sed 替换、修改链接文件注意问题

1. sed 替换、修改链接文件注意问题
3. #系统与版本
4. [root@ localhost ~]# cat /etc/redhat-release
5. CentOS release 6.8 (Final)
6. [root@ localhost ~]# sed --version
7. GNU sed version 4.2.1
8. Copyright (C) 2009 Free Software Foundation, Inc.
9. This is free software; see the source for copying conditions. There is NO
10. warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE,
11. to the extent permitted by law.
13.  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：
14.  [root@ localhost ~]# ll /etc/sysconfig/selinux
15. lrwxrwxrwx. 1 root root 17 Dec 20 11:31 /etc/sysconfig/selinux -> ../selinux/config
16. [root@ localhost ~]# ll /etc/selinux/config
17. -rw-r--r--. 1 root root 458 Dec 20 11:31 /etc/selinux/config
20. [root@ localhost ~]# sed -i "s/SELINUX=enforcing/SELINUX=disabled/g" /etc/sysconfig/selinux
21. [root@ localhost ~]# ll /etc/sysconfig/selinux
22. -rw-r--r--. 1 root root 457 Dec 22 19:18 /etc/sysconfig/selinux
24. 我们发现链接文件不再是链接文件了，后来查看sed --help选项时发现如下选项说明
26. --follow-symlinks
27.               follow symlinks when processing in place; hard links will still be broken.
29. -i[SUFFIX], --in-place[ = SUFFIX]
30.               edit files in place (makes backup if extension supplied). The default operation mode is to
31.               break symbolic and hard links. This can be changed with --follow-symlinks and --copy.
33. -c, --copy
34.               use copy instead of rename when shuffling files in -i mode. While this will avoid breaking
35.               links (symbolic or hard), the resulting editing operation is not atomic. This is rarely the
36.               desired mode; --follow-symlinks is usually enough, and it is both faster and more secure.
37.             
38.             
40. #测试
41. [root@ localhost ~]# echo "test" >>test
42. [root@ localhost ~]# ln -s test test-zsq
43. [root@ localhost ~]# ll -i test-zsq #链接文件
44. 260727 lrwxrwxrwx. 1 root root 4 Dec 22 19:21 test-zsq -> test
45. [root@ localhost ~]# cat test-zsq
46. test
48. #如果直接-i删除，链接文件将失效
49. [root@ localhost ~]# sed -i "s/test//g" test-zsq
50. [root@ localhost ~]# ll -i test-zsq
51. 260726 -rw-r--r--. 1 root root 1 Dec 22 19:29 test-zsq
53. #重新添加再测试，加上-c选项
54. [root@ localhost ~]# rm -rf test-zsq
55. [root@ localhost ~]# ln -s test test-zsq
56. [root@ localhost ~]# ll -i test-zsq
57. 260726 lrwxrwxrwx. 1 root root 4 Dec 22 19:33 test-zsq -> test
58. [root@ localhost ~]# echo "test" >>test
59. [root@ localhost ~]# sed -i -c '/test/d' test-zsq
60. [root@ localhost ~]# ll -i test-zsq
61. 260726 lrwxrwxrwx. 1 root root 4 Dec 22 19:33 test-zsq -> test
63. #--follow-symlinks选项
64. [root@ localhost ~]# rm -rf test-zsq
65. [root@ localhost ~]# ln -s test test-zsq
66. [root@ localhost ~]# echo "test" >> test
67. [root@ localhost ~]# cat test
68. test
69. [root@ localhost ~]# sed -i --follow-symlinks '/test/d' test
70. [root@ localhost ~]# ls -l test-zsq
71. lrwxrwxrwx. 1 root root 4 Dec 22 19:50 test-zsq -> test
72. [root@ localhost ~]# cat test
75. --follow-symlinks比-c选项更快更安全
76.   -c, --copy
77.                  use copy instead of rename when shuffling files in -i mode.
78.                  While this will avoid breaking links (symbolic or hard), the
79.                  resulting editing operation is not atomic. This is rarely
80.                  the desired mode; --follow-symlinks is usually enough, and
81.                  it is both faster and more secure.
84. 链接文件/etc/rc.local也是/etc/rc.d/rc.local的软连接，用sed添加删除启动服务要特别注意
85. 有可能配置的链接文件失效而导致服务起不来
88. 在centos 5.x系列中运行相同的操作没有出现类似的现象
89. 经查是sed的版本不同造成的影响，centos 5系列的还是使用老版本的sed，没有--follow-symlinks类似的选项
91. [root@DNS ~]# sed --version
92. GNU sed version 4.1.5
93. Copyright (C) 2003 Free Software Foundation, Inc.
94. This is free software; see the source for copying conditions. There is NO
95. warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE,
96. to the extent permitted by law.
97. [root@DNS ~]# sed --help
98. Usage: sed [OPTION]... {script-only-if-no-other-script} [input-file]...
100.   -n, --quiet, --silent
101.                  suppress automatic printing of pattern space
102.   -e script, --expression = script
103.                  add the script to the commands to be executed
104.   -f script-file, --file = script-file
105.                  add the contents of script-file to the commands to be executed
106.   -i[SUFFIX], --in-place[ = SUFFIX]
107.                  edit files in place (makes backup if extension supplied)
108.   -c, --copy
109.                  use copy instead of rename when shuffling files in -i mode
110.          (avoids change of input file ownership)
111.   -l N, --line-length = N
112.                  specify the desired line-wrap length for the `l' command
113.   --posix
114.                  disable all GNU extensions.
115.   -r, --regexp-extended
116.                  use extended regular expressions in the script.
117.   -s, --separate
118.                  consider files as separate rather than as a single continuous
119.                  long stream.
120.   -u, --unbuffered
121.                  load minimal amounts of data from the input files and flush
122.                  the output buffers more often
123.       --help display this help and exit
124.       --version output version information and exit

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