Set script to be executed at/after boot, with only write permissions, but no shell access on the target

Question

Suppose you have write access to a Linux system, with root rights, but no interactive shell.

You can only write new files or overwrite existing files. Creation of (sub)directories is possible. It's possible to define file permissions.

How would you create a ba(sh) script, to be run after or at system boot?

edit: I do not have read permissions on the system either - only write permissions

Answer 1

That depends on what init setup the Linux system in question uses. All the more commonplace ones use text files (and lots of symbolic links) to determine what needs to be run at boot and in what order, whether that's a bash script for sysvinit or a systemd unit file or something for runit or upstart or whatnot.

Figure out which init system is being used, then write an init file of the appropriate type with the instructions you want, and drop it into the appropriate directory.

Without more information than you're providing in your hypothetical question it's hard to give a more specific answer.