(coreutils.info)Header lines

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8.3.5 Header lines
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The ‘--header’ option can be used when the files to join have a header
line which is not sorted:

     $ cat file1
     Name     Age
     Alice    25
     Charlie  34

     $ cat file2
     Name   Country
     Alice  France
     Bob    Spain

     $ join --header -o auto -e NA -a1 -a2 file1 file2
     Name     Age   Country
     Alice    25    France
     Bob      NA    Spain
     Charlie  34    NA

   To sort a file with a header line, use GNU ‘sed -u’.  The following
example sort the files but keeps the first line of each file in place:

     $ ( sed -u 1q ; sort -k2b,2 ) < file1 > file1.sorted
     $ ( sed -u 1q ; sort -k2b,2 ) < file2 > file2.sorted
     $ join --header -o auto -e NA -a1 -a2 file1.sorted file2.sorted > file3