Does resizing an ext4 partition formats it?

You must boot your system from a live Linux distro, then use gparted, available either as default or as extra package on all distros. There are guides to do this all over Google, you can find one here just as an example. However, despite all assurances, you should backup all of your data in any case, there is alway Murphy's law.

At the cost of sounding banal, let me just state you caot shrink a partition to a size smaller than the amount currently stored on it, without losing data.

Don't you wish now you had used LVM instead, ab initio?

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