what text editor may be used in rescue mode. i am trying to edit "fstab" - this after changes in the physical setup of hard and optical drives messed up my ubuntu installation. tried invoking nano but the shell gave me this: "Error opening terminal: bterm." vim, on the other, would not give me the insert mode. thanks in advance.

How about plain old "vi".

OR nano login and enter 'sudo nano'

Not sure what you mean by rescue mode.

vim is an enhance vi like editor. If your session vi/vim while you were editing the file you just need to use the "-r" flag to tell it to recover the last session. vi/vim actually makes a copy of the file you're editing and does its work on that until you issue the :w (or :wq or either with ! or shift ZZ etc... - i.e. you tell it to save).

The above default functionality allows for rescue from crashed/improperly terminated editing session because your original file was never changed.

If what you mean is you want to be able to recover a file from before you made the changes that you saved then you simply should copy the file before you start the edit. This isn't Windoze so you're not going to be asked if you're really really sure before it does what you tell it.

Typically for most files I do "cp -p file file.datestamp" before I do the edit. The file of course is whatever file (fstab for example) and the datestamp is today's date (20050918 for example). The "-p" tells it to preserve permissions and modify times so the file is exactly the same as the original (except its on a different inode but that only matters rarely for weird key files). If you later need to recover you simply do "cp -p file.datestamp file" to overwrite the one you mucked up.