The join command will join or merge similar lines in two different files. The join command without any options will combine lines that begin with an identical string until the first whitespace. For example, let's consider two files that contain lines that begin with an identical string (Employee).
# file1.txt Employee name=John Doe # file2.txt Employee occupation=Engineer
The join command followed by the two files will join the lines that begin with the identical string.
~]# join file1.txt file2.txt Employee name=John Doe occupation=Engineer
The lines in the files being joined must be sorted. If the files are not sorted, "not sorted" will be displayed when attempting to join the files.
~]# join file1.txt file2.txt join: file1.txt is not sorted
Use the sort command to sort the files being joined.
~]# sort file1.txt -o file1.txt ~]# sort file2.txt -o file2.txt ~]# join file1.txt file2.txt Employee name=John Doe occupation=Engineer Employee name=Jane Doe occupation=Sales Manager