bash while loop failing while using sed - sed

Question

I am facing an issue with sed in a while-loop.using sed. I want to read the 2nd column of file1, compare it with the content of file2, and if the string is matched, i want to replace the matched string of file1 with file2 string.
I tried with the following code, but it is not returning any output.
cat file1 | while read a b; do
sed -i "s/$b/$(grep $b file2)/g" file1 > file3;
done
Example input:
file_1 content:
1 1234
2 8765
file2 content:
12345
34567
87654
Expected output:
1 12345
2 87654

Answer 1

Your script is very inefficient. Using the while-loop you read each line of file1. This is N operations. Per line you process with the while loop, you reproscess the full file1, making it an N*N process. However, in the sed, you grep file2 constantly. If file2 has M lines, this becomes an N*N*M process. This is very inefficient.
On top of that there are some issues:
You updated file1 inplace because you use the -i flag. An inplace update does not provide any output, so file3 will be empty.
You are reading file1 with the while-loop and at the same time you update file1 with sed. I don't know how this will react, but I don't believe it is healthy.
If $b is not in file2 you would, according to your logic, have a line with only a single column. This is not what you expect.
A fix of your script, would be this:
while read -r a b; do
c=$(grep "$b" file2)
[[ "$c" == "" ]] || echo "$a $c"
done < file1 > file3
which is still not efficient, but it is already M*N. The best way is using awk
note: as a novice, always parse your script with http://www.shellcheck.net
note: as a professional, always parse your script with http://www.shellcheck.net

Answer 2

Could you please try following.
awk 'FNR==NR{a[$2]=$1;next} {for(i in a){if(match($0,"^"i)){print a[i],$0;continue}}}' file1 file2
Adding a non-one liner form of solution:
awk '
FNR==NR{
a[$2]=$1
next
}
{
for(i in a){
if(match($0,"^"i)){
print a[i],$0
continue
}
}
}
' Input_file1 Input_file2
Explanation: Adding detailed explanation for above code.
awk ' ##Starting awk code from here.
FNR==NR{ ##Checking condition if FNR==NR then do following.
a[$2]=$1 ##Creating array a whose index is $2 and value is $1.
next ##next will skip all further statements from here.
}
{ ##Statements from here will run for 2nd Input_file only.
for(i in a){ ##Traversing through array a all elements here.
if(match($0,"^"i)){ ##Checking condition if current line matches index of current item from array a then do following.
print a[i],$0 ##Printing array a whose index is i and current line here.
continue ##Again take cursor to for loop.
}
}
}
' Input_file1 Input_file2 ##Mentioning all Input_file names here.