Purpose of specifying several UNIX commands in a single RUN instruction in Dockerfile

Question

I have noticed that many Dockerfiles try to minimize the number of instructions by several UNIX commands in a single RUN instruction. So i

Answer 1

In addition to the space savings, it's also about correctness

Consider your first dockerfile (a common mistake when working with debian-like systems which utilize apt):

FROM ubuntu 
MAINTAINER demousr@example.com 

RUN apt-get update 
RUN apt-get install –y nginx 
CMD ["echo", "Image created"] 

If two or more images follow this pattern, a cache hit could cause the image to be unbuildable due to cached metadata

• let's say I built an image which looks similar to that ~a few weeks ago
• now I'm building this image today. there's a cache present up until the RUN apt-get update line
• the docker build will reuse that cached layer (since the dockerfile and base image are identical) up to the RUN apt-get update
• when the RUN apt-get install line runs, it will use the cached apt metadata (which is now weeks out of date and likely will error)