Using value inside a variable without expanding - bash

Question

I am trying to find and replace a specific text content using the sed command and to run it via a shell script.
Below is the sample script that I am using:
fp=/asd/filename.txt
fd="sed -i -E 's ($2).* $2:$3 g' ${fp}"
eval $fd
and executing the same by passing the arguments:
./test.sh update asd asdfgh
But if the argument string contains $ , it breaks the commands and it is replacing with wrong values, like
./test.sh update asd $apr1$HnIF6bOt$9m3NzAwr.aG1Yp.t.bpIS1.
How can I make sure that the values inside the variables are not expanded because of the $?
Updated
sh file test.sh
set -xv
fp="/asd/filename.txt"
sed -iE "s/(${2//'$'/'\$'}).*/${2//'$'/'\$'}:${3//'$'/'\$'}/g" "$fp"
text file filename.txt
hello:world
Outputs
1)
./test.sh update hello WORLD
sed -iE "s/(${2//'$'/'\$'}).*/${2//'$'/'\$'}:${3//'$'/'\$'}/g" "$fp"
++ sed -iE 's/(hello).*/hello:WORLD/g' /asd/filename.txt
2)
./test.sh update hello '$apr1$hosgaxyv$D0KXp5dCyZ2BUYCS9BmHu1'
sed -iE "s/(${2//'$'/'\$'}).*/${2//'$'/'\$'}:${3//'$'/'\$'}/g" "$fp"
++ sed -iE 's/(hello).*/hello:'\''$'\''apr1'\''$'\''hosgaxyv'\''$'\''D0KXp5dCyZ2BUYCS9BmHu1/g' /asd/filename.txt
In both the case , its not replacing the content

Answer 1

You don't need eval here at all:
fp=/asd/filename.txt
sed -i -E "s/(${2//'$'/'\$'}).*/\1:${3//'$'/'\$'}/g" "$fp"
The whole sed command is in double quotes so variables can expand.
I've replaced the blank as the s separator with / (doesn't really matter in the example).
I've used \1 to reference the first capture group instead of repeating the variable in the substitution.
Most importantly, I've used ${2//'$'/'\$'} instead of $2 (and similar for $3). This escapes every $ sign as \$; this is required because of the double quoting, or the $ get eaten by the shell before sed gets to see them.
When you call your script, you must escape any $ in the input, or the shell tries to expand them as variable names:
./test.sh update asd '$apr1$HnIF6bOt$9m3NzAwr.aG1Yp.t.bpIS1.'

Answer 2

Put the command-line arguments that are filenames in single quotes:
./test.sh update 'asd' '$apr1$HnIF6bOt$9m3NzAwr.aG1Yp.t.bpIS1'

Answer 3

must protect all the script arguments with quotes if having space and special shell char, and escape it if it's a dollar $, and -Ei instead of -iE even better drop it first for test, may add it later if being really sure
I admit i won't understant your regex so let's just get in the gist of solution, no need eval;
fp=/asd/filename.txt
sed -Ei "s/($2).*/$2:$3/g" $fp
./test.sh update asd '\$apr1\$HnIF6bOt\$9m3NzAwr.aG1Yp.t.bpIS1.'