quotation does not look alright. this needs testing :)

The confusing thing about the quotation is that the Arduino IDE seems to handle quotes differently when they are next to a placeholder:

"outer string "inner string {placeholder} more inner string" more outer string"

gets converted into

"outer string "inner string value more inner string" more outer string"

which is obviously incorrect. On the other hand,

"outer string "{placeholder}" more outer string"

becomes

"outer string \"value\" more outer string"

which is correct in this case.

This is kind of uncommon, though, and I couldn't find any mention of this behavior in the platform specification docs.

I've tested this using the Arduino IDE v1.8.9 and 1.8.13 on Ubuntu 20.04, as well as with the arduino-cli directly for my CI builds and it works correctly. The bash commands it generates look correct to me (enable verbose output in the preferences to see them, the output is also in my previous message) and the partitions.csv file gets copied to the build folder if it exists, no matter if the sketch path contains spaces or not.

what if we pre-escape the whole paths so that partitions.csv is also escaped. I also need to test this with Sloeber and make sure that paths expand as they should

Why does partitions.csv need to be escaped? It doesn't contain any spaces or special characters.

Bash should handle this correctly, multiple strings (quoted or not quoted) that are right next to each other are combined into a single string.

For example:

$ echo "quoted&:'string"suffix
> quoted&:'stringsuffix
$ ls "$HOME"/.profile
> /home/pieter/.profile

because the commands are used by other builders and IDEs. It makes most sense that the full path is escaped, not just part of it.