Comparing two files and printing the content

Question

I want to compare two files and want the output with the unique contents of both files + common content from the first file.

Example :-

File A.txt

A=x
B=y
C=z

File B.txt

A=p
D=q
E=r

Output :-

A=x
B=y
C=z
D=q
E=r

Kindly help me in resolving this. I want to perform this on Linux server so any command or shell script for this.

Answer 1

You can use awk:

$ awk -F= '{a[$1]++; if(a[$1]==1){print}}' A.txt B.txt  
A=x
B=y
C=z
D=q
E=r

Explanation

• a[$1]++; : saves the 1st field as a key in the array a and increments the value associated with that key by one. This means that the first time a key is seen (for example, the first A=x), the value will be 1 but the second it will be 2. This lets us count unique elements.

• if(a[$1]==1){print} : if the first field has only been seen once, print.

Because we're processing B.txt after A.txt, the values in A.txt are the ones that are kept.