Performing JOINs on text files

Linux comes equipped with a handy utility aptly called JOIN which performs join operations on text files.

Performing an inner join is as simple as running join file1.txt file2.txt

Producing an outer join can be a bit more verbose. The following command is an example of an outer join.

join -t $’\t’ -o 0,1.2,2.2 -a 1 -a 2 file1.txt file2.txt

Let’s break that down a bit.

-t $’\t’ the -t indicates to join what character to use as the field-separator. In this example, the leading $ allows us to pass the tab character, instead of just a backslash-escaped t.
-o 0,1.2,2.2 specifies the fields to output. In this example, we want the joined field 0, then the second field form file 1, then the second field from file 2.
-a 1 -a 2 specifies that we want to join all the fields from file 1 and all the fields from file 2.