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