(coreutils.info)Sorting files for join

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8.3.2 Pre-sorting
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‘join’ requires sorted input files.  Each input file should be sorted
according to the key (=field/column number) used in ‘join’.  The
recommended sorting option is ‘sort -k 1b,1’ (assuming the desired key
is in the first column).

Typical usage:
     $ sort -k 1b,1 file1 > file1.sorted
     $ sort -k 1b,1 file2 > file2.sorted
     $ join file1.sorted file2.sorted > file3

   Normally, the sort order is that of the collating sequence specified
by the ‘LC_COLLATE’ locale.  Unless the ‘-t’ option is given, the sort
comparison ignores blanks at the start of the join field, as in ‘sort
-b’.  If the ‘--ignore-case’ option is given, the sort comparison
ignores the case of characters in the join field, as in ‘sort -f’:

     $ sort -k 1bf,1 file1 > file1.sorted
     $ sort -k 1bf,1 file2 > file2.sorted
     $ join --ignore-case file1.sorted file2.sorted > file3

   The ‘sort’ and ‘join’ commands should use consistent locales and
options if the output of ‘sort’ is fed to ‘join’.  You can use a command
like ‘sort -k 1b,1’ to sort a file on its default join field, but if you
select a non-default locale, join field, separator, or comparison
options, then you should do so consistently between ‘join’ and ‘sort’.

To avoid any locale-related issues, it is recommended to use the ‘C’
locale for both commands:

     $ LC_ALL=C sort -k 1b,1 file1 > file1.sorted
     $ LC_ALL=C sort -k 1b,1 file2 > file2.sorted
     $ LC_ALL=C join file1.sorted file2.sorted > file3