Docker Exec Format Error

When working with Docker containers, you might come across the “exec format error” when attempting to run a command inside a container. This error mainly occurs when you try to run a specific instruction in the container.

In this tutorial, we will learn about the Docker “exec format error”, why it occurs, and how you can fix it.

What Causes the Docker Exec Format Error?

Although the cause of the exec format error may vary depending on your container configurations, the commands that you wish to execute, the Dockerfile layout, and more, this error occurs when Docker is unable to find the correct binary format for the executed command.

The main cause of this is when you attempt to execute a binary with mismatched architectures for your container.

Other Causes:

There are other reasons that can lead to the Docker “exec format error”. Some of the reasons why this might occur include:

1. Missing Shebang (#!/bin/bash) – If the script or binary lacks a proper shebang (e.g., #!/bin/bash for a shell script), Docker may not be able to determine how to execute it.
2. File Corruption – In other cases, file corruption can lead to the Docker “exec format error”. Ensure that the file that you wish to execute is not damaged.

How to Fix the Exec Format Error

The main and most common method of fixing the Docker “exec format error” is by adding a shebang on the entrypoint of your script. The shebang is basically a special set of characters in a script file that tells the system which program we wish to use to execute the script.

The shebang is the first line in a script and is made up of “#!” followed by the path to the binary that you wish to use.

Once the kernel encounters shebang, it automatically uses the specified binary to run all the other commands that are defined in the script.

Most popular shebangs include:

Bash Script (Linux/Unix Shell)

Python 3

Python 2

Perl

Ruby

Node.js (JavaScript)

PHP

Java

Go

Rust

Choose one that suits your needs and set it as the execution program for your script.

You can also tell Docker the entrypoint of your script in the “docker run” command as follows:

This should execute the specified command with the target binary.

Conclusion

We looked at the cause and potential solutions to the Docker “exec format error” when running a command inside a Docker container.