Question

Given these two files

file1.txt
------
foo 
bar

file2.txt
------
foo 1
foo 2
bar 31

How can I obtain the first line of successful join in file2.txt?
The expected result after joining is:

foo 1 
bar 31

I tried this but didn’t work:

join file1.txt file2.txt

What’s the right join command?

Answer 1

The join you tried will print both instances of foo from file2. If you want to pick only one, you could use sort to ensure there are unique entries in both files before you do the actual join:

join <(sort file1) <(sort -k1,1 -u file2)