join

Join lines on a common field, writes to standard output a line for each pair of input lines that have identical join fields.

Syntax
      join [Options]... File1 File2

Options

   -a FILE-NUMBER
       Print a line for each unpairable line in file FILE-NUMBER (either '1' or '2'),
       in addition to the normal output.

   -e STRING
       Replace those output fields that are missing in the input with STRING.

   -i
   --ignore-case
      Ignore differences in case when comparing keys.  With this option, the lines of the
      input files must be ordered in the same way.  Use 'sort -f' to produce this ordering.

   -1 FIELD
   -j1 FIELD
      Join on field FIELD (a positive integer) of file 1.

  -2 FIELD
   -j2 FIELD
      Join on field FIELD (a positive integer) of file 2.

   -j FIELD
      Equivalent to '-1 FIELD -2 FIELD'.

   -o FIELD-LIST...
      Construct each output line according to the format in FIELD-LIST.
      Each element in FIELD-LIST is either the single character '0' or has the form M.N where
      the file number, M, is '1' or '2' and N is a positive field number.

      A field specification of '0' denotes the join field.  In most cases, the functionality
      of the '0' field spec can be reproduced using the explicit M.N that corresponds to the join field.
      However, when printing unpairable lines (using either of the '-a' or '-v' options), there is no
      way to specify the join field using M.N in FIELD-LIST if there are unpairable lines in both files.
      To give 'join' that functionality, POSIX invented the '0' field specification notation.

      The elements in FIELD-LIST are separated by commas or blanks.
      Multiple FIELD-LIST arguments can be given after a single '-o' option; the values of
      all lists given with '-o' are concatenated together.
      All output lines - including those printed because of any -a or -v option - are subject to
      the specified FIELD-LIST.

   -t CHAR
      Use character CHAR as the input and output field separator.

   -v FILE-NUMBER
      Print a line for each unpairable line in file FILE-NUMBER (either '1' or '2'),
      instead of the normal output.

Either File1 or File2 (but not both) can be '-', meaning standard input. File1 and File2 should be already sorted in increasing textual order on the join fields, using the collating sequence specified by the 'LC_COLLATE' locale.

Unless the '-t' option is given, the input should be sorted ignoring blanks at the start of the join field, as in 'sort -b'. If the '--ignore-case' option is given, lines should be sorted without regard to the case of characters in the join field, as in 'sort -f'.

The defaults are:
The join field is the first field in each line;
Fields in the input are separated by one or more blanks, with leading blanks on the line ignored;
Fields in the output are separated by a space;
Each output line consists of the join field, the remaining fields from File1, then the remaining fields from File2.

“A nation is not in danger of financial disaster merely because it owes itself money” ~ Andrew William Mellon

Related Linux commands

csplit - Split a file into context-determined pieces.
cut - Divide a file into several parts.
fmt - Reformat paragraph text.
fold - Wrap input lines to fit in specified width.
head - Output the first part of file(s).
paste - Merge lines of files.
split - Split a file into fixed-size pieces.
tail - Output the last part of files.
Equivalent Windows command: FC - Compare two files.