Docker is a containerization tool that helps developers create and manage portable, consistent Linux containers.
When developing or deploying containers you’ll often need to look inside a running container to inspect its current state or debug a problem. To this end, Docker provides the
docker exec command to run programs in containers that are already running.
In this tutorial we will learn about the
docker exec command and how to use it to run commands and get an interactive shell in a running Docker container.
This tutorial assumes you already have Docker installed, and your user has permission to run
docker. If you need to run
docker as the root user, please remember to prepend
sudo to the commands in this tutorial.
For more information on using Docker without
sudo access, please see the Executing the Docker Command Without Sudo section of our How To Install Docker tutorial.
Starting a Test Container
To use the
docker exec command, you will need a running Docker container. If you don’t already have a container, start a test container with the following
docker run command:
docker run -d --name container-name alpine watch "date >> /var/log/date.log"
We use the
-d flag to detach the container from our terminal and run it in the background.
--name container-name will name the container
container-name. You could choose any name you like here, or leave this off entirely to have Docker automatically generate a unique name for the new container.
Next we have
alpine, which specifies the image we want to use for the container.
And finally we have
watch "date >> /var/log/date.log". This is the command we want to run in the container.
watch will repeatedly run the command you give it, every two seconds by default. The command that
watch will run in this case is
date >> /var/log/date.log.
date prints the current date and time, like this:
OutputFri Jul 23 14:57:05 UTC 2021
>> /var/log/date.log portion of the command redirects the output from
date and appends it to the file
/var/log/date.log. Every two seconds a new line will be appended to the file, and after a few seconds it will look something like this:
OutputFri Jul 23 15:00:26 UTC 2021 Fri Jul 23 15:00:28 UTC 2021 Fri Jul 23 15:00:30 UTC 2021 Fri Jul 23 15:00:32 UTC 2021 Fri Jul 23 15:00:34 UTC 2021
In the next step we’ll learn how to find the names of Docker containers. This will be useful if you already have a container you’re targeting, but you’re not sure what its name is.
Finding the Name of a Docker Container
We’ll need to provide
docker exec with the name (or container ID) of the container we want to work with. We can find this information using the
docker ps command:
This command lists all of the Docker containers running on the server, and provides some high-level information about them:
OutputCONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 76aded7112d4 alpine "watch 'date >> /var…" 11 seconds ago Up 10 seconds container-name
In this example, the container ID and name are highlighted. You may use either to tell
docker exec which container to use.
If you’d like to rename your container, use the
docker rename command:
docker rename container-name new-name
Next, we’ll run through several examples of using
docker exec to execute commands in a running Docker container.
Running an Interactive Shell in a Docker Container
If you need to start an interactive shell inside a Docker Container, perhaps to explore the filesystem or debug running processes, use
docker exec with the
-i flag keeps input open to the container, and the
-t flag creates a pseudo-terminal that the shell can attach to. These flags can be combined like this:
docker exec -it container-name sh
This will run the
sh shell in the specified container, giving you a basic shell prompt. To exit back out of the container, type
exit then press
If your container image includes a more advanced shell such as
bash, you could replace
Running a Non-interactive Command in a Docker Container
If you need to run a command inside a running Docker container, but don’t need any interactivity, use the
docker exec command without any flags:
docker exec container-name tail /var/log/date.log
This command will run
tail /var/log/date.log on the
container-name container, and output the results. By default the
tail command will print out the last ten lines of a file. If you’re running the demo container we set up in the first section, you will see something like this:
OutputMon Jul 26 14:39:33 UTC 2021 Mon Jul 26 14:39:35 UTC 2021 Mon Jul 26 14:39:37 UTC 2021 Mon Jul 26 14:39:39 UTC 2021 Mon Jul 26 14:39:41 UTC 2021 Mon Jul 26 14:39:43 UTC 2021 Mon Jul 26 14:39:45 UTC 2021 Mon Jul 26 14:39:47 UTC 2021 Mon Jul 26 14:39:49 UTC 2021 Mon Jul 26 14:39:51 UTC 2021
This is essentially the same as opening up an interactive shell for the Docker container (as done in the previous step with
docker exec -it container-name sh) and then running the
tail /var/log/date.log command. However, rather than opening up a shell, running the command, and then closing the shell, this command returns that same output in a single command and without opening up a pseudo-terminal.
Running Commands in an Alternate Directory in a Docker Container
To run a command in a certain directory of your container, use the
--workdir flag to specify the directory:
docker exec --workdir /tmp container-name pwd
This example command sets the
/tmp directory as the working directory, then runs the
pwd command, which prints out the present working directory:
pwd command has confirmed that the working directory is
Running Commands as a Different User in a Docker Container
To run a command as a different user inside your container, add the
docker exec --user guest container-name whoami
This will use the guest user to run the
whoami command in the container. The
whoami command prints out the current user’s username:
whoami command confirms that the container’s current user is guest.
Passing Environment Variables into a Docker Container
Sometimes you need to pass environment variables into a container along with the command to run. The
-e flag lets you specify an environment variable:
docker exec -e TEST=sammy container-name env
This command sets the
TEST environment variable to equal
sammy, then runs the
env command inside the container. The
env command then prints out all the environment variables:
OutputPATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin HOSTNAME=76aded7112d4 TEST=sammy HOME=/root
TEST variable is set to
To set multiple variables, repeat the
-e flag for each one:
docker exec -e TEST=sammy -e ENVIRONMENT=prod container-name env
If you’d like to pass in a file full of environment variables you can do that with the
First, make the file with a text editor. We’ll open a new file with
nano here, but you can use any editor you’re comfortable with:
.env as the filename, as that’s a popular standard for using these sorts of files to manage information outside of version control.
KEY=value variables into the file, one per line, like the following:.env
Save and close the file. To save the file and exit
ENTER to save, then
CTRL+X to exit.
Now run the
docker exec command, specifying the correct filename after
docker exec --env-file .env container-name env
OutputPATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin HOSTNAME=76aded7112d4 TEST=sammy ENVIRONMENT=prod HOME=/root
The two variables in the file are set.
You may specify multiple files by using multiple
--env-file flags. If the variables in the files overlap each other, whichever file was listed last in the command will override the previous files.