My root's HOME (which is /) is on read-only file system.
When I run fish, got errors

error: Your history will not be saved.
error: Unable to locate data directory derived from $HOME: '//.local/share/fish'.
error: The error was 'Read-only file system'.
error: Please set $HOME to a directory where you have write access.
error: Your personal settings will not be saved.
error: Unable to locate config directory derived from $HOME: '//.config/fish'.
error: The error was 'Read-only file system'.
error: Please set $HOME to a directory where you have write access.

fish --version: fish, version 3.1.0
OS: Android (LineageOS 16)
echo $TERM: xterm-256color
uname -a: Linux localhost 4.9.188-perf+ #1 SMP PREEMPT Mon Feb 17 23:58:32 UTC 2020 aarch64
su -c 'env HOME=$(mktemp -d) fish': good, no errors

How to reproduce the problem:

Open Termux
Run su -c fish