file1.txt    file2.txt
a 7 nah      a anau
b 0 blah     c bau
c 5 bah      d cau
d 1 gah
e 0 hah

a 7 nah anau
b 0 blah null
c 5 bah bau
d 1 gah cau
e 0 hah null

join -o1.1,1.2,1.3,2.2 -a1 -e"null" <(sort file1.txt) <(sort file2.txt)

a 7 nah anau
b 0 blah null
c 5 bah bau
d 1 gah cau
e 0 hah null

join file1 file2 -e null -o auto -a 1 2>/dev/null