Merge three file in to a single master file by excluding the header in 2nd and 3rd files

Here are a couple of ways:

• Using paste and cut :

  $ paste -d ' ' <(cut -d' ' -f 1 ABC.txt ) <(cut -d' ' -f 1 PQR.txt ) > output.txt
  ABC PQR
  

  If your system does not support process substitution, use this instead:

  $ cut -d' ' -f 1 ABC.txt > /tmp/aa; cut -d' ' -f 1 PQR.txt > /tmp/bb; paste -d ' ' /tmp/aa /tmp/bb
  

• Using awk (thanks @Costas):

  awk 'FNR==NR{a[FNR]=$1; next}{print a[FNR],$1}' ABC.txt PQR.txt > output.txt
  

  The special variable FNR is the line number of the current input file and NR is the line number of the input in general, whatever file it came from. The two are equal only while the first input file is being read. So, the first fields of the first file are saved in the a array (a[FNR]=$1) whose keys are line numbers and whose values are the 1st fields. Then, when the second file is reached, we print the value corresponding to its line number (a[NR]) and the current line's 1st field.

An awk solution:

$ awk 'NR==FNR{a[$2]=$3"\t"$4"\t"$5"\t"$6"\t"$7"\t"$8"\t"$9; next} 
              {
                if($1 in a){
                    print $0,a[$1]
                }
               }' file2 file1
UN          ID    St      M1    M2       SE    DOF  PV        PA            FC TID  X   E   GG7 J   O   
17127159    0   -5.9    297.3   765.7   0.22    4   0.003   0.00389231  2.57536 16657436    353.568 335.295 221.717 815.654 684.85  
17127163    2   -3.87   189.914 492.307 0.3548  4   0.0179  0.01795     2.59226 16657450    221.647 226.774 136.274 431.32  392.533 

Explanation

Awk splits each input line into fields (at whitespace, by default), making the 1st field $1 the 2nd $2 etc. The special variable NR is the current input line number and FNR is the current line number of the file being read. Therefore, when processing multiple files, the two are equal only while the first file is being read.

• NR==FNR{a[$2]=$3"\t"$4"\t"$5"\t"$6"\t"$7"\t"$8"\t"$9; next} : if we're reading the first file, save fields 3 through 9 (joined by tabs) as the value in the array a whose key is the 2nd field. Then, skip to the next line.

• The next ensures that the rest of the script will not be run for the first file (file2) but only the second (file1).

• if($1 in a){ print $0,a[$1] } : we're now in the second file (file1). If the first field exists as a key in the a array (if($1 in a)), then print the current line $0 and the value stored in a for $1: fields 3 through 9 from file2.