This behavior is still present in Octave 4.2.0.

The other way (for tab completion involving folder/file names) would be to incorporate file browser capabilities using e.g., glob().
That would require recognition by ?readline? that a path is to be completed, while AFAIU currently it just searches the command history. (I may be very wrong here)
An advantage would be that non-existing paths can be weeded out before <Return> is pressed and relevant error messages are generated.

AFAICS that is the way that Matlab does it.  I attached a screenshot of ML r2016a prerelease where I started with "cd Prog" and then presses TAB repeatedly.  As can be seen on that screenshot, I started from a completely different folder.

Yes, I was naively expecting a final quote after the auto completion of a folder name with a space sign inside. In my presented usecase of a "cd" command in the interactive console, this seemed the obvious behavior to me.

But you are right. If we add a closing quote in this case, it would keep the user from expanding the path further into subdirectories by simply typing some more characters and pressing tab again. I had not thought about this other usecase before. (I think the behavior of first adding a closing quote, which would lateron be automaticly removed when the user continues typing, is too weired. This would most probably only irritate most users.)

I personally would be fine with just closing this bug report now. I have no other and still consistent idea how to better handle this.

Just for completeness, here is what other readlines do in this case:

• The linux bash shell autocompletes "cd my[tab]" to "cd my\ data/". I think this only works because in this shell space signes can be escaped with a bashlash. This is a very elegant solution to our problem. But currently it does not work in Octave, because slashes and bashlashes are both used for folder name separation (as is necessary to support linux and windows file name conventions).

• In the Windows command console 'cd my[tab]' autocompletes to 'cd "my data"'. This is what I was naively expecting. But you are right that this keeps the user from doing auto completion in a nested way, which is a valid use case as well.

• I haven't tested what  Matlab does.

Here is how to reproduce the behavior:

• start Octave
• navigate to a folder, that has a SUBfolder with a space in the folder name, e.g. a subfolder "my dataset"
• type "cd my" into the command line and press the tab key for auto completion
• the auto-completed result in the command line will be "cd 'my dataset\". Note that the quote sign at the end is missing.
• pressing the enter key now, will obviously result in an error message

So the auto completion of folder names does not work properly.

Note that the auto completion of FILE names does work properly. (This error on folder names might be due to the trailing backslash that is appended to folder names.)

This happens on Linux and Windows. It happens with Octave-4.0.1-rc3 as well as with the 4.0.0 release.