I am running QGIS 3.4.14 on Ubuntu Linux 18.02. I cannot see the SAGA tool in the QGIS Processing window. I can confirm that I have SAGA 7.3 installed and working as a stand-alone GUI application. I have gone into the processing options in QGIS and enabled SAGA as a provider.
This QGIS website states if you are running Linux, SAGA binaries are not included with Processing, so you have to download and install the software yourself. Please check the SAGA website for more information. In this case, there is no need to configure the path to the SAGA executable, and you will not see those folder entries. Instead, you must make sure that SAGA is properly installed and its folder is added to the PATH environment variable. Just open a console and type saga_cmd to check that the system can find where the SAGA binaries are located.
I can confirm sage_cmd will open the command line version of SAGA. I have added PATH="$HOME/bin:$PATH:/opt/saga-7.4.0" to ~/.profile.
Still, I do not see SAGA in the processing window. How do I get the SAGA tool available in QGIS 3.4?