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

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<script type="text/x-red" data-help-name="exec">
    <p>Calls out to a system command.<br/></p>
    <p><b>Inputs</b></p>
    <ul><li><code>msg.payload</code> - optionally appended to the configured command.</li></ul>
    <ul><li><code><i>msg.kill</i></code> - can be used to kill a running command, see below.</li></ul>
    <ul><li><code><i>msg.pid</i></code> - can be used to kill a specific running command, see below.</li></ul>
    <p><b>Outputs</b></p>
    <ol>
        <li>stdout, <code>msg.payload</code> containing the returned output from the command.</li>
        <li>stderr, <code>msg.payload</code> containing any error output from the command.</li>
        <li>return code, <code>msg.payload</code> containing the return (see below).</li>
    </ol>
    <p><b>Details</b></p>
    <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 succesful 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 a numeric return code
    on the 3rd port. For example a successful command should return <code>0</code>.</p>
    <p>The optional <b>append</b> gets added to the command after <code>msg.payload</code> - so you can do
    things like pipe the result to another command.</p>
    <p>Commands or parameters with spaces should be enclosed in quotes - <i>"This is a single parameter"</i></p>
    <p>The returned <code>payload</code> is usually a <i>string</i>, unless non-UTF8 characters are detected, in which
    case it contains a <i>buffer</i>.</p>
    <p>The blue status icon and PID will be visible while the node is active. This can be read by a <code>status</code> node.</p>
    <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, e.g. "SIGINT", "SIGQUIT", "SIGHUP", etc. Defaults to "SIGTERM" if blank ("").
    If there is 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><b>Tip</b>: 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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