Execute in a Docker Container

The short answer

In Docker, to execute a command inside a container running in detached mode (i.e., the background), you can use the docker exec command as follows:

$ docker exec <container> <command>

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Where:

• container is the name or the identifier of a container.
• command is the command you want to run inside of the container.

For example, the following command will execute the ls command inside of the container identified by the name http-server and write the output of the command to the standard output of the terminal:

$ docker exec http-server ls

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Easily retrieve this command using Warp’s AI Command Suggestions

If you’re using Warp as your terminal, you can easily retrieve this command using the Warp AI Command Suggestions feature:

Entering the execute command in docker container in the AI Command Suggestions will prompt a docker command that can be quickly inserted into your shell by pressing CMD+ENTER.

Executing commands as a non-root user

By default, the specified command will be executed inside of the container as the root user.

To execute a command as a non-root user, you can use the docker exec command with the -u flag (short for --user) as follows:

$ docker exec -u >username< -it >container< >command<

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Where:

• username is the username or UID of the user the command will be executed as in the form of <name|uid>[:<group|gid>].

For example, the following command will execute the ls command inside of the container named ubuntu as the johndoe user:

 $ docker exec -u johndoe ubuntu ls

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Executing commands in a specific directory

By default, the specified command will be executed inside of the container in the home directory of the user specified through the -u flag, or by default, in the root directory (i.e., /).

To change the working directory the command will be executed in, you can use the -w flag (short for --workdir) as follows:

 $ docker exec -w <path> <container> <command>

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Where:

• path is the absolute path of the working directory.

For example, the following command will output the content of the 20240321.log file located in the /app/data/logs/ directory using the cat command in the container identified by the ID 729fae70158c:

 $ docker exec -w /app/data/logs 729fae70158c cat 20240321.log

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Starting an interactive shell session

To facilitate the execution of commands in a container for debugging, performing administrative tasks, or simply testing, you can start an interactive shell session that provides direct access to the container's shell using the -it flags (short for --interactive and --tty) as follows:

 $ docker exec -it <container> <shell>

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Where:

• -it is used to interactively execute commands inside a container.
• shell is the absolute path to a shell binary installed in the container.

For example, the following command will start an interactive shell session using Bash in the container named ubuntu:

$ docker exec -it ubuntu /bin/bash

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You can learn more about containers and Bash by reading our other article on how to run a Bash shell in Docker.

Using shell expansions

By default, any shell expansions specified in the command executed by the docker exec command will be expanded by the shell the command is executed from on the local machine.

For example, when running the following command, the ~ expression will be expanded into the path of the user's home directory on the local machine:

$ docker exec 729fae70158c ls ~

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To get around this problem, you can pass your command as sub-command of the /bin/bash -c command as follows:

$ docker exec <container> /bin/bash -c '<command>'

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For example, when running the following command, the ~ expression will be expanded into the path of the johndoe user's home directory (i.e., /home/johndoe) in the container:

$ docker exec -u johndoe 729fae70158c /bin/bash -c 'ls ~'

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Executing a command with temporary environment variables

By default, the commands executed through the docker exec command inherit the environment variables set at the time the container is created.

To override existing environment variables or set additional ones, you can use the -e flag (short for --env) as follows:

$ docker exec -e <variable>=<value> [-e <variable>=<value> …] <container> <command>

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Where:

• variable is the name of the environment variable.
• value is the value assigned to that variable.

For example, the following command will set the NODE_ENV environment variable as development and execute the node index.js command in the /app directory of the container named web-server:

$ docker exec -w /app -e NODE_ENV=development web-server node index.js

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Using an environment file

Alternatively, you can use the --env-file flag to pass a file containing a list of environment variables as follows:

$ docker exec --env-file <path> <container> <command>

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Where:

• path is the relative or absolute path to the environment file.

For example, the following command will use the variables specified in the .env.dev file located in the current working directory and execute the npm run test command in the /app directory of the container named web-server:

$ docker exec -w /app --env-file .env.dev web-server npm run test

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If you want to save the changes made to a container using the docker exec command, you can read our other article on how to save a Docker container as an image.