1.12.2.4               1
1.12.2.7               12
1.12.2.2               5
1.12.2.4               4
1.12.2.6               67
1.12.2.12              5

awk 'FNR==NR { array[$1]=$2; next } { for (i in array) gsub(i, array[i]) }1' master.txt file.txt

The Id of the customer is 5. He is from Grg.
The Name of the machine is ASB
The id is 4. He is from Psg.

awk 'FNR==NR { array[$1]=$2; next } { for (i in array) gsub(i, array[i]) }1' master.txt file.txt > name_of_your_output_file.txt

FNR==NR { ... }   # FNR is the current record number, NR is the record number
                  # so FNR==NR simply means: "while we process the first file listed
                  # in this case it's "master.txt"
array[$1]=$2      # add column 1 to an array with a value of column 2
next              # go onto the next record

{                 # this could be written as: FNR!=NR
                  # so this means "while we process the second file listed..."
for (i in array)  # means "for every element/key in the array..."
gsub(i, array[i]) # perform a global substitution on each line replacing the key
                  # with it's value if found
}1                # this is shorthand for 'print'

awk 'FNR==NR { array[$1]=$2; next } { for (i in array) gsub("\<"i"\>", array[i]) }1' master.txt file.txt

cat << EOF > mappings
1.12.2.4               1
1.12.2.7               12
1.12.2.2               5
1.12.2.4               4
1.12.2.6               67
1.12.2.12              5
EOF

sed -r -e 's:([^ ]*) +(.*):s/1/2/g:' mappings

sed -r -e 's:([^ ]*) +(.*):s/\b1\b/2/g:' mappings | sed -f - infile

> cat temp
1.12.2.4  1
1.12.2.7  12
1.12.2.2  5
1.12.2.4  4
1.12.2.6  67
1.12.2.12  5

> cat temp2
The Id of the customer is 1.12.2.12. He is from Grg.
The Name of the machine is ASB
The id is 1.12.2.4. He is from Psg.

> temp.pl
The Id of the customer is 5. He is from Grg.
The Name of the machine is ASB
The id is 4. He is from Psg

>

#!/usr/bin/perl

use strict;
use warnings;

my %hsh=();

open (MYFILE, 'temp');
open (MYFILE2, 'temp2');

while (<MYFILE>) {
my@arr = split/s+/;
$hsh{$arr[0]} = $arr[1];
}
my $flag;
while(<MYFILE2>)
{
$flag=0;
my $line=$_;
foreach my $key (keys %hsh)
{
   if($line=~/$key/)
   {
    $flag=1;
    $line=~s/$key/$hsh{$key}/g;
    print $line;
   }
}
  if($flag!=1)
  {
  print $line;
  $flag=0;
  }
}
close(MYFILE);
close(MYFILE2);