find and replace with variables - variables

Question

I'm trying to find and replace with variables, but it doesn't work.
Here is the code. I need to append -C -w 10% -c 5% -p /u0 to append to the end of a matching line. I do not know how to suppress the (-) Any ideas? Thank you.
OLD=$(command[check_disk]=/usr/local/nagios/libexec/check_disk -w 10% -c 5% -p / -p /var -p /tmp -p /home -p /boot -p /usr -A -e)
NEW=$(command[check_disk]=/usr/local/nagios/libexec/check_disk -w 10% -c 5% -p / -p /var -p /tmp -p /home -p /boot -p /usr -A -e -C -w 10% -c 5% -p /u0)
sed -i "s/$OLD/$NEW/" /home/scripts/nrpe.cfg

Answer 1

Try this (assumes bash):
OLD='command[check_disk]=/usr/local/nagios/libexec/check_disk -w 10% -c 5% -p / -p /var -p /tmp -p /home -p /boot -p /usr -A -e'
NEW='command[check_disk]=/usr/local/nagios/libexec/check_disk -w 10% -c 5% -p / -p /var -p /tmp -p /home -p /boot -p /usr -A -e -C -w 10% -c 5% -p /u0'
oldEscaped=$(sed 's/[^^]/[&]/g; s/\^/\\^/g' <<<"$OLD")
newEscaped=$(sed 's/[\\&/]/\\&/g' <<<"$NEW")
sed -i "s/$oldEscaped/$newEscaped/" /home/scripts/nrpe.cfg
Your first problem was that you mistook $(...) for a string-quoting mechanism (it is not; it's used for command substitution (executing the enclosed command and replacing the construct with the command's output)).
To assign literal strings, simply use single quotes as above.
Your second problem was that you can't blindly pass strings to sed's s (string-substitution) command, because certain characters have special meaning to sed, so to use them literally they have to be escaped - the most obvious problem being the / instances in the strings, which get mistaken for the delimiters of the s/.../.../ command.
Therefore, 2 auxiliary sed commands are used to perform the requisite escaping:
sed 's/[^^]/[&]/g; s/\^/\\^/g' <<<"$OLD" escapes the old string so that none of its characters can be mistaken for the regex delimiter or special regular-expression characters.
sed 's/[\\&/]/\\&/g' <<<"$NEW" escapes the new string so that none of its characters can be mistaken for the regex delimiter or backreferences (such as &, or \1).
Finally, note that it's better not to use all-uppercase shell variable names such as $OLD, so as to avoid conflicts with environment variables.