How to read docker logs

Docker is the most popular containerization technology now, and logs are an indispensable and important component in the running process of the container. Understanding how to view Docker logs plays a vital role in the normal operation of the container and troubleshooting. This article will introduce a general method to view the logs of Docker containers.

1. The log of the Docker container is mainly divided into two parts: standard output and standard error. Standard output (stdout) is used by the container to output normal information, including the application's standard output information and access log information. Standard error (stderr) is used by the container to output error information, including the application's error output information, exception information, etc.

2. Commonly used commands to view Docker logs

1. docker logs

This is the most commonly used method to view Docker logs. This command can view the latest log information of the standard output and standard error output of the specified container, as follows:

docker logs [OPTIONS] CONTAINER

OPTIONS Parameter description:

-f: Continuously output logs, displayed in reverse chronological order.

-t: Display log timestamp.

--tail [number]: Only display the information of the last number line, the default is all.

Example:

# 查看最后100行容器日志
docker logs --tail 100 container_name

# 持续查看容器日志
docker logs -f container_name

# 显示时间戳
docker logs -t container_name

2. journalctl

journalctl is a system log management tool that can view system logs, container logs, service logs, etc. This command can view the log information of the specified container name, as follows:

journalctl CONTAINER_NAME=container_id

Example:

# 查看所有系统日志
journalctl

# 查看指定docker容器日志
journalctl CONTAINER_NAME=mydocker

3. In some cases, the log of the container cannot be displayed, and you need to manually enter the container to view it. The method is as follows:

1. Enter the container through docker exec

You can enter the container through the docker exec command, and then view the log inside the container. As follows:

docker exec -ti container_name /bin/bash

2. View the log file inside the container

Docker outputs the container log to a file in the /var/log/ directory. You can view the container log through the command .

# 查看启动日志
sudo tail -f /var/log/syslog | grep "docker/container_name"

# 查看应用程序日志
sudo tail -f /var/log/docker/container_name/*.log

4. Summary

Viewing Docker container logs is an important function of Docker. When a problem occurs when a container is running, the problem can be located in time by viewing the container log. In daily operation and maintenance, we should actively use the log viewing commands provided by Docker, and remember to use appropriate methods to output container logs.

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