We use a smart service called "Append Text to File" from the "Text File Utilities" plug-in. It seems to have stopped working since the latest release a few days ago. Are there any alternatives?
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Depending on your use case, the "Text from Template" smart service might work. I allows you to create a template with placeholders like "###text###". To append text to the same file in iterations, I just write that placeholders as the last line of my actual content.