Match 2 fields in different files

Question

I have 2 files like this:

File A:

1,x
2,z
3,y

File B:

7,b
3,c
9,t
1,m

I would like to loop through File A (first columns) and see if there any matches in File B (first columns) using awk .

The expected will be:

1,m
3,c

Answer 1

Just using awk :

$ awk -F, 'NR==FNR{a[$1];next}($1 in a)' file1 file2
3,c
1,m

Pipe to sort for ordered output:

$ awk -F, 'NR==FNR{a[$1];next}($1 in a)' file1 file2 | sort
1,m
3,c

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Alternatively this is what join does (requires input files to be sorted) :

$ join <(sort file1) <(sort file2) -j1 -t, -o"2.1,2.2"
1,m
3,c

Answer 2

join -t, -o 1.1,1.2 <(sort fileA) <(sort fileB)