(coreutils.info)Paired and unpaired lines

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8.3.4 Controlling ‘join’’s field matching
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In this section the ‘sort’ commands are omitted for brevity.  Sorting
the files before joining is still required.

   ‘join’’s default behavior is to print only lines common to both input
files.  Use ‘-a’ and ‘-v’ to print unpairable lines from one or both
files.

All examples below use the following two (pre-sorted) input files:

     $ cat file1                          $ cat file2
     a 1                                  a A
     b 2                                  c C

Command                              Outcome
                                     
--------------------------------------------------------------------------
     $ join file1 file2              common lines (_intersection_)
     a 1 A                           
     $ join -a 1 file1 file2         common lines _and_ unpaired lines
     a 1 A                           from the first file
     b 2                             
     $ join -a 2 file1 file2         common lines _and_ unpaired lines
     a 1 A                           from the second file
     c C                             
     $ join -a 1 -a 2 file1 file2    all lines (paired and unpaired)
     a 1 A                           from both files (_union_).
     b 2                             see note below regarding ‘-o
     c C                             auto’.
                                     
     $ join -v 1 file1 file2         unpaired lines from the first file
     b 1                             (_difference_)
                                     
     $ join -v 2 file1 file2         unpaired lines from the second
     c C                             file (_difference_)
                                     
     $ join -v 1 -v 2 file1 file2    unpaired lines from both files,
     b 2                             omitting common lines (_symmetric
     c C                             difference_).
                                     

The ‘-o auto -e X’ options are useful when dealing with unpaired lines.
The following example prints all lines (common and unpaired) from both
files.  Without ‘-o auto’ it is not easy to discern which fields
originate from which file:

     $ join -a 1 -a 2 file1 file2
     a 1 A
     b 2
     c C

     $ join -o auto -e X -a 1 -a 2 file1 file2
     a 1 A
     b 2 X
     c X C

   If the input has no unpairable lines, a GNU extension is available;
the sort order can be any order that considers two fields to be equal if
and only if the sort comparison described above considers them to be
equal.  For example:

     $ cat file1
     a a1
     c c1
     b b1

     $ cat file2
     a a2
     c c2
     b b2
     $ join file1 file2
     a a1 a2
     c c1 c2
     b b1 b2