How to get contents from 2 files and append that contents to a new file using a shell script

Assuming file2.txt has just one line as shown, how about a paste solution:

paste file1.txt <(yes $(<file2.txt) | head -n $(wc -l <file1.txt))

• yes $(<file2.txt) repeats the line of file2.txt.
• $(wc -l <file1.txt) returns the line count of file1.txt.
• head -n $(wc -l <file1.txt) prints as many lines as file1.txt.

Assuming that file2.txt has always exactly 1 line, then you might exploit GNU sed's hold space, consider following simple example let file1.txt content be

123
456
789

and file2.txt file content be

ABC

then

sed -e '1{h;d}' -e 'G;s/\n/ /' file2.txt file1.txt

gives output

123 ABC
456 ABC
789 ABC

Explanation: I register two actions, for 1 line (globally, that is first line of first of mentioned file, observe that it is file2.txt file1.txt not in reverse) I instruct GNU sed to save line into hold space (h) and immediately go further without output anything (d), for other lines I instruct GNU sed to append newline and content of hold space (G) and then replace said newline with space characters.

(tested in GNU sed 4.5)

awk 'NR==FNR{v=$0; next} {print $0, v}' file2.txt file1.txt

Set OFS to \t or pipe the output to column -t to get whatever separators/alignment you like if a blank isn't adequate.

paste expects two file of equal length. I'm guessing you basically want this common Awk two-liner:

awk 'BEGIN { FS=OFS="\t" } NR==FNR { a[++i] = $0; next }
    { print $0, a[(FNR-1) % i + 1] }' file2.txt file1.txt

(I have assumed your files are tab-separated; it's not clear from your question.)

assuming file2 only has 1 line

cat temptest_f_00_01.txt 
123
456
789

cat temptest_f_00_02.txt
ABC

mawk -F'^$' 'FNR<NR || _*(ORS=" "$_ RS)' f2.txt f1.txt

123 ABC
456 ABC
789 ABC