4. join

The command join, like paste, adds columns to a file. However it does it in a way that is similar to the join operation in relational databases. It joins data from multiple files based on a shared key field.

To demonstrate join let's make a couple of files with a shared key. The first file will contain the release dates and the release names:

cut -f 1,1 distros-by-date.txt > distros-names.txt

paste \
    distros-dates.txt \
    distros-names.txt \
    > distros-key-names.txt

head distros-key-names.txt

The second file will contain the release dates and the version numbers:

cut -f 2,2 distros-by-date.txt > distros-vernums.txt

paste \
    distros-dates.txt \
    distros-vernums.txt \
    > distros-key-vernums.txt

head distros-key-vernums.txt

Both of these files have the release date as a common field. Let's join them:
```
join \
    distros-key-names.txt \
    distros-key-vernums.txt \
    | head
```
It is important that the files must be sorted on the key field for join to work properly.

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