node-red/nodes/core/core/75-exec.html

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        &nbsp;msg.payload
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<script type="text/x-red" data-help-name="exec">
    <p>Runs a system command and returns its output.</p>
    <p>The node can be configured to either wait until the command completes, or to
    send its output as the command generates it.</p>
    <p>The command that is run can be configured in the node or provided by the received
    message.</p>

    <h3>Inputs</h3>
    <dl class="message-properties">
        <dt class="optional">payload <span class="property-type">string</span></dt>
        <dd>if configured to do so, will be appended to the executed command.</dd>
        <dt class="optional">kill <span class="property-type">string</span></dt>
        <dd>the type of kill signal to send an existing exec node process.</dd>
        <dt class="optional">pid <span class="property-type">number|string</span></dt>
        <dd>the process ID of an existing exec node process to kill.</dd>
    </dl>

    <h3>Outputs</h3>
    <ol class="node-ports">
        <li>Standard output
            <dl class="message-properties">
                <dt>payload <span class="property-type">string</span></dt>
                <dd>the standard output of the command.</dd>
            </dl>
        </li>
        <li>Standard error
            <dl class="message-properties">
                <dt>payload <span class="property-type">string</span></dt>
                <dd>the standard error of the command.</dd>
            </dl>
        </li>
        <li>Return code
            <dl class="message-properties">
                <dt>payload <span class="property-type">object</span></dt>
                <dd>an object containing the return code, and possibly <code>message</code>, <code>signal</code> properties.</dd>
            </dl>
        </li>
    </ol>
    <h3>Details</h3>
    <p>By default uses the <code>exec</code> system call which calls the command, waits for it to complete, and then
    returns the output. For example a successful command should have a return code of <code>{ code: 0 }</code>.</p>
    <p>Optionally can use <code>spawn</code> instead, which returns the output from stdout and stderr
    as the command runs, usually one line at a time. On completion it then returns an object
    on the 3rd port. For example, a successful command should return <code>{ code: 0 }</code>.</p>
    <p>Errors may return extra information on the 3rd port <code>msg.payload</code>, such as a <code>message</code> string,
    <code>signal</code> string.</p>
    <p>The command that is run is defined within the node, with an option to append <code>msg.payload</code> and a further set of parameters.</p>
    <p>Commands or parameters with spaces should be enclosed in quotes - <code>"This is a single parameter"</code></p>
    <p>The returned <code>payload</code> is usually a <i>string</i>, unless non-UTF8 characters are detected, in which
    case it is a <i>buffer</i>.</p>
    <p>The node&apos;s status icon and PID will be visible while the node is active. Changes to this can be read by the <code>Status</code> node.</p>
    <h4>Killing processes</h4>
    <p>Sending <code>msg.kill</code> will kill a single active process. <code>msg.kill</code> should be a string containing
    the type of signal to be sent, for example, <code>SIGINT</code>, <code>SIGQUIT</code> or <code>SIGHUP</code>.
    Defaults to <code>SIGTERM</code> if set to an empty string.</p>
    <p>If the node has more than one process running then <code>msg.pid</code> must also be set with the value of the PID to be killed.</p>
    <p>Tip: if running a Python app you may need to use the <code>-u</code> parameter to stop the output being buffered.</p>
</script>

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