A bit of detail about FS in awk

In awk, the details of FS are learned in EffectiveawkProgram. Although shell is a conventional weapon at work, it is not very familiar with the use of shell, so I cannot understand it deeply, or record what I did not notice, after all, each time you read an English document, it is not very important. Some details about FS in Effe... awk
When learning objective awk Program. Although shell is a conventional weapon at work, it is not very familiar with the use of shell, so I cannot understand it deeply, or record what I did not notice, after all, each time you read an English document, it is not very important.
Invalid awk Program Chapter 3 Using Regular Expression to Separate Fields in Reading Input Files mentions an interesting phenomenon.
Echo "a B c d" | awk '{print $2 }'
Echo "a B c d" | awk 'In in {FS = "[\ t \ n]"} {print $2 }'
Whether the two outputs are consistent. before I learned this chapter, I thought that the outputs are the same, both of which are B. Actually:

We can see that the output of the first command is different from that of the second command. The reason is that, by default, FS is a space. in this case, before processing, the strip will first drop the leading space and tab, as well as the trailing space and tab, however, if FS is changed to [\ t \ n], the blank characters in the header and tail are not strip. if there is a space in the header, we can see that $1 is null or empty.

Another interesting phenomenon is that record re-creation will lead to a blank character strip in the header and tail.
 
We can see that only the seemingly meaningless operation $2 = $2 is executed, and the space in the header is dropped by strip. In fact, the two spaces at the end are also dropped by strip. Because the value assignment operation triggers a string rebuild, and the rebuild process needs to find $1, $2... $ NF: link up. the process of searching for $1 is equivalent to $1 when FS = "". Blank characters (spaces and tabs) are ignored, concatenated string has no blank characters in the header and tail.