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CentOS

1. When system booting, press Space bar to interrupt the GRUB process;

2. Press "a" and ;

3. When see "rhgb quiet", add " 1" after "quiet" and press ;

Now CentOS boot in single user mode, you login as root, use passwd to reset password.

Mint

1. When system booting to the GRUB 2 menu, select the system entry you want to recover, press "e";

2. In the start script, find the line "linux ... ro quiet splash $vt_handoff";

3. Modify "ro" to the end of line to "rw init=/bin/bash";

4. Press Ctrl-x to continue system start;

5. Now you are in root shell, reset root password with passwd;

6. Press Ctrl-Alt-Del to restart machine;

Verified on Linux Mint 17.

Ref: Reset forgotten/lost main user password in Linux Mint 12+

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