How to find out common elements between two files?

Question

For an example, I have 2 files having following info:

File #1:

12
13
14
15

File #2:

12 1
13 2
14 2
15 6
16 7
17 8

Output File:

1
2
2
6

In the output file, I want only the second column values of file #2 which has matched with the first column of file #1. Is there any utility function for that in Linux? I'm new in shell scripting, can anyone help me out?

Solution

you could use the join command to join the files on a specific column, and awk to parse the output.

To join these files on column 1 pass the parameters -j 1 to the join command:

usr@srv % join -j 1 test test2 
12 1

13 2

14 2

15 6

Afterwards use awk to print only the second column:

usr@srv % join -j 1 test test2 | awk '{print $2}'
1

2

2

6

Comments

[Stéphane Chazelas]

No need for awk: join -o 2.2 test test2

[German]

Please improve your answer by adding an explanation of what your code does.

[Madhan]

Is there a way to specify standard output as stdin? I.e. the purpose is check common elements between all three files? Not just two? join f1 f2 > fr; join fr f3 -- something like join f1 f2 f3?