Join lines of two sorted files on a common field.

# To join two files on the first (default) field:
join <file1> <file2>

# To join field3 of file1 with field1 of file2:
join -1 3 -2 1 <file1> <file2>

# To produce a line for each unpairable line for file1:
join -a 1 <file1> <file2>