Combining RUN commands in Docker - Commands & Configuration

Introduction

When building Docker images, each RUN command creates a new layer. Combining multiple commands into one RUN reduces image size and speeds up builds by minimizing layers.

When you want to install multiple packages in one step to keep the image small

When you need to run several setup commands that depend on each other

When you want to clean temporary files after installation in the same step

When you want to reduce the number of layers to improve build speed

When you want to keep your Dockerfile clean and efficient

Config File - Dockerfile

Dockerfile

FROM ubuntu:22.04

RUN apt-get update && apt-get install -y \
    curl \
    git \
    && rm -rf /var/lib/apt/lists/*

CMD ["bash"]

FROM sets the base image.

RUN combines update, install, and cleanup commands using && and line continuation \ for readability.

This reduces image layers and removes temporary files to keep the image small.

CMD sets the default command to run a bash shell.

Commands

Builds the Docker image named 'combined-run-example' using the Dockerfile in the current directory. This runs the combined RUN commands in one layer.

Terminal

docker build -t combined-run-example .

Expected Output

[+] Building 5.3s (7/7) FINISHED

 => [internal] load build definition from Dockerfile

 => => transferring dockerfile: 123B

 => [internal] load .dockerignore

 => => transferring context: 2B

 => [internal] load metadata for docker.io/library/ubuntu:22.04

 => [1/3] FROM docker.io/library/ubuntu:22.04@sha256:...

 => CACHED [2/3] RUN apt-get update && apt-get install -y curl git && rm -rf /var/lib/apt/lists/*

 => [3/3] CMD ["bash"]

 => exporting to image

 => => exporting layers

 => => writing image sha256:...

 => => naming to docker.io/library/combined-run-example

→

-t - Assigns a name and optionally a tag to the image

Lists the Docker images filtered by the name 'combined-run-example' to verify the image was built.

Terminal

docker images combined-run-example

Expected Output

REPOSITORY             TAG       IMAGE ID       CREATED          SIZE

combined-run-example   latest    abcdef123456   10 seconds ago   88MB

Runs a container from the image and checks that curl and git are installed by printing their versions.

Terminal

docker run --rm combined-run-example bash -c "curl --version && git --version"

Expected Output

curl 7.81.0 (x86_64-pc-linux-gnu) libcurl/7.81.0 OpenSSL/3.0.2 zlib/1.2.11

Release-Date: 2022-03-30

Protocols: dict file ftp ftps gopher http https imap imaps pop3 pop3s rtsp smb smbs smtp smtps telnet tftp

Features: AsynchDNS HTTPS-proxy IPv6 Largefile NTLM NTLM_WB SSL TLS-SRP UnixSockets

git version 2.34.1

→

--rm - Automatically removes the container after it exits

Key Concept

If you remember nothing else from this pattern, remember: combining multiple commands in one RUN reduces image size and speeds up builds by creating fewer layers.

Common Mistakes

Writing multiple RUN commands separately instead of combining them

Each RUN creates a new layer, increasing image size and build time.

Combine commands with && in a single RUN to reduce layers and clean up temporary files in the same step.

Not cleaning up temporary files after installing packages

Temporary files remain in the image, making it larger than necessary.

Use commands like rm -rf /var/lib/apt/lists/* in the same RUN to remove temporary files.

Summary

Use one RUN command with && to combine multiple shell commands.

Cleaning up temporary files in the same RUN keeps the image small.

Fewer RUN commands mean fewer image layers and faster builds.