Editing Files from dockerfile

Question

I need to add several lines to /etc/sysctl.conf in a Docker image.

Is there an idempotent way to do this via a Dockerfile rather than editing manually and using the docker commit approach?

Someone already requested this: https://github.com/moby/moby/issues/12193 — Augustin Riedinger, Nov 28 '17 at 14:22

score 183 · Accepted Answer · edited May 23 '17 at 12:25

183

I would use the following approach in the Dockerfile

RUN   echo "Some line to add to a file" >> /etc/sysctl.conf

That should do the trick. If you wish to replace some characters or similar you can work this out with sed by using e.g. the following:

RUN   sed -i "s|some-original-string|the-new-string |g" /etc/sysctl.conf

However, if your problem lies in simply getting the settings to "bite" this question might be of help.

edited May 23 '17 at 12:25

Community

answered Dec 30 '14 at 23:27

wassgren

Can you also add a environment variable to this file: - RUN echo $ENV_VAR1 >> /etc/sysctl.conf – Righto Jan 10 '18 at 15:37
8

how to append content contains with newline. – tree em Feb 21 '18 at 04:06
4

what if I have to write more than one line, is writing echo that many number of times a good idea.? – viveksinghggits May 12 '19 at 12:52
this worked great for the one line I needed to add. To add on a new line I just put in \r\n ... so the example above would be RUN echo "\r\nSome line to add to a file" >> /etc/sysctl.conf – James Nelli Apr 08 '20 at 19:41
how can you use this in docker-compose.yml with `command: echo "line" >> /etc/file.conf` ? – Ing. Michal Hudak Apr 27 '21 at 19:22
1

What about multi-line? – Ken Ingram Apr 30 '21 at 18:39
@righto "Can you also add and environment variable?" You can use -e to set the environment as part of your docker run command. There's also a yaml syntax for a docker-compose.yml file to do the same thing at runtime. – Ed Greenberg May 04 '22 at 15:19
For multiline, you can do the `RUN echo` for each line or do `RUN echo -e 'line1\nline2' >> /path/file` – Timo Richter Mar 20 '23 at 13:57

score 17 · Answer 2 · edited Feb 10 '22 at 10:45

sed work pretty well to replace stuff, if you need to append, you can user double redirect

sed -i 's/origin text/new text/g' /etc/sysctl.conf
bash -c 'echo hello world' >> /etc/sysctl.conf

-i is a non-standard option of GNU sed for inline editing (alleviating the need for dealing with temporary files).

The s is the substitute command of sed for find and replace

The g means global replace i.e. find all occurrences of origin text and replace with new text using sed

score 1 · Answer 3 · answered Mar 14 '23 at 16:05

To complement the answers that already explain how you can append a single line, you can append multiple lines to a file like this:

RUN echo -e '\
Hello World!\n\
This is an example on how to\n\
append multiple lines to a file from within a Dockerfile.'\
>> /etc/sysctl.conf

Use double quotes if you want to expand variables.

3 Answers3