How can I extract records with a column in common?

I have two tables with different numbers of columns and rows. I want to find rows using a common column (column B is in common). Here is an example. Could you please help?

file1.txt

A  B    C   D
a  b    c   d
i  ii  iii iV
*  **   #  ##

file2.txt

E  B  
f  ff 
h  b
g  gg
k  ii

output:

A  B    C   D  E
a  b    c   d  h
i  ii  iii iV  k

Solution 1:

You can do this kind of thing by building a hash / associative array / lookup table e.g. using Awk:

$ awk 'NR==FNR{B[$2]=$1; next} $2 in B {print $0,B[$2]}' file2.txt file1.txt
A  B    C   D E
a  b    c   d h
i  ii  iii iV k

There's also the join command - but that requires inputs to be sorted on the common field.

Solution 2:

To add to the answer by @steeldriver, to do it with sort and join:

join -j 2 -o 1.1,0,1.3,1.4,2.1 <(sort -k 2 file1.txt)  <(sort -k 2 file2.txt)

• -j 2 tells join which field is the key.
• -o gives the order of the fields in the output, where 0 is the common key, and the others are FILENUM.FIELD. See man join for details.
• -k 2 tells sort which field is the key.
• <( ) is bash process substitution.

Output is:

a b c d h
A B C D E
i ii iii iV k