Add a space or tab before the last character in each line

You can do:

sed -E 's/.(.)$/\1/' file.txt  

To edit the file in place, without backup:

sed -Ei 's/.(.)$/\1/' file.txt 

To edit the file in place, with original file backed up with .bak extension:

sed -Ei.bak 's/.(.)$/\1/' file.txt 

POSIX-ly:

sed 's/.\(.\)$/\1/' file.txt

Short awk command:

awk '/[13579]$/' file > hostnames_odd.txt

• [13579] - character class representing the list of allowed digits (odd numbers)
• $ - the end of the string/line

Result:

$ cat hostnames_odd.txt 
abc123
abc435

Or the same with grep:

grep '[13579]$' file  > hostnames_odd.txt

In case if there could possibly be a whitespace(s) at the end of some line(s) change the crucial pattern to the following: [13579][[:space:]]*$