(coreutils.info)join invocation

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8.3 ‘join’: Join lines on a common field
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‘join’ writes to standard output a line for each pair of input lines
that have identical join fields.  Synopsis:

     join [OPTION]... FILE1 FILE2

   Either FILE1 or FILE2 (but not both) can be ‘-’, meaning standard
input.  FILE1 and FILE2 should be sorted on the join fields.

     $ cat file1
     a 1
     b 2
     e 5

     $ cat file2
     a X
     e Y
     f Z

     $ join file1 file2
     a 1 X
     e 5 Y

‘join’’s default behavior (when no options are given):
   • the join field is the first field in each line;
   • fields in the input are separated by one or more blanks, with
     leading blanks on the line ignored;
   • fields in the output are separated by a space;
   • each output line consists of the join field, the remaining fields
     from FILE1, then the remaining fields from FILE2.

General options in join

Options which affect general program behavior.

Sorting files for join

Using ‘sort’ before ‘join’.

Working with fields

Joining on different fields.

Paired and unpaired lines

Controlling ‘join’’s field matching.

Header lines

Working with header lines in files.

Set operations

Union, Intersection and Difference of files.