Unix Shell Script: Finding unique lines of a file without sorting the file

Let's take an example file as follows:

$ cat file

AAAA

FFFF

BBBB

BBBB

CCCC

AAAA

FFFF

DDDD

If you want to find unique lines of this file, it can be done as

$ cat file|sort|uniq

AAAA

BBBB

CCCC

DDDD

FFFF

However, it changes the order of occurrence of lines.

Following "awk" command will find unique lines of a file without sorting the file:

$ awk '!x[$0]++' file

AAAA

FFFF

BBBB

CCCC

DDDD

awk 'x[$0]++' keep on incrementing the hash array "x" of line from 0 or undefined to 1,2 and so on. Since there is no code for awk in {}, so it by default prints the line. When ! is appended in front of x[$0]++ , it will only print line when value of its hash array "x" is 0 or undefined.