sed -i   修改链接文件注意事项   破坏了原文件

因为sed -i /etc/sysconfig/selinux(selinux文件是/etc/selinux/config的软链接)配置文件重启SELINUX没有关闭，才发现原来sed -i是不能直接修改软链接文件的，如下我修改之后的后果：

[root@node1 ~]# ll /etc/sysconfig/selinux
lrwxrwxrwx. 1 root root 19 2月 20 20:34 /etc/sysconfig/selinux -> /etc/selinux/config
[root@node1 ~]# sed -i “s/SELINUX=enforcing/SELINUX=disabled/g” /etc/sysconfig/selinux
[root@node1 ~]# ll /etc/sysconfig/selinux
-rw-r–r– 1 root root 457 2月 20 22:50 /etc/sysconfig/selinux
[root@node1 ~]#

我们发现链接文件不再是链接文件了，后来查看sed man选项时发现如下选项说明
–follow-symlinks

follow symlinks when processing in place; hard links will still be broken.
-i[SUFFIX], –in-place[=SUFFIX]

edit files in place (makes backup if extension supplied). The default operation mode is to

break symbolic and hard links. This can be changed with –follow-symlinks and –copy.
-c, –copy

use copy instead of rename when shuffling files in -i mode. While this will avoid breaking

links (symbolic or hard), the resulting editing operation is not atomic. This is rarely the

desired mode; –follow-symlinks is usually enough, and it is both faster and more secure.

以上说明就不作过多解释了，说的很明显，看下面实例
[root@node1 ~]# echo “test” >>test
[root@node1 ~]# ln -s ~/test ~/test_soft
[root@node1 ~]# ln ~/test ~/test_hard
[root@node1 ~]# ll -i test*
271653 -rw-r–r– 2 root root 5 2月 20 23:04 test
271653 -rw-r–r– 2 root root 5 2月 20 23:04 test_hard
271655 lrwxrwxrwx 1 root root 10 2月 20 23:04 test_soft -> /root/test
[root@node1 ~]# sed -i “s/test/hard/g” test_hard
[root@node1 ~]# sed -i “s/test/soft/g” test_soft
[root@node1 ~]# ll -i test*
271653 -rw-r–r– 1 root root 5 2月 20 23:04 test
271656 -rw-r–r– 1 root root 5 2月 20 23:05 test_hard
271657 -rw-r–r– 1 root root 5 2月 20 23:06 test_soft
[root@node1 ~]#

很明显如man中所说-i选项对软链接和硬链接都会使受到破坏

[root@node1 ~]# rm -rf test*
[root@node1 ~]# echo “test” >>test
[root@node1 ~]# ln -s ~/test ~/test_soft
[root@node1 ~]# ln ~/test ~/test_hard
[root@node1 ~]# ll -i test*
271653 -rw-r–r– 2 root root 5 2月 20 23:08 test
271653 -rw-r–r– 2 root root 5 2月 20 23:08 test_hard
271655 lrwxrwxrwx 1 root root 10 2月 20 23:08 test_soft -> /root/test
[root@node1 ~]# sed -i -c “s/test/soft/g” test_soft
[root@node1 ~]# sed -i -c “s/test/soft/g” test_hard
[root@node1 ~]# ll -i test*
271653 -rw-r–r– 2 root root 5 2月 20 23:11 test
271653 -rw-r–r– 2 root root 5 2月 20 23:11 test_hard
271655 lrwxrwxrwx 1 root root 10 2月 20 23:08 test_soft -> /root/test
–follow-symlinks选项只对软链接有效，硬链接还是会被破坏，建议使用-c选项，这里就不举例了
后来朋友在RHEL5上运行相同的操作居然没有出现类似的现象，运行结果如下：
$ echo “test” >> test
$ ln -s ~/test ~/test1
$ ll ~/test1
lrwxrwxrwx 1 sxkj sxkj 15 02-21 13:26 /home/sxkj/test1 -> /home/sxkj/test
$ sed -i “s/test/test1/g” ~/test1
$ ll ~/test1
lrwxrwxrwx 1 sxkj sxkj 15 02-21 13:26 /home/sxkj/test1 -> /home/sxkj/test
经查是sed的版本不同造成的影响，RHEL5系列的还是使用老版本的sed，没有–follow-symlinks类似的选项，笔者之前实验的版本是RHEL6.3，所以出现之前的一系列问题了