How to replace a column in .fam with .txt in unix - bash

Question

I am looking for some options in unix (may be awk or sed ) through which I can replace the last column in my .fam file with the last column (v8) of a .txt file. Something similar to the merge function in R.
My .fam file looks like this
20481 20481 0 0 2 -9
20483 20483 0 0 1 1
20488 20488 0 0 2 1
20492 20492 0 0 1 1
and my .txt file looks like this.
V1 V2 V3 V4 V6 V7_Pheno V8
2253792 20481 NA DNA 1 Yes 2
2253802 20483 NA DNA 4 Yes 2
2253816 20488 NA DNA 0 No 1
2253820 20492 NA DNA 4 Yes 2
My outcome.fam file should looks like this
20481 20481 0 0 2 2
20483 20483 0 0 1 2
20488 20488 0 0 2 1
20492 20492 0 0 1 2

Answer 1

paste merges the lines
awk allow you to select column, so
paste foo.fam bar.txt | awk '{ print $1 " " $2 " " $3 " " $4 " " $13 }'
should do what you want
If you want to suppress the header line of .txt file, you can call tail to skip the first line:
tail -n +2 bar.txt
You can hence integrate it in you command line (assuming you use bash)
paste foo.fam <(tail -n +2 bar.txt) | awk '{ print $1 " " $2 " " $3 " " $4 " " $13 }'

Answer 2

awk can do it alone.
$: awk 'BEGIN{ getline < "f.txt" }
{ gsub("[^ ]+$",""); l=$0; getline < "f.txt"; print l$7; }' f.fam
20481 20481 0 0 2 2
20483 20483 0 0 1 2
20488 20488 0 0 2 1
20492 20492 0 0 1 2
The BEGIN reads the header record on the .txt.
Then for each line of the .fam, strip off the last field and save to l.
getline used this way parses to fields also, so print l$7; prints the shortened record from .fam and adds the last field from .txt.