RHCE command practice (I)

RHCE Command practice （ One ）

One 、at command
at Configure in 5 Hours later, , take “This is a at task” write in root Home directory at_test In file

[[email protected] etc]# at now + 5 hours
warning: commands will be executed using /bin/sh
at> echo "This is a at task" >> /root/at_test<EOT>
job 1 at Sun Jul 24 02:58:00 2022

Two 、crontab command
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

[[email protected] etc]# crontab -e
30 9 * * 6-7 echo "Have a RHCE Lesson" >> /root/cron_test
0 14 * * 6-7 echo "Have a RHCE Lesson" >> /root/cron_test

3、 ... and 、chrony service
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
The first Linux：

[[email protected] etc]# systemctl enable chronyd
[[email protected] etc]# systemctl start chronyd
[[email protected] etc]# vim /etc/chrony.conf 
# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
#pool 2.rhel.pool.ntp.org iburst
server utp.aliyun.com iburst
[[email protected] etc]# systemctl restart chronyd

The second one Linux：

[[email protected] etc]# systemctl enable chronyd
[[email protected] etc]# systemctl start chronyd
[[email protected] etc]# vim /etc/chrony.conf 
# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
#pool 2.rhel.pool.ntp.org iburst
server 192.168.106.129 iburst     						# The first Linux Of ip
[[email protected] etc]# systemctl restart chronyd

Four 、 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 ： The key used for encryption and decryption is the same key （ Once the key is compromised , Then the overall information of all files using this key will be cracked ）
Commonly used algorithm ：DES、3DES、TDEA、Blowfish、RC2、RC4、RC5、IDEA、SKIPJACK

Asymmetric encryption ： Everyone will independently generate public key and private key . Give the public key to the party that needs encryption , The encrypted file can only be decrypted by the party holding the private key corresponding to the public key .
This is equivalent to the beginning A There is a safe with a lock （ It's not locked ）（ Public key ） And a corresponding key （ Private key ）, here B to A Ask for a safe （ Public key ）, other party B Put the document into the safe and lock it , Then hand it back , This lock only has A Corresponding key , It's just A To open . Even if it is intercepted in this process , That is, I just got the safe but couldn't open it to get the contents of the files .
Commonly used algorithm ：RSA、DSA、ECDSA