Special variable IFS in the shell

As I get used to Python's writing style, I often want to use bash to iterate through the rows of a file and process the rows that are read.

But a weird situation if my text resembles this

The first line.
The second line.
The third line.

For line in ' cat filename ' Doecho $linedone

When I traverse each line of the file in the way above, I find that the data read is this:

The
First
Line.
The
Second
Line.
The
Third
Line.

Obviously, Bash doesn't follow my idea of breaking a newline character as a single line delimiter, but instead of a space as a separator character. This is certainly not the result of my hope, and I am baffled by it.

Until one day I accidentally saw a system variable--ifs about the shell.

With regard to FS, often awk is not unfamiliar, awk has rs,ors,FS, OFS 4 variables that can define delimiters. The IFS in the shell also defines the delimiter.

If you want to achieve the effect I want, then you just need to assign the IFS to a delimiter. But there is a more fastidious, so the question arises

Ifs= ' \ n ' ifs=$ "\ n" ifs=$ ' \ n '

These three assignments appear to be more like "assigning a newline character to ifs," but actually only the last one is the result I want.

Ifs= ' \ n '//character n as the line break of the IFS.

ifs=$ "\ n"//This is actually converted to a newline character by $, but only when interpreted (or executed) is converted to a newline character.

ifs=$ ' \ n '//This is the true line break.

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Special variable IFS in the shell