RHCE's at, crontab's basic operations, the Chrony service, symmetric encryption and asymmetric encryption

a, First at There are many later time formats
at 4pm + 3 days
at 10am Jul 31
at 1am tomorrow
 HH:MM
midnight, noon,  teatime  (4pm) 
AM or PM
month-name day with an optional year
MMDD[CC]YY, MM/DD/[CC]YY, DD.MM.[CC]YY or [CC]YY-MM-DD
now + count time-units
minutes, hours,days,  or weeks
today,tomorrow

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

[[email protected] ~]# at now + 5 hours
warning: commands will be executed using /bin/sh
at> echo "This  is a at task" > /root/at_test
at> <EOT>
job 8 at Sun Jul 24 02:46:00 2022
[[email protected] ~]# atq
8	Sun Jul 24 02:46:00 2022 a root
[[email protected] ~]# date
Sat Jul 23 21:47:12 CST 2022

 

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

[[email protected] ~]# crontab -e
crontab: installing new crontab
[[email protected] ~]# crontab -l
0 9,14 * * 6,7 echo " It's time to go RHCE Class of " > /root/cron_test

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

1> First we need to download and install chrony, because chrony There are dependent software , So we can use yum download .

yum -y install chrony

2> After installation , We need to get into /etc/chrony.conf This file is used to configure

Our first one linux The server needs to synchronize time from Alibaba cloud server , Alibaba cloud time service center website is ntp.aliyun.com

3> After this, we need to modify other options , As shown in the figure below

4> Set boot up , Restart the service

systemctl enable chronyd
systemctl restart chronyd
Turn on network time synchronization
timedatectl set-ntp true

 

At this time, the first one has been completed

5> We also need another server , We can clone the first one , In this way, I have chrony Software  

 

6> The operation is the same as that of the first server , Yes /etc/chrony.conf Editing , Only need to ip Change to the first server ip Can

  Then is

Set boot up , Restart the service

systemctl enable chronyd
systemctl restart chronyd
Turn on network time synchronization
timedatectl set-ntp true

  At this point... Has been completed

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 ： Encryption and decryption use the same key

Symmetric encryption algorithm ：DES、3DES、AES、DESX、Blowfish、、RC4、RC5、RC6.

Asymmetric encryption ： The encryption and decryption keys are different , But it's a pair

Asymmetric encryption algorithm ：RSA、DSA（ For digital signature ）、ECC（ For mobile devices ）、Diffie-Hellman、El Gamal.