In 2022, a list of commonly used commands for primary operation and maintenance, practical landing scenarios + solutions, and 9 practical scenarios

This is a practical collection for junior operation and maintenance engineers. There is no complicated list, only various scenarios and corresponding solutions in actual combat.
The following commands are based on the CentOS operating system, and the command lines of different systems are slightly different. It is recommended to compare and study.

To check the system version, use the following command

cat /etc/redhat-release

The result is as follows, CentOS 7.5 distribution

CentOS Linux release 7.5.1804 (Core) 

Add the above cat command

catCommand: Generally used to view the content of the file, the format is as follows

cat [-AbeEnstTuv] [--help] [--version] fileName

Don't memorize a lot of it, generally used forcat 文件名即可

9+ scene

1. System firewall status

When deploying a Web site, it is often necessary to check the status of the system firewall and check the open port number of the server. The relevant commands are as follows

Check the system firewall
The command line is as follows

systemctl status firewalld

The results obtained note the green and red areas

Turn off the firewall
The command line is as follows

systemctl stop firewalld

After closing, you can use the above view status command again to get the status of the firewall at this time as follows

inactive (dead)

Restart the firewall
The command line looks like this

systemctl start firewalld

Here you can expand the knowledge point of learning is the systemctlcommand , the specific learning direction is as follows

Commands related to managing system startup and managing system services are systemctlimplemented .

• systemctl start [unit type]:start up;
• systemctl stop [unit type]:stop;
• systemctl restart [unit type]: restart;
• systemctl status[unit type]: View status;
• systemctl reload [unit type]:Reload;

2. System ports

As a back-end operation and maintenance, you will encounter a lot of scenarios and need to check the open services and ports in the system. The command line is as follows

netstat -ntlp

The command to learn here is netstat, if you don't have it, use yum install -y net-toolsto install it.

netstatThe common parameters of the command are described as follows:

• -a:all;
• -t: Display the TCP port;
• -u: Display the UDP port;
• -p: Display the process identifier and program name;
• -l: show only listening socket programs;
• -n: Do not perform DNS polling, display IP.

At this point netstat -ntlp, it is easy to understand if you are looking at it. You can combine various commands for use.
Here, with the filter command, you can improve the efficiency of use.

netstat -ntulp |grep 80

The grep command is as follows

grep is used to perform keyword searches in text. For example, the query result of the above command is:

Similarly, you can modify it to grep 27get the following result

key parameter:

• -b: Search executable files (binaries) as text files;
• -c: only display the number of lines found;
• -i: ignore case;
• -n: display line number;
• -v: Inverse selection, listing lines without keyword.

For example, view python related port data

netstat -ntulp |grep python

View the specific situation of a port

lsof -i tcp:80

This involves a command called lsof(list open files), which is a tool used to list the current system open files

The parameters involve the following:

• -a: List the processes that have opened the file;
• -i<条件>: List eligible processes, which can be protocol, :port, @IP;

At this time, the following commands can be re-learned.
listen on port 22

lsof -i :22 

Listen on ports 1~1024

ls -i:1-1024 

3. Process related

In centos, you need to view the status of the processes running in the system. The relevant commands are as follows

View the detailed information of a process The
ps command is the most common process viewing tool in Linux systems. It is mainly used to display static snapshots containing complete information about the currently running processes. For example, the following commands

ps 8000

To view all processes, use the following command

ps aux

The command parameters are as follows:

• a: Display all process information under the current terminal, including processes of other users;
• u: User-dominated process status
• x: usually used together with parameter a to display the process information of the current user under all terminals

ps auxThere is another command similar to ps -ef, which is different from the previous command in the System V display style.

ps -ef

In addition to the pscommand , the topprocess will also be used. This command will display the process ranking in the current terminal in a full- screen interactive interface, and track the CPU, memory and other system resource usage in time. By default, it refreshes every 3 seconds, similar to Windows The task manager in the system.

Aborting the process
Viewing the process, killing the process, are all common operations, and the killing process uses the killcommand :

kill -9 8000

-9Indicates to force the process to stop immediately.

4. File permissions chmod

The chmod command is used to view and modify file permissions.

The following permissions exist in CentOS, and files have three identities:

• owner: file owner
• group: the same user group
• others: other non-user groups.
  When modifying permissions, the three identities of each file (folder is also a file) have three permissions respectively.
• read: read
• host：写
• execute: execute

Use ls -alto view the permissions of all files in the current directory, as shown in the following figure

The permission characters of the file are -rwxrwxrwx, each 3 is a group, and the corresponding identity is owner\group\others.

Permissions can be represented by numbers, among them r=4, w=2, x=1, -=0, and then look at 777 when setting permissions, is it easy to understand?

• The numbers for settings rwx (可读写运行）permission settings are 4 + 2 + 1 = 7;
• The numbers for settings rw- （可读写不可运行）permission settings are 4 + 2 = 6;
• The number of settings r-x （可读可运行不可写）permission settings is 4 +1 = 5.

The command format is as follows

chmod  xyz file文件名

The xyzcorresponding three permissions represent the permissions of User, Group, and Other.
There is another way of writing, as follows:

chmod 777 file  (相同的命令 chmod u=rwx,g=rwx,o=rwx file 或  chmod a=rwx file)

chomdNeeds more practice to use (but not very often)

The help manual for viewing commands in the CentOS system can be added later --help, for example

chmod --help
ps --help

5. Create soft links

Soft links (similar to shortcuts in Windows operating systems) can be configured in CentOS.

Create soft links

ln  -s  [源文件或目录]  [目标文件或目录]

remove soft link

rm -rf /home/python

Don't add it at the end, /otherwise it will delete the file (running with a bucket)

6. Search related

which command
which is used to search the path and alias of the command

The syntax of the whereis command is as follows

whereis [option] name

Find the path to the executable, source code files of a specific program

The syntax of the find command is as follows

find [搜索范围] [搜索条件]

The locate command
locate command find -nameis another way of writing, but it is faster than the latter because it does not search a specific directory, but a database /var/lib/locatedbthat contains all local file information.

Before use, you need to install locate first. The installation command is as follows:

yum -y install mlocate

The following error will occur during the first execution, which needs to be configured

[root@xxxx ~]# locate install.sh 
locate: can not stat () `/var/lib/mlocate/mlocate.db': No such file or directory

Execute the updatedbcommand and locate it again.

7. RAM and HDD

During the operation and maintenance time, it is often necessary to check the memory and hard disk usage. Use the following command
top command
. As mentioned above, you can use to qexit .

View memory usage

free -m

The meaning of each column of the output is

• used(used memory);
• free(free memory);
• buff/cache (buffer cache/page cache);

Hard disk usage

df -h

8. Run commands without hanging up

Use nohup(no hang up) to run the program without hanging up. Note that there is no background running function. The command syntax is as follows

nohup command [arg...] [&]

The &last means to run in the background, but when the user exits (suspends), the command exits automatically.

The case is as follows:

nohup command  > file.log 2>&1 &   

• 0 – stdin (standard input) standard input
• 1 – stdout (standard output) Standard output
• 2 – stderr (standard error) standard error output

2>&1is to redirect standard error (2) to standard output (&1), and standard output (&1) is redirected file.logto .

9. Other small commands

pwd command to
query the path of the user's location

cd command to
change directory

mkdir command to
create directory files

tree command
displays the directory structure in a tree structure

cp command
Copy command

mv command
move command

touch command to
create file or update file timestamp

stat command to
view file attributes

tar command to
compress and package

power on command

reboot # 重启机器
poweroff # 关闭系统

record time

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