It does so for a reason because it is not advisable to use the "-i" switch at all, even in the few versions of sed not adhering to the standards which have it, because the the standards also do NOT have the switch at all.

The reason is that "behind the curtain" the "-i" switch does the same as i said: creating a temporary file and then copying it over the original. It does NOT really do "in-place-editing" as the documentation says, it does in fact the same every script programmer does. This in itself is not bad but: if you do it yourself you have control over where the temporary file goes and that it is removed afterwards. With the "-i" switch you don't have this control and in case of an error it may well be that it is left stuck somewhere and such a leftover chunk of junk is filling up valuable disk space.

Furthermore it changes file permissions and ownership, which a real in-place-editing wouldn't do. Consider the following example:

Code:

$ id
uid=1000(edstevens) gid=1000(edstevens) groups=1000(edstevens),999(users)

$ ls -l
total 189960
-rw-rw-r-- 1 bakunin users       90 Nov 13 15:16 script.original

That means you can edit script.original because you are in the group "users" which has write privilege.

Now run this - and guess, what:

Code:

$ sed -i '...<whatever, it doesn't matter>...' script.original

$ ls -l
total 189960
-rw-rw-r-- 1 edstevens edstevens       99 Jan 04 18:00 script.original

Did you notice? The ownership is changed! Now, that happens when you create a temporary file too, but there you are aware that you are dealing with a new file. Here you have no reason to suspect because you have been (wrongly) told it will be the same file plus your changes. It isn't. It will have a new inode number, it will be located somewhere else on the disk physically, etc.. This is why you should stay away from this switch. Have i mentioned that it doesn't conform to the standard either? If you want to create portable script which will not work somewhere but mysteriously not so somewhere else you better adhere to the standards slavishly. They define what you can rightfully expect from a system. If you expect more than chances are you will be disappointed sooner or later.

I hope this helps.

bakunin