How to Compare two files and then append line that is not a partial match?

You can parse the files with awk

awk -F '/' '
    FNR == NR {seen[$1] = $0; next}
    {if ($1 in seen) print seen[$1]; else missing[$1]}
    END {for (x in missing) print x}
' Temp.txt Extensions.txt

Output:

1234/sip:[email protected]:5060 9421b96c5e Avail 1.480
4321/sip:[email protected]:5060 e9b6b979a4 Avail 1.855
111

• Set field separator to slash, -F '/'
• The action after FNR == NR is executed for the lines of the first input file. We store the lines in the associative array seen as keys, and go to next line.
• The second action is executed for the second file, when FNR != NR. If the first field matches, we print the stored line, else we save the field into another array missing.
• At the END, we print the missing lines.

You could read the contents of Extensions.txt into an array, delete the partial matches, then print whatever remains:

$ awk -F/ '
    NR==FNR {a[$1]; next} {for(i in a) if($1 ~ i) delete a[i]} END{for(i in a) print i} 1
  ' Extensions.txt Temp.txt
1234/sip:[email protected]:5060  9421b96c5e   Avail   1.480
4321/sip:[email protected]:5060  e9b6b979a4   Avail   1.855
111

Using grep+cut:

grep -xvFf <(cut -d'/' -f1 tmp) ext >> tmp

---

Here we are safe in grep using tmp for the input in process-substitution as the patterns feed and write the result back into same tmp file in append mode; see the explanation in below link:

Using same filename for the input in sub-shell and also as output in parent shell will conflict?