1. Create a dummy file which has the same file name, same header line as you real data files. If your real data file is an Excel file like below, you also need to make sure it has the same sheet name as the real data file.
After that, Input one dummy data line such as just to input a "dummy" word into the first column like below.
2. Place the dummy file you created at step 1 to the network directory folder that you selected in [Search in] in Tableau Prep Builder like below.
For this example, it is a folder named as "test" under the network directory \\<domain name>\xxxxx\
3. In Tableau Prep Builder, open your flow and switch your Source to [Single table] via Input > Tables tab.
4. In Tableau Prep Builder, select the dummy file that you placed in to "test" folder at step 2 via [Browser] button or just input the directory directly.
5. In Tableau Prep Builder, switch your Source to [Union multiple tables] via Input > Tables tab, then check the [Include subfolders] option.
Make sure the dummy file is displayed in [Included Tables]>[Files] column without any error and no subfolder in its File column. After that, click [Apply].
6. In Tableau Prep Builder, make sure the folder has been set to "test" folder in [Search in], and no subfolder in the File column of dummy file.
7. In Tableau Prep Builder, on the clean step which is next to the Input step, remove the dummy data line that you input in your dummy file by right clicking the dummy data line and clicking [Exclude].
8. In Tableau Prep Builder, publish the flow to your Tableau Server on Linux with Direct Connection option.
9. In your Tableau Server on Linux, open the published flow via [Edit Flow] and confirm the subfolder information is included in "File Paths" field