因為sed -i /etc/sysconfig/selinux(selinux檔案是/etc/selinux/config的軟連結)配置檔案重啟SELINUX沒有關閉，才發現原來sed -i是不能直接修改軟連結檔案的，如下我修改之後的後果：

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

我們發現連結檔案不再是連結檔案了，後來檢視sed man選項時發現如下選項說明

--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.

以上說明就不作過多解釋了，說的很明顯，看下面例項

[[email protected] ~]# echo "test" >>test
[[email protected] ~]# ln -s ~/test ~/test_soft
[[email protected] ~]# ln ~/test ~/test_hard
[[email protected] ~]# 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
[[email protected] ~]# sed -i "s/test/hard/g" test_hard 
[[email protected] ~]# sed -i "s/test/soft/g" test_soft 
[[email protected] ~]# 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
[[email protected] ~]# 

很明顯如man中所說-i選項對軟連結和硬連結都會使受到破壞

[[email protected] ~]# rm -rf test*
[[email protected] ~]# echo "test" >>test
[[email protected] ~]# ln -s ~/test ~/test_soft
[[email protected] ~]# ln ~/test ~/test_hard
[[email protected] ~]# 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
[[email protected] ~]# sed -i -c  "s/test/soft/g" test_soft 
[[email protected] ~]# sed -i -c  "s/test/soft/g" test_hard 
[[email protected] ~]# 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選項只對軟連結有效，硬連結還是會被破壞，建議使用-c選項，這裡就不舉例了

後來朋友在RHEL5上執行相同的操作居然沒有出現類似的現象，執行結果如下：

$ 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

經查是sed的版本不同造成的影響，RHEL5系列的還是使用老版本的sed，沒有--follow-symlinks類似的選項，筆者之前實驗的版本是RHEL6.3，所以出現之前的一系列問題了