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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

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up vote 4 down vote accepted

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.

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You are absolutely right. However I do not want to give the script-executing user +w on /etc – Mo. Oct 29 '09 at 14:14
Oh ha... I do like the idea of the symlink! Thanks a lot! – Mo. Oct 29 '09 at 14:16
If you go the symlink route, make sure you lock down "my_directory" so somebody else can't replace the file. There is a reason why /etc/ is locked down as hard as it is, and you subvert that at your own risk. – Paul Tomblin Oct 29 '09 at 14:46
Note that you do not need any special privileges to create a symlink in, for example, /tmp that points to a file in /etc (so the "You'll need root powers to create the link" comment is bogus). The rest is OK. – Jonathan Leffler Oct 29 '09 at 22:21
@JonathanL, it's not bogus. The method is to create the symlink in /etc pointing to somewhere else (that requires write access to /etc), not the other way around as you seem to have misunderstood. – paxdiablo Oct 30 '09 at 13:04

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.

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That's the way I go for now. This does not feel very "elegant" to me, but it works... – Mo. Oct 29 '09 at 14:15
Use cp instead of cat, I suggest - it is a more direct expression of what you are doing. – Jonathan Leffler Oct 29 '09 at 22:18
@JonathanLeffler No, it looks like it needs to be cat if you don't have write permission on the directory. – Tom Wadley Jan 8 at 11:40

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