Assigning a value having semicolon (';') to a variable in bash

You don't need sed (or any other regex engine) for this at all:

s='hello;'echo "${s//;/\;}"

This is a parameter expansion which replaces ; with \;.

That said -- why are you trying to do this? In most cases, you don't want escape characters (which are syntax) to be inside of scalar variables (which are data); they only matter if you're parsing your data as syntax (such as using eval), which is a bad idea for other reasons, and best avoided (or done programatically, as via printf %q).

I find it interesting that the use of back-ticks gives one result (your result) and the use of $(...) gives another result (the wanted result):

$ echo "hello;" | sed 's/\([^\\]\);/\1\\;/g'hello\;$ z1=$(echo "hello;" | sed 's/\([^\\]\);/\1\\;/g')$ z2=`echo "hello;" | sed 's/\([^\\]\);/\1\\;/g'`$ printf "%s\n" "$z1" "$z2"hello\;hello;$

If ever you needed an argument for using the modern x=$(...) notation in preference to the older x=`...` notation, this is probably it. The shell does an extra round of backslash interpretation with the back-ticks. I can demonstrate this with a little program I use when debugging shell scripts called al (for 'argument list'); you can simulate it with printf "%s\n":

$ z2=`echo "hello;" | al sed 's/\([^\\]\);/\1\\;/g'`$ echo "$z2"seds/\([^\]\);/\1\;/g$ z1=$(echo "hello;" | al sed 's/\([^\\]\);/\1\\;/g')$ echo "$z1"seds/\([^\\]\);/\1\\;/g$ z1=$(echo "hello;" | printf "%s\n" sed 's/\([^\\]\);/\1\\;/g')$ echo "$z1"seds/\([^\\]\);/\1\\;/g$ 

As you can see, the script executed by sed differs depending on whether you use x=$(...) notation or x=`...` notation.

s/\([^\]\);/\1\;/g            # ``s/\([^\\]\);/\1\\;/g          # $()

Summary

Use $(...); it is easier to understand.

You need to use four (three also work). I guess its because it's interpreted twice, first one by the sed command and the second one by the shell when reading the content of the variable:

result=`echo "hello;" | sed 's/\([^\\]\);/\1\\\\;/g'`

And

echo "$result"

yields:

hello\;