sed 替换、修改链接文件注意问题

sed 替换、修改链接文件注意问题

#系统与版本
[root@localhost ~]# cat /etc/redhat-release
CentOS release 6.8 (Final)
[root@localhost ~]# sed --version
GNU sed version 4.2.1
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE,
to the extent permitted by law.

 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@localhost ~]#  ll /etc/sysconfig/selinux
lrwxrwxrwx. 1 root root 17 Dec 20 11:31 /etc/sysconfig/selinux -> ../selinux/config
[root@localhost ~]# ll /etc/selinux/config
-rw-r--r--. 1 root root 458 Dec 20 11:31 /etc/selinux/config


[root@localhost ~]# sed -i "s/SELINUX=enforcing/SELINUX=disabled/g" /etc/sysconfig/selinux
[root@localhost ~]#  ll /etc/sysconfig/selinux
-rw-r--r--. 1 root root 457 Dec 22 19:18 /etc/sysconfig/selinux

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

--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.



#测试
[root@localhost ~]# echo "test" >>test
[root@localhost ~]# ln -s test test-zsq
[root@localhost ~]# ll -i test-zsq  #链接文件
260727 lrwxrwxrwx. 1 root root 4 Dec 22 19:21 test-zsq -> test
[root@localhost ~]# cat test-zsq
test

#如果直接-i删除，链接文件将失效
[root@localhost ~]# sed -i "s/test//g" test-zsq
[root@localhost ~]# ll -i test-zsq
260726 -rw-r--r--. 1 root root 1 Dec 22 19:29 test-zsq

#重新添加再测试，加上-c选项
[root@localhost ~]# rm -rf test-zsq
[root@localhost ~]# ln -s test test-zsq
[root@localhost ~]# ll -i test-zsq
260726 lrwxrwxrwx. 1 root root 4 Dec 22 19:33 test-zsq -> test
[root@localhost ~]# echo "test" >>test
[root@localhost ~]# sed -i -c '/test/d' test-zsq
[root@localhost ~]# ll -i test-zsq
260726 lrwxrwxrwx. 1 root root 4 Dec 22 19:33 test-zsq -> test

#--follow-symlinks选项
[root@localhost ~]# rm -rf test-zsq
[root@localhost ~]# ln -s test test-zsq
[root@localhost ~]# echo "test" >> test
[root@localhost ~]# cat test
test
[root@localhost ~]# sed -i --follow-symlinks '/test/d' test
[root@localhost ~]# ls -l test-zsq
lrwxrwxrwx. 1 root root 4 Dec 22 19:50 test-zsq -> test
[root@localhost ~]# cat test


--follow-symlinks比-c选项更快更安全
  -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.


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
有可能配置的链接文件失效而导致服务起不来


在centos 5.x系列中运行相同的操作没有出现类似的现象
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

[root@DNS ~]# sed --version
GNU sed version 4.1.5
Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE,
to the extent permitted by law.
[root@DNS ~]# sed --help
Usage: sed [OPTION]... {script-only-if-no-other-script} [input-file]...

  -n, --quiet, --silent
                 suppress automatic printing of pattern space
  -e script, --expression=script
                 add the script to the commands to be executed
  -f script-file, --file=script-file
                 add the contents of script-file to the commands to be executed
  -i[SUFFIX], --in-place[=SUFFIX]
                 edit files in place (makes backup if extension supplied)
  -c, --copy
                 use copy instead of rename when shuffling files in -i mode
         (avoids change of input file ownership)
  -l N, --line-length=N
                 specify the desired line-wrap length for the `l' command
  --posix
                 disable all GNU extensions.
  -r, --regexp-extended
                 use extended regular expressions in the script.
  -s, --separate
                 consider files as separate rather than as a single continuous
                 long stream.
  -u, --unbuffered
                 load minimal amounts of data from the input files and flush
                 the output buffers more often
      --help     display this help and exit
      --version  output version information and exit