match values in first column of two files and join the matching lines in a new file

Question

I need to find matches with the string in column 1 ($1) in file1.txt with the string in column 1 ($1) in file2.txt. Then I want to join the lines where there was a match in

Answer 1

awk 'BEGIN {
  FS = OFS = "\t"
  }
NR == FNR {
  # while reading the 1st file
  # store its records in the array f
  f[$1] = $0
  next
  }
$1 in f {
  # when match is found
  # print all values
  print f[$1], $0
  }' file1 file2 

Answer 2

If you don't mind the output being ordered by the first column then you can use this invocation of the join command:

join <(sort file1.txt) <(sort file2.txt) >out.txt