Question

I need to output the lines of a file where the first column’s values match the firsts column’s values of the second using awk. Easier explained with an example:

file1.txt
1
2
3
4
5

file2.txt
1   dog
4   fish
6   cat
9   rabbit

I want file3.txt to be:

1   dog
4   fish

Any help appreciated

Answer 1

You can try join command:

$ join file{1,2}.txt

Or use awk

$ awk 'FNR==NR{dict[$1]; next}; $1 in dict' file{1,2}.txt