dockerfile RUN echo multiline and with variables

Question

I have something like this in my Dockerfile

docker-compose.yml

version: '3.7'
services:   
  backuppc-app:
    image: tiredofit/backuppc
    container_name: backuppc-app
    build:
      context: .
      dockerfile: ./Dockerfile
    volumes:
      - /mnt/TOSHIBA-BACKUP/backuppc/data:/var/lib/backuppc
      - /mnt/TOSHIBA-BACKUP/backuppc/conf/etc/:/etc/backuppc
      - /mnt/TOSHIBA-BACKUP/backuppc/conf/home/:/home/backuppc
    ports:
      - "8081:80"
      - "8082:10050"
    environment:
      - BACKUPPC_UUID=1000
      - BACKUPPC_GUID=1000
      - SMTP_HOSTNAME=${SMTP_HOSTNAME}
      - SMTP_PORT=${SMTP_PORT}
      - SMTP_USERNAME=${SMTP_USERNAME}
      - SMTP_PASSWORD=${SMTP_PASSWORD}
    restart: always

Dockfile

FROM tiredofit/backuppc

RUN echo $'defaults \n\
auth on \n\
tls on \n\
account gmail \n\
host $SMTP_HOSTNAME \n\
port $SMTP_PORT \n\
user $SMTP_USERNAME \n\
from $SMTP_USERNAME \n\
password $SMTP_PASSWORD \n\
account default : gmail \n' > /root/.msmtprc

Checking into /root/.msmtprc variables not evaluated.. tryed many tests.. using ${VAR_NAME} doesn't work. Using double quotes variables are evaluated but carriage return not.

Also when i try to write that string into /home/backuppc/.msmtprc file is not overwritten

-rw-r--r--    1 root     backuppc       200 Jan 17 11:19 .msmtprc

Does this answer your question? [docker-compose build environment variable](https://stackoverflow.com/questions/52429984/docker-compose-build-environment-variable) — blami, Jan 17 '21 at 11:56

EDG956 · Answer 1 · 2021-01-18T10:36:10.487

To counter the fact that the shell is ignoring the \n when using double quotes and that using single quotes performs no variable expansion, you can create a variable to hold the newline character and expand it in a string surrounded by double quotes.

The following should work:

RUN nl=$'\n' && echo "\
foobar $nl\
$YOUR_VAR $nl\
hello world" > file.txt

After building the image, you should have the contents of file with the newlines as you wish.

Adding on @blami's comment, you should pass these variables as args to your build section in your docker-compose and declare them in your Dockerfile before using them with ARG var_name, like the link he posted recommends.

So at the end, you should have a Dockerfile similar to:

FROM tiredofit/backuppc

ARG YOUR_VAR

RUN nl=$'\n' && echo "\
first line $nl\
$YOUR_VAR $nl\
third line " > file.txt

You can try to run now

> docker build . -t test --build-arg YOUR_VAR="second line"
> docker run test cat file.txt

And get something like

first line
second line
third line

As for the docker-compose, I haven't really tried this, but I think it should look something like this:

version: '3.7'
services:   
  backuppc-app:
    container_name: backuppc-app
    image: tiredofit/backuppc
    build:
      context: .
      dockerfile: ./Dockerfile
      args:
         - YOUR_VAR=${YOUR_VAR}
...

Assuming you've set the YOUR_VAR environment variable.

@AugustoMurri I have updated my answer. Have you tried that? I got it working with a Dockerfile starting from the same image and the docker build / docker run way, but didn't try the full case. — EDG956, Jan 18 '21 at 10:24

dockerfile RUN echo multiline and with variables

1 Answers1