Join files by key
Joining two files by key in the first column of files can be easily done by using UNIX command below.
join -j 1 file1.txt file2.txt
This is very useful command, but the output is space-delimited by default.
In order to get the output by tab-delimited, following option for join is effective.
join -j 1 -t "$(printf '\011')" file1.txt file2.txt
Two files must be sorted and non-redundant before running join command.
sort command with -u (unique) option should be applied.
sort -u file1.txt > file1s.txt
mv file1s.txt file1.txt