During system operation and maintenance, it may be necessary to perform specific tasks at a time preset by someone.

Tasks such as sending emails regularly, backing up and clearing log files, etc.

The content of the task can be regarded as a series of commands or a script, and we need to execute it at a specific time.

Schedule execution, run once at a specific time: at

Executed regularly, multiple times per cycle: cron

The at command can specify Linux to run a script at a specific time

atd is the daemon process of at, which will run in background mode when the system starts.

The atd process regularly checks the /var/spool/at directory on the system to obtain tasks written through the at command.

systemctl status atd #status

Install and start:

at command

Set time - absolute timing

Set time - relative timing

execute permission

cron operating mechanism

crontab command

crontab file

Example: Execute the command every Monday at 8:00 am 00 08 * * 1 commands

Example: Every two hours between 11 pm and 8 am, add sleep) text to the /tmp/tet.txt file. The corresponding lines in the crontab file are as follows: * 23-8/2 * * * echo "sleepy" >> /tmp/test.txt

Each time after editing a user's cron settings, cron automatically generates a file with the same name as the user under /var/spool/cron. This user's cron information is recorded in this file. This file cannot be edited directly and can only be edited with crontab -e. Users can also create another file and use the "cron file name" command to import cron settings.

After cron starts, it reads the crontab file every one minute to check whether the commands in it need to be executed. Therefore, there is no need to restart the cron service after the file is modified.

Edit configuration file

equipment

broadcast address

interface

subnet mask

routing

link

The configuration path recognized by the network device: /etc/sysconfig/network-scripts/

Configuration file: ifcfg-*

ls -l /etc/sysconfig/network-scripts/

Use the editor to modify the configuration file directly, and make a backup before modifying it.

After the configuration is modified, it will not take effect immediately. You need to restart the NetworkManger service process or restart the system.

systemctl reload NetworkManager

Use the ss command to view socket statistics

ss-ta

NetworkManager is a daemon that dynamically controls and configures the network in the system. It is used to keep the current network devices and connections in working condition.

NetworkManager can be controlled through the command line tool nmcli, which is comprehensive, powerful and complex

Key concepts

nmcli connection show --active

Display details of a single connection: ncmcli con show "connection name", similar to the ip link command

To display device status, use: ncmcli dev status

To display device information, use: ncmcli dev show [device name]

Create connection via nmcli

Modify connection through nmcli

Routing

route

nmcli configure static routing

The host name is the name that uniquely identifies a machine in a local area network; this machine is not necessarily a physical machine, it can also be a virtual machine

cat /etc/hostname

Set hostname

There are some hosts in the LAN, and we can access them through IP addresses. As the number of hosts in the LAN increases, the P address is difficult to remember. We hope to directly access other hosts through the host name. At this time, we can record the host through a Find these hosts in the table of names and IP addresses. This table is hosts.

cat /etc/hosts

As the number of hosts in the network increases, it is difficult for a single hosts file to carry numerous mapping relationships.

When the IP corresponding to the domain name cannot be found in hosts, the host will submit the domain name to the domain name resolution server (DNS), and then the DNS returns the IP to the host. This process is called: domain name resolution

DNS is like a public hosts file/distributed database

nslookup exam.openEuler.com

Process is the entity of a running program in the computer and is a specific implementation of the program.

When each Linux process is created, it is assigned a memory space, that is, the system allocates a certain logical address space to the process.

Each incoming process has a unique process D (PID), which is used to track the process

Any process can create a child process by copying its own address space (fork). The child process records the ID of the parent process (PPID).

The first system process is systemd, all other processes are its descendants

Running status (TASKRUNNING)

Interruptible sleep state (light sleep) (TASKINTERRUPTIBLE)

Uninterruptible sleep state (deep sleep state) (TASKUNINTERRUPTIBLE)

Paused state (TASKSTOPPED)

TASK ZOMBIE

The CPU resource (time slice) allocation of a process refers to the priority of the process (priority)

Processes with higher priority have priority execution rights

Configuring the priority of the process is very useful for Linux in a multi-tasking environment and can improve system performance.

Use the ps command to view the process ps -l

Process priority is determined by dynamic priority (PRI) and static priority (PR), which is a number that determines the order of processes in the CPU. The higher the priority process, the greater the chance of being executed by the processor.

Based on the behavior of the process, the kernel uses heuristics to decide whether to turn dynamic priority on or off. You can directly modify the static priority of the process through the nice level. The process with a higher static priority will get a longer time slice (the time slice is the execution time of the process in the processor)

Linux supports nice levels from 19 (lowest priority) to -20 (highest priority), and the default is just 0. Only root users can adjust the nice level of a process to a negative number (giving it a higher priority)

Nice command syntax, priority (adjustment value range -19 ~ 20): nice [-n adjustment] [command [arg..]]

For example, to set the priority of vi running to -18: nice -n -18 vi &

The syntax of the renice command is the same as that of nice. The target objects are program group-g, process-p, and user-u. The syntax format is: renice [-n adjustment] [-] <pid>

For example, set the priority of vi running to 12: renice -n 12 -p 9700

View the nice description: nice-help; view the renice description: renice-h

You can also use top to change the NI value of the process: Press "r" after entering top -> Enter the process PID -> Enter the nice value

The background process hardly interacts with the user and has a slightly lower priority.

The Linux daemon is a special background process that is independent of the terminal and periodically performs tasks or waits for wake-up.

The foreground process interacts with users and requires higher response speed and higher priority.

The foreground process is the process that the user uses to control the terminal.

Guarding means not being controlled by the terminal.

Most Linux servers are implemented using daemon processes. For example, Internet server inetd, Web server httpd, etc. At the same time, the daemon process completes many system tasks. For example, job planning process crond, printing process lpd, etc.

Daemon processes are generally used as system services. You can use crotab to submit, edit or delete corresponding jobs.

Daemon process is also a daemon process. Most service processes in Linux are implemented through daemon processes. For example, process No. 0 (scheduling process), process No. 1 (process). It starts running when the machine is started and stops when it is turned off.

&: Put & at the end of the command to execute the command in the background

Ctrl z: When a command is being executed in the foreground, press Ctrl z to put it in the background and pause it.

Ctrl c: Interrupt the currently executing command

jobs: View how many commands are currently running in the background

fg: Move the command in the background to the foreground to continue running

bg: wake up the background task and run it in the background

Commonly used ps aux or bs-ef to view process information

The current user must be the owner of the process or the root user

kill sends a signal to the process through the PID. The TERM signal is sent by default. Use the -S parameter to specify other signals: #kill 3389; #kill-s HUP 3389

killall signals the process through the process name and supports wildcards, for example #killall python*