join

file1 with 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 whitespace.
When file1 or file2 is - read standard input.

Usage: 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 whitespace.  
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 EMPTY          replace missing input fields with EMPTY
  -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     end lines with 0 byte, not newline
      --help     display this help and exit
      --version  output version information and exit

file1 and file2 must be sorted on the join fields.
Use sort -k 1b,1 if join has no options,
or use join -t '' if sort has no options.

If the input is not sorted and some lines cannot be joined, a warning message will be given.

Comparisons honor the rules specified by $LC_COLLATE.

Example
Seperator is the dot.

cat 0 
0
111tatc.222
0.bash.222.txt
00-2222222222.html

cat 00
0
111tatc.222
0.bash.222.txt
00-2222222222.html

join -e xxx   -t . 0 00
0
111tatc.222.222
0.bash.222.txt.bash.222.txt
00-2222222222.html.html

Useful for Log and System Analysis: Combining separate logs (e.g., matching a system health log to an IP address mapping log using the hostname as the key).
User and Inventory Management: Merging employee names with salary tables using an Employee ID, or matching product descriptions with prices using SKU numbers.
Report Generation: Aggregating metrics from different system tools into a single, comprehensive text or CSV report.

Default: Performs an Inner Join (only outputs lines where keys match in both files) using the first column as the key.join file1.txt file2.txt

names.txt:
101 Alice
102 Bob
103 Charlie

departments.txt
101 Engineering
102 Marketing
104 Sales

join names.txt departments.txt
Output (Inner Join):
101 Alice Engineering 
102 Bob Marketing 
=== occurs in both files.
    === taken from file1.
          === taken from file2.


keep lines from names.txt even if they don't have a matching department (Left Outer Join):
        join -a 1 names.txt departments.txt
Output:
101 Alice Engineering
102 Bob Marketing
103 Charlie