 | 来源：Linux系统中文网
ubunt
1,开机到grub时，用上下键移到第二行的恢复模式，按e（注意不是回车）
即Ubutn，kernel 2.6.12-9-386（recovery mode）
2，把ro single 改成rw single init=/bin/bash 然后按ctrl+x
RHEL 6/centos 6
1. grub
进入启动画面之后，敲入“e”，把光标移动到kernel ...那一行，再敲入“e”，在kernel 一行的最后加上空格single，回车
敲入“b”，启动系统，即进入单用户模式，
这个时候就可以用passwd命令改密码了。
passwd root
改完以后reboot重启。
2. lilo
进入lilo命令行模式，敲入linux single，即进入单用户模式。
REHL7/Centos 7 (grub2)
If you have lost or forgot root password of RHEL7, you can follow this step to change to a new password, this is different with the RHEL6 single-usermode,
1, press e button in GRUB 2 boot screen.
2, at the end of Linux line add init=/bin/sh and remove 'rhgb' and 'quiet'
3, Ctrl+x to boot the system with the parameter
4, remount the file system as writable, run the mount -o remount, rw / command
5, passwd to change password.
6, make sure SELinux context of the files that were modified is restored, touch /.autorelabel
7, last step, resume the initialization and finish the system boot. exec /sbin/init
1. 添加init=/bin/sh 移除'rhgb' and 'quiet'
2. Ctrl+x
3. mount -o remount, rw /
4. passwd
5. touch /.autorelabel {selinux 开启时}
6. exec /sbin/init
7. 进入单用户显示乱码使用：LANG=zh-cn.utf8
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