Replace all white spaces with commas in a text file

Something like:

sed '/^[[:blank:]]*B$/{n;s/Hello/Hi/g;}'

That assumes there are no consecutive Bs (one B line followed by another B line).

Otherwise, you could do:

awk 'last ~ /^[[:blank:]]*B$/ {gsub("Hello", "Hi")}; {print; last=$0}'

The sed equivalent would be:

sed 'x;/^[[:blank:]]*B$/{
       g;s/Hello/Hi/;b
     }
     g'

To replace the second word after B, or to replace world with universe only if two lines above contained B:

awk 'l2 ~ /B/ {gsub("world","universe")}; {print; l2=l1; l1=$0}'

To generalise it to n lines above:

awk -v n=12 'l[NR%n] ~ /B/ {gsub("foo", "bar")}; {print; l[NR%n]=$0}'

One approach is to pick a different delimiter besides /. For example, using |:

sed -ie "s|queue_directory = /var/spool/postfix-secondary|queue_directory = /var/spool/postfix-$newnumber|g" /etc/postfix-$newnumber/main.cf

Another approach is to backslash-escape your other slashes:

sed -ie "s/queue_directory = \/var\/spool\/postfix-secondary/queue_directory = \/var\/spool\/postfix-$newnumber/g" /etc/postfix-$newnumber/main.cf

Two more suggestions for your particular usage. First, I don't think you need the g flag, unless you anticipate the substitution appearing multiple times in the same line. Second, if you are just trying to change a directive, you could potentially just change it no matter what the previous value. For example:

sed -ie "s|^queue_directory =.*|queue_directory = /var/spool/postfix-$newnumber|" /etc/postfix-$newnumber/main.cf