Combining RUN commands in Docker - Step-by-Step Execution

Process Flow - Combining RUN commands

Start Dockerfile

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RUN command 1

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RUN command 2

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RUN command 3

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Single image layer created

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Next Dockerfile instruction

Docker executes combined RUN commands in one layer by chaining them with && or \ to reduce image layers.

Execution Sample

Docker

RUN apt-get update && apt-get install -y curl \
    && echo "Installation done"

This RUN command updates packages, installs curl, and prints a message in one layer.

Process Table

Step	Command Part	Execution	Result	Layer Created
1	apt-get update	Runs package list update	Success: package lists updated	No
2	apt-get install -y curl	Installs curl package	Success: curl installed	No
3	echo "Installation done"	Prints message	Output: Installation done	Yes

💡 All commands combined in one RUN, so one image layer created after last command

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
Image Layers	0	0	0	1
Packages Updated	No	Yes	Yes	Yes
Curl Installed	No	No	Yes	Yes
Output Message	None	None	None	Installation done

Key Moments - 2 Insights

Why do we use && or \ to combine RUN commands?

Using && or \ combines commands into one RUN instruction, creating only one image layer as shown in execution_table step 3, which keeps the image smaller.

What happens if one command fails in the combined RUN?

If any command fails, the whole RUN fails and the image build stops, because commands are chained with && which stops on failure (see execution_table steps).

Visual Quiz - 3 Questions

Look at the execution table, how many image layers are created after all RUN commands?

A One

B Two

C Three

D Zero

💡 Hint

Check the 'Layer Created' column in the execution_table rows.

At which step is curl installed according to the execution table?

A Step 1

B Step 3

C Step 2

D None

💡 Hint

Look at the 'Command Part' and 'Result' columns for curl installation.

If the first command fails, what happens to the image build?

A It continues to next commands

B It stops immediately

C It skips the failed command

D It creates a partial layer

💡 Hint

Recall that commands are chained with && which stops on failure, as explained in key_moments.

Concept Snapshot

RUN commands in Dockerfile can be combined using && or \ to run multiple commands in one layer.
This reduces image size by creating fewer layers.
If any command fails, the whole RUN fails and build stops.
Use backslash \ for line continuation for readability.
Example: RUN cmd1 && cmd2 && cmd3

Full Transcript

In Dockerfiles, combining RUN commands means chaining multiple commands with && or using backslash \ to write them on multiple lines. This way, Docker executes all commands in one RUN instruction, creating a single image layer. This keeps the image smaller and more efficient. The execution table shows each command running step by step, with the final layer created after all succeed. If any command fails, the build stops immediately. This method is common to optimize Docker images.