The join command in Linux is used to combine lines of two files based on a common field. When it encounters unpairable lines—lines that do not have a matching key in the other file—it simply ignores those lines by default.
For example, if you have two files:
file1.txt:
1 Alice
2 Bob
3 Charliefile2.txt:
1 Electronics
2 BooksIf you run the command:
join file1.txt file2.txtThe output will be:
1 Alice Electronics
2 Bob BooksThe line with "3 Charlie" from file1.txt is ignored because there is no matching key in file2.txt.
If you want to include unpairable lines, you can use the -a option to specify which lines to include. For example, -a 1 will include all lines from the first file, and -a 2 will include all lines from the second file, even if they do not have matches.
