How to remove duplicates from a file and write to the same file?

Question

I know my title is not much self-explanatory but let me try to explain it here.

I have a file name test.txt which has some duplicate lines. Now, what I want

Answer 1

Use Sponge for Reading/Writing to Same File

You can use the sponge utility from moreutils to soak up standard output before writing the file. This prevents you from having to shuffle files around, and approximates an in-place edit. For example:

sort -u test.txt | sponge test.txt

Sample Output

Using your corpus, this results in the expected output.

$ cat test.txt 
AAAA
BBBB
CCCC