sed -i destroys symlink

[Miek Gieben]

When using 'sed -i' on a symlink the symlink gets destroyed.
Observe:

% ls -l myfile symlink
-rw-rw-r-- 1 miekg miekg 12 Apr 26 10:53 myfile
lrwxrwxrwx 1 miekg miekg 6 Apr 26 10:53 symlink -> myfile

# do an edit
% sed -i 's/hello//' symlink

% ls -l myfile symlink
-rw-rw-r-- 1 miekg miekg 12 Apr 26 10:53 myfile
-rw-rw-r-- 1 miekg miekg 7 Apr 26 10:54 symlink

And gone is your symlink.

This either a feature or a bug :)

[Miek Gieben (miek)]

This is due to using rename(), in the file lib/utils.c (is sed's source) at line 324 in the function:
ck_rename (from, to, unlink_if_fail):

       int rd = rename (from, to);

from is now a tmp-file and to is the original file. According to the manpage of rename(2):

       If oldpath refers to a symbolic link the link is renamed; if newpath refers to a symbolic link
       the link will be overwritten.

So the later thing happens.

[Miek Gieben (miek)]

The following "patch" fixed the problem. It does so by looking at the target. If it is an symlink the tmp file will be copied
to the target of the symlink. Patch also included as attachment.

elektron.m% diff -ur sed-4.1.5/lib/utils.c sed-4.1.5-modified/lib/utils.c
--- sed-4.1.5/lib/utils.c 2005-06-21 16:09:40.000000000 +0200
+++ sed-4.1.5-modified/lib/utils.c 2009-04-27 11:04:38.003830559 +0200
@@ -321,7 +321,19 @@
   const char *from, *to;
   const char *unlink_if_fail;
 {
- int rd = rename (from, to);
+ /* if to is a symlink, shouldn't we move the the link name instead? */
+ char *buf[255];
+ int rd;
+ ssize_t sym;
+ if ((sym = readlink(to, buf, 254)) != -1)
+ {
+ buf[sym] = '\0';
+ rd = rename (from, buf);
+ }
+ else
+ {
+ rd = rename (from, to);
+ }
   if (rd != -1)
     return;

[Paolo Bonzini (bonzini)]

sed 4.2 will provide --follow-symlinks, for now it is a feature that you cannot turn off. Sorry.

[Paolo Bonzini (bonzini)]

Fixed in karmic thanks to update to sed-4.2.1

[Aditya Gnaneshwar (aditya-28041991)]

can we have a similar option for a hard link file?