I encountered this code:

```
file=$(<filename)

```

This reads the file from the filename.

My question is how does this work?

I read from this post:

[How to use double or single brackets, parentheses, curly braces](https://stackoverflow.com/questions/2188199/bash-double-or-single-bracket-parentheses-curly-braces)

It tells me, single parentheses can function as:

```
- Sub bash execution
- Array construction

```

But in the case above, I do not know how this explains.

Besides this question, I want to know that why when I do `echo $file`, the file content concatenate into one line?