• Restart Docker Daemon Ubuntu 20.04
• Restart Docker Engine Ubuntu
• Docker Service Restart Ubuntu
• Restart Docker Command Ubuntu

Docker is an application that simplifies the process of managing application processes in containers. In this tutorial, you'll install and use Docker Community Edition (CE) on Ubuntu 18.04. Docker is an application that simplifies the process of managing application processes in containers. In this tutorial, you'll install and use Docker Community Edition (CE) on Ubuntu 20.04.

Estimated reading time: 11 minutes

After successfully installing and starting Docker, the dockerd daemonruns with its default configuration. This topic shows how to customizethe configuration, start the daemon manually, and troubleshoot and debug thedaemon if you run into issues.

Start the daemon using operating system utilities

On a typical installation the Docker daemon is started by a system utility,not manually by a user. This makes it easier to automatically start Docker whenthe machine reboots.

The command to start Docker depends on your operating system. Check the correctpage under Install Docker. To configure Dockerto start automatically at system boot, seeConfigure Docker to start on boot.

Start the daemon manually

If you don’t want to use a system utility to manage the Docker daemon, orjust want to test things out, you can manually run it using the dockerdcommand. You may need to use sudo, depending on your operating systemconfiguration.

When you start Docker this way, it runs in the foreground and sends its logsdirectly to your terminal.

To stop Docker when you have started it manually, issue a Ctrl+C in yourterminal.

Configure the Docker daemon

There are two ways to configure the Docker daemon:

• Use a JSON configuration file. This is the preferred option, since it keepsall configurations in a single place.
• Use flags when starting dockerd.

You can use both of these options together as long as you don’t specify thesame option both as a flag and in the JSON file. If that happens, the Dockerdaemon won’t start and prints an error message.

To configure the Docker daemon using a JSON file, create a file at/etc/docker/daemon.json on Linux systems, or C:ProgramDatadockerconfigdaemon.jsonon Windows. On MacOS go to the whale in the taskbar > Preferences > Daemon > Advanced.

Here’s what the configuration file looks like:

With this configuration the Docker daemon runs in debug mode, uses TLS, andlistens for traffic routed to 192.168.59.3 on port 2376.You can learn what configuration options are available in thedockerd reference docs

You can also start the Docker daemon manually and configure it using flags.This can be useful for troubleshooting problems.

Here’s an example of how to manually start the Docker daemon, using the sameconfigurations as above:

You can learn what configuration options are available in thedockerd reference docs, or by running:

Many specific configuration options are discussed throughout the Dockerdocumentation. Some places to go next include:

Docker daemon directory

The Docker daemon persists all data in a single directory. This tracks everythingrelated to Docker, including containers, images, volumes, service definition,and secrets.

By default this directory is:

• /var/lib/docker on Linux.
• C:ProgramDatadocker on Windows.

You can configure the Docker daemon to use a different directory, using thedata-root configuration option.

Since the state of a Docker daemon is kept on this directory, make sureyou use a dedicated directory for each daemon. If two daemons share the samedirectory, for example, an NFS share, you are going to experience errors thatare difficult to troubleshoot.

Troubleshoot the daemon

You can enable debugging on the daemon to learn about the runtime activity ofthe daemon and to aid in troubleshooting. If the daemon is completelynon-responsive, you can alsoforce a full stack trace of allthreads to be added to the daemon log by sending the SIGUSR signal to theDocker daemon.

Restart Docker Daemon Ubuntu 20.04

Troubleshoot conflicts between the daemon.json and startup scripts

If you use a daemon.json file and also pass options to the dockerdcommand manually or using start-up scripts, and these options conflict,Docker fails to start with an error such as:

If you see an error similar to this one and you are starting the daemon manually with flags,you may need to adjust your flags or the daemon.json to remove the conflict.

Note: If you see this specific error, continue to thenext section for a workaround.

If you are starting Docker using your operating system’s init scripts, you mayneed to override the defaults in these scripts in ways that are specific to theoperating system.

Use the hosts key in daemon.json with systemd

One notable example of a configuration conflict that is difficult to troubleshootis when you want to specify a different daemon address fromthe default. Docker listens on a socket by default. On Debian and Ubuntu systems using systemd,this means that a host flag -H is always used when starting dockerd. If you specify ahosts entry in the daemon.json, this causes a configuration conflict (as in the above message)and Docker fails to start.

To work around this problem, create a new file /etc/systemd/system/docker.service.d/docker.conf withthe following contents, to remove the -H argument that is used when starting the daemon by default.

There are other times when you might need to configure systemd with Docker, such asconfiguring a HTTP or HTTPS proxy.

Restart Docker Engine Ubuntu

Note: If you override this option and then do not specify a hosts entry in the daemon.jsonor a -H flag when starting Docker manually, Docker fails to start.

Run sudo systemctl daemon-reload before attempting to start Docker. If Docker startssuccessfully, it is now listening on the IP address specified in the hosts key of thedaemon.json instead of a socket.

Important: Setting hosts in the daemon.json is not supported on Docker Desktop for Windowsor Docker Desktop for Mac.

Out Of Memory Exceptions (OOME)

If your containers attempt to use more memory than the system has available,you may experience an Out Of Memory Exception (OOME) and a container, or theDocker daemon, might be killed by the kernel OOM killer. To prevent this fromhappening, ensure that your application runs on hosts with adequate memory andseeUnderstand the risks of running out of memory.

Read the logs

The daemon logs may help you diagnose problems. The logs may be saved in one ofa few locations, depending on the operating system configuration and the loggingsubsystem used:

Operating system	Location
RHEL, Oracle Linux	/var/log/messages
Debian	/var/log/daemon.log
Ubuntu 16.04+, CentOS	Use the command journalctl -u docker.service or /var/log/syslog
Ubuntu 14.10-	/var/log/upstart/docker.log
macOS (Docker 18.01+)	~/Library/Containers/com.docker.docker/Data/vms/0/console-ring
macOS (Docker <18.01)	~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/console-ring
Windows	AppDataLocal

Enable debugging

There are two ways to enable debugging. The recommended approach is to set thedebug key to true in the daemon.json file. This method works for everyDocker platform.

1. Edit the daemon.json file, which is usually located in /etc/docker/.You may need to create this file, if it does not yet exist. On macOS orWindows, do not edit the file directly. Instead, go toPreferences / Daemon / Advanced.

2. If the file is empty, add the following:

   If the file already contains JSON, just add the key 'debug': true, beingcareful to add a comma to the end of the line if it is not the last linebefore the closing bracket. Also verify that if the log-level key is set,it is set to either info or debug. info is the default, and possiblevalues are debug, info, warn, error, fatal.

3. Send a HUP signal to the daemon to cause it to reload its configuration.On Linux hosts, use the following command.

   On Windows hosts, restart Docker.

Instead of following this procedure, you can also stop the Docker daemon andrestart it manually with the debug flag -D. However, this may result in Dockerrestarting with a different environment than the one the hosts’ startup scriptscreate, and this may make debugging more difficult.

Force a stack trace to be logged

If the daemon is unresponsive, you can force a full stack trace to be loggedby sending a SIGUSR1 signal to the daemon.

• Linux:

• Windows Server:

  Download docker-signal.

  Get the process ID of dockerd Get-Process dockerd.

  Run the executable with the flag --pid=<PID of daemon>.

This forces a stack trace to be logged but does not stop the daemon.Daemon logs show the stack trace or the path to a file containing thestack trace if it was logged to a file.

The daemon continues operating after handling the SIGUSR1 signal anddumping the stack traces to the log. The stack traces can be used to determinethe state of all goroutines and threads within the daemon.

View stack traces

The Docker daemon log can be viewed by using one of the following methods:

• By running journalctl -u docker.service on Linux systems using systemctl
• /var/log/messages, /var/log/daemon.log, or /var/log/docker.log on olderLinux systems

Note: It is not possible to manually generate a stack trace on Docker Desktop forMac or Docker Desktop for Windows. However, you can click the Docker taskbar icon andchoose Diagnose and feedback to send information to Docker if you run intoissues.

Look in the Docker logs for a message like the following:

The locations where Docker saves these stack traces and dumps depends on youroperating system and configuration. You can sometimes get useful diagnosticinformation straight from the stack traces and dumps. Otherwise, you can providethis information to Docker for help diagnosing the problem.

Check whether Docker is running

The operating-system independent way to check whether Docker is running is toask Docker, using the docker info command.

Docker Service Restart Ubuntu

You can also use operating system utilities, such assudo systemctl is-active docker or sudo status docker orsudo service docker status, or checking the service status using Windowsutilities.

Finally, you can check in the process list for the dockerd process, usingcommands like ps or top.

Restart Docker Command Ubuntu

docker, daemon, configuration, troubleshooting