The RUN instruction

Learn to run commands at build time with RUN.

The RUN instruction

RUN runs a command during the build and adds the result to the image as a new layer. It is used for installing packages, compiling and creating directories:

FROM ubuntu:24.04
RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

update and install must be in the same RUN; written separately, the install may use a stale cached package list. The trailing rm shrinks the layer by deleting the package lists.

Layer count and size

Each RUN is a separate layer. A file downloaded in one layer and deleted in the next doesn't shrink the image, since the earlier layer doesn't change. That is why installing and cleaning up are done in the same RUN:

# bad: 3 layers, cache files stay in the image
RUN apt-get update
RUN apt-get install -y curl
RUN rm -rf /var/lib/apt/lists/*

# good: a single layer
RUN apt-get update \
    && apt-get install -y --no-install-recommends curl \
    && rm -rf /var/lib/apt/lists/*

Exercise Installing in one layer

Write a Dockerfile based on ubuntu:24.04; chain apt-get update, apt-get install -y curl git and rm -rf /var/lib/apt/lists/* with && in a single RUN instruction.

Show example solution

FROM ubuntu:24.04
RUN apt-get update && apt-get install -y curl git && rm -rf /var/lib/apt/lists/*

Test yourself

What is the difference between RUN and CMD?
- RUN only works on Linux
- CMD runs several times
- RUN runs during the build, CMD when the container starts
- They are the same
Show answer

RUN runs during the build, CMD when the container starts — RUN changes the image; CMD only records the default start command.

Install a package without asking for confirmation.

RUN apt-get update && apt-get install ____ curl

Show answer

RUN apt-get update && apt-get install -y curl

Match.

RUN

Adds a new layer
&&

Chains commands in one RUN
\ (end of line)

Continues the line
rm -rf /var/lib/apt/lists/*

Shrinks the layer

Next lesson: COPY and ADD

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