Temporary "jailbreak" to gain temp root access to iOS file system?

I'm wondering if there's a way to gain root access to the iPhone file system without a full jailbreak? I.e. I do not want/need Cydia, non Apple AppStore Apps, etc. I do not want/need the change in the storage partitioning to make room for additional programs, etc. I'd just like to get at some of the existing files in the root file system and be able to modify their content as well as their settings (esp. file access rights and ownership). After having modified some files I'd be happy to continue using iOS without any tethered or untethered Jailbreak. To me it seem this is much simpler as a "full" jailbreak - question is, if it is possible and if so how... If I had to write a shell script to modify the files that'd be also OK - doesn't need top be "live access" - on the other hand - AFAIK before a jailbreak there is no shell, no command-line, tools, etc. Any advice and ideas appreciated...