What exactly is echo '[local]\nlocalhost\n' > /etc?

Question

RUN echo '[local]\nlocalhost\n' > /etc/ansible/hosts

This line is in my dockerfile, and I assume it just means add localhost to the ansible/hosts file?

Can someone break down what [local]\nlocalhost\n is in a bit more detail?

Answer 1

This line tells Docker to run the shell command: echo '[local]\nlocalhost\n' > /etc/ansible/hosts

That shell command will print the following text block in the file /etc/ansible/hosts:

[local]
localhost

It does this because \n is the "newline" character, signaling the beginning of a new text line. That means that echo has been instructed to do the following: print the text [local], then start a new line, print localhost, and finally start another new line. This output is redirected using the special > character, and the destination after redirection is specified as /etc/ansible/hosts. While that file may not have a .txt extension, you can think of it as a plain text file.

That file is probably specified because it is used by Ansible to communicate with different hosts (in this case localhost) through its inventory functionality.

So yes, you were essentially correct -- this adds localhost to the Ansible hosts file. It's worth noting that this command will overwrite any existing file at /etc/ansible/hosts.

Answer 2

The line in question:

RUN echo '[local]\nlocalhost\n' > /etc/ansible/hosts

appears to run the echo command and use the shell redirect character > to send the output of the echo command into the file /etc/ansible/hosts.

Two things to note here:

• In the echo command, the \n sequence stands for a newline character (Enter key). So [local] and localhost are actually being echoed to the file on consecutive lines. This corresponds to the proper format for a stanza in /etc/ansible/hosts as described here. (I don't know enough about ansible to tell you why the hostname localhost is used here instead of the IP address 127.0.0.1.)

• The shell redirect character > will overwrite (that is, it will erase and replace) the contents of /etc/ansible/hosts rather than appending this new stanza to the end of the file. (To redirect and append to a file, you would use >> instead.)