/usr/bin/join

Usage: /usr/bin/join [OPTION]... FILE1 FILE2
For each pair of input lines with identical join fields, write a line to
standard output.  The default join field is the first, delimited by blanks.

When FILE1 or FILE2 (not both) is -, read standard input.
  -a FILENUM
         also print unpairable lines from file FILENUM,
         where FILENUM is 1 or 2, corresponding to FILE1 or FILE2
  -e STRING
         replace missing (empty) input fields with STRING;
         I.e., missing fields specified with '-12jo' options
  -i, --ignore-case
         ignore differences in case when comparing fields
  -j FIELD
         equivalent to '-1 FIELD -2 FIELD'
  -o FORMAT
         obey FORMAT while constructing output line
  -t CHAR
         use CHAR as input and output field separator
  -v FILENUM
         like -a FILENUM, but suppress joined output lines
  -1 FIELD
         join on this FIELD of file 1
  -2 FIELD
         join on this FIELD of file 2
      --check-order
         check that the input is correctly sorted, even
         if all input lines are pairable
      --nocheck-order
         do not check that the input is correctly sorted
      --header
         treat the first line in each file as field headers,
         print them without trying to pair them
  -z, --zero-terminated
         line delimiter is NUL, not newline
      --help
         display this help and exit
      --version
         output version information and exit

Unless -t CHAR is given, leading blanks separate fields and are ignored,
else fields are separated by CHAR.  Any FIELD is a field number counted
from 1.  FORMAT is one or more comma or blank separated specifications,
each being 'FILENUM.FIELD' or '0'.  Default FORMAT outputs the join field,
the remaining fields from FILE1, the remaining fields from FILE2, all
separated by CHAR.  If FORMAT is the keyword 'auto', then the first
line of each file determines the number of fields output for each line.

Important: FILE1 and FILE2 must be sorted on the join fields.
E.g., use "sort -k 1b,1" if 'join' has no options,
or use "join -t ''" if 'sort' has no options.
Comparisons honor the rules specified by 'LC_COLLATE'.
If the input is not sorted and some lines cannot be joined, a
warning message will be given.

Report bugs to: <bug-coreutils@gnu.org>.
GNU coreutils home page: <https://www.gnu.org/software/coreutils/>
General help using GNU software: <https://www.gnu.org/gethelp/>
Full documentation <https://www.gnu.org/software/coreutils/join>
or available locally via: info '(coreutils) join invocation'