Detailed explanation of at and crontab commands of RHCE and deployment of Chrony

Catalog

1.at Configure in 5 Hours later, , take "This is a at task" write in root Home directory at_test In file

2.crontab To configure , Every Saturday , Sunday 9 Half past six ,14 spot Remind me to go RHCE course write in root Home directory cron_test In file

​3. On two Linux Upper Department chrony service ： requirement ： The first one synchronizes the time from Alibaba cloud server , The second synchronizes the time from the first

4. What is symmetric encryption , What is asymmetric encryption , What are the encryption algorithms of symmetric encryption , What encryption algorithms are needed for asymmetric encryption

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1.at Configure in 5 Hours later, , take "This is a at task" write in root Home directory at_test In file

2.crontab To configure , Every Saturday , Sunday 9 Half past six ,14 spot Remind me to go RHCE course write in root Home directory cron_test In file

​3. On two Linux Upper Department chrony service ： requirement ： The first one synchronizes the time from Alibaba cloud server , The second synchronizes the time from the first

First, there are two linux

The first

  The second one

  Then restart

4. What is symmetric encryption , What is asymmetric encryption , What are the encryption algorithms of symmetric encryption , What encryption algorithms are needed for asymmetric encryption

Symmetric encryption

Both encryption and decryption use the same key

Symmetric encryption algorithm

Symmetric encryption algorithms mainly include DES、3DES（TripleDES）、AES、RC2、RC4、RC5 and Blowfish etc. .

Asymmetric encryption

Encryption and decryption use different secret key Encryption algorithm

Asymmetric encryption algorithm

RSA、Elgamal、 Knapsack algorithm 、Rabin、D-H、ECC（ Elliptic curve encryption algorithm ）.