soft link
ln -s aaa bbbb
It can be understood as creating a shortcut named bbb in the aaa folder
hard link
ln aaa bbb
It can be understood that there are two identical folders or files on the hard disk, and the content between them is synchronized in real time
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contro+alt+f6 switch between graphical and command line pages
against + alt + f7
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ssh principle
In the Linux environment, it is very common for one host to remote another host.
The host sends a link request to the remote host, the remote host will send a public key, and the remote host encrypts the login password with the public key. It is decrypted by the remote host with its own private key and can log in if it is correct. Because the public key does not have a cs certificate issuing center like https, there is a man-in-the-middle attack, that is, when a host sends a request to a remote host, it is intercepted by an intermediate third-party host. Then return a public key to the host. The host thinks that it is still connected to the remote host, and then enters the password of the remote host. At this time, the third-party host knows the password of the remote host. For the above phenomenon of the password, the remote host will publicize the public key, but because it is too long to be recognized, it is encrypted with md5 and publicized in order to identify the correctness of the public key.
In addition, it is possible to remember the password after entering the password once when connecting to the remote host. The host will generate a pair of secret keys, and mainly put the public key in the trusted public key store of the remote host. When the host is on the remote host, the remote host first sends a string of characters, the host encrypts it with its own private key and returns it to the remote host, and the remote host decrypts it with the host's public key. If the characters are found to be the same as when sent, the host can be identified. Province, you can log in without a password.
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Remote file copy command (-r means to iterate subdirectories)
scp -r /etc/xx user@jiqiming:/etc