Munging Dockerfiles using sed

 So I had a requirement to update a Dockerfile, which I'd pulled from a GitHub repository, without actually adding my changes ( via git add and git commit ) to that repo ...

Specifically, I wanted to add a command to update the to-be-built Alpine image.

Here's how I solved it ...

Having cloned the target repo, which included the Dockerfile ( example below ): -

FROM alpine:3.7
RUN apk add --no-cache mysql-client
ENTRYPOINT ["mysql"]

At present, this is what happens when I build the image: -

docker build -f Dockerfile .

Sending build context to Docker daemon  2.048kB
Step 1/3 : FROM alpine:3.7
3.7: Pulling from library/alpine
5d20c808ce19: Pull complete 
Digest: sha256:8421d9a84432575381bfabd248f1eb56f3aa21d9d7cd2511583c68c9b7511d10
Status: Downloaded newer image for alpine:3.7
 ---> 6d1ef012b567
Step 2/3 : RUN apk add --no-cache mysql-client
 ---> Running in 07bc9ca0e14a
fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/community/x86_64/APKINDEX.tar.gz
(1/6) Installing mariadb-common (10.1.41-r0)
(2/6) Installing ncurses-terminfo-base (6.0_p20171125-r1)
(3/6) Installing ncurses-terminfo (6.0_p20171125-r1)
(4/6) Installing ncurses-libs (6.0_p20171125-r1)
(5/6) Installing mariadb-client (10.1.41-r0)
(6/6) Installing mysql-client (10.1.41-r0)
Executing busybox-1.27.2-r11.trigger
OK: 41 MiB in 19 packages
Removing intermediate container 07bc9ca0e14a
 ---> 43862371f8a4
Step 3/3 : ENTRYPOINT ["mysql"]
 ---> Running in d8b08c967cc1
Removing intermediate container d8b08c967cc1
 ---> 1ee30800ffbd
Successfully built 1ee30800ffbd

I wanted to add a command to run apk upgrade before the ENTRYPOINT entry ( so, in essence, inserting it between lines 2 and 3 )

This is what I did: -

sed -i '' -e "2s/^//p; 2s/^.*/RUN apk --no-cache upgrade/" Dockerfile

which results in: -

FROM alpine:3.7

RUN apk add --no-cache mysql-client

RUN apk --no-cache upgrade

ENTRYPOINT ["mysql"]

Now the build looks like this: -

docker build -f Dockerfile .

Sending build context to Docker daemon  2.048kB

Step 1/4 : FROM alpine:3.7

3.7: Pulling from library/alpine

5d20c808ce19: Pull complete 

Digest: sha256:8421d9a84432575381bfabd248f1eb56f3aa21d9d7cd2511583c68c9b7511d10

Status: Downloaded newer image for alpine:3.7

 ---> 6d1ef012b567

Step 2/4 : RUN apk add --no-cache mysql-client

 ---> Running in b29668e70377

fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/main/x86_64/APKINDEX.tar.gz

fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/community/x86_64/APKINDEX.tar.gz

(1/6) Installing mariadb-common (10.1.41-r0)

(2/6) Installing ncurses-terminfo-base (6.0_p20171125-r1)

(3/6) Installing ncurses-terminfo (6.0_p20171125-r1)

(4/6) Installing ncurses-libs (6.0_p20171125-r1)

(5/6) Installing mariadb-client (10.1.41-r0)

(6/6) Installing mysql-client (10.1.41-r0)

Executing busybox-1.27.2-r11.trigger

OK: 41 MiB in 19 packages

Removing intermediate container b29668e70377

 ---> 971a3d538edf

Step 3/4 : RUN apk --no-cache upgrade

 ---> Running in 8dfa10b481ad

fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/main/x86_64/APKINDEX.tar.gz

fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/community/x86_64/APKINDEX.tar.gz

(1/2) Upgrading musl (1.1.18-r3 -> 1.1.18-r4)

(2/2) Upgrading musl-utils (1.1.18-r3 -> 1.1.18-r4)

Executing busybox-1.27.2-r11.trigger

OK: 41 MiB in 19 packages

Removing intermediate container 8dfa10b481ad

 ---> 35d7cbec77c0

Step 4/4 : ENTRYPOINT ["mysql"]

 ---> Running in c0d7d310d396

Removing intermediate container c0d7d310d396

 ---> 4ad02b88f4d4

Successfully built 4ad02b88f4d4

In essence, the sed command duplicates line 2: -

RUN apk add --no-cache mysql-client

as line 3, and then replaces the newly duplicated text with the required replacement: -

RUN apk --no-cache upgrade

Neat, eh ?

I've got this in a script, and it works a treat .....