sed 替換、修改鏈接文件注意問題

sed 替換、修改鏈接文件注意問題

1. sed 替換、修改鏈接文件注意問題
   
2. 
   
3. #系統與版本
   
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.
    
12. 
    
13. 因為sed -i /etc/sysconfig/selinux(selinux文件是/etc/selinux/config的軟鏈接)配置文件重啟SELINUX沒有關閉，才發現原來sed -i是不能直接修改軟鏈接文件的，如下我修改之後的後果：
    
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
    
18. 
    
19. 
    
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
    
23. 
    
24. 我們發現鏈接文件不再是鏈接文件了，後來查看sed --help選項時發現如下選項說明
    
25. 
    
26. --follow-symlinks
    
27. follow symlinks when processing in place; hard links will still be broken.
    
28. 
    
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.
    
32. 
    
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. 
    
39. 
    
40. #測試
    
41. [root@localhost ~]# echo "test" >>test
    
42. [root@localhost ~]# ln -s test test-zsq
    
43. [root@localhost ~]# ll -i test-zsq #鏈接文件
    
44. 260727 lrwxrwxrwx. 1 root root 4 Dec 22 19:21 test-zsq -> test
    
45. [root@localhost ~]# cat test-zsq
    
46. test
    
47. 
    
48. #如果直接-i刪除，鏈接文件將失效
    
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
    
52. 
    
53. #重新添加再測試，加上-c選項
    
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
    
62. 
    
63. #--follow-symlinks選項
    
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
    
73. 
    
74. 
    
75. --follow-symlinks比-c選項更快更安全
    
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.
    
82. 
    
83. 
    
84. 鏈接文件/etc/rc.local也是/etc/rc.d/rc.local的軟連接，用sed添加刪除啟動服務要特別注意
    
85. 有可能配置的鏈接文件失效而導致服務起不來
    
86. 
    
87. 
    
88. 在centos 5.x系列中運行相同的操作沒有出現類似的現象
    
89. 經查是sed的版本不同造成的影響，centos 5系列的還是使用老版本的sed，沒有--follow-symlinks類似的選項
    
90. 
    
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]...
    
99. 
    
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