sed on not-owned file

Question

is it possible, to change a file in /etc with sed -i that I am not the owner of? However, I do have write permissions to that file.

Or as an alternative, how do I change a line in a file in /etc without having su-priviliges from within a script.

Thanks a lot

Answer 1

It's very likely that sed -i will create a brand new file then replace the original with it. That means you would have to have write access to the directory /etc, not just the file.

None of the tools that create a new file will work without that power, and that is rightly so: /etc is a very important directory and you probably shouldn't be playing there without permission :-)

One solution is to put your file somewhere where you can modify it, then just create a soft link to it from /etc. something like:

ln -s /my_directory/my_file /etc/my_file

You'll need root powers to create the link of course. The soft link will prevent the linkage from disappearing should you do a new-file/rename operation, since the soft link is simply a file containing the target path (which, unlike the inode, won't change with sed -i).

And this limits their power to changing that file and that file only. No possibility for security holes by opening up the permissions on /etc.

Answer 2

Instead of using "sed -i", you could try

sed ... > /tmp/foo && cat /tmp/foo > /etc/file

That way you're doing the actual changing of the file into a file in /tmp, and then writing that result to the file that you have write access but not replace access.