Let us say that you've installed a library via postInstall script, and now you want to find a file name ending in the extension .so.
Run a command like find / -name *.so.   This will look through all files -- starting in the /  directory, which is root -- for files with the characters .so  in their filenames. (The *  is a wildcard, instructing the computer to look for all possible combination of things ending in .so .)
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Here is a lifewire article explaining the 'find' command in more detail.
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You can do this from either a shell script or an interactive terminal session.