). Data as JSON: /api/errors/18b0833edcb5c1c5. Report an issue: GitHub.

Appendix: source

Thrown at Process.php:1197

    {
        return $this->input;
    }

    /**
     * Sets the input.
     *
     * This content will be passed to the underlying process standard input.
     *
     * @param string|resource|\Traversable|self|null $input The content
     *
     * @return $this
     *
     * @throws LogicException In case the process is running
     */
    public function setInput(mixed $input): static
    {
        if ($this->isRunning()) {
            throw new LogicException('Input cannot be set while the process is running.');
        }

        $this->input = ProcessUtils::validateInput(__METHOD__, $input);

        return $this;
    }

    /**
     * Performs a check between the timeout definition and the time the process started.
     *
     * In case you run a background process (with the start method), you should
     * trigger this method regularly to ensure the process timeout
     *
     * @throws ProcessTimedOutException In case the timeout was reached
     */
    public function checkTimeout(): void
    {
        if (self::STATUS_STARTED !== $this->status) {

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