Escape dollar sign in string by shell script

As you know, a dollar sign marks a variable. You have to take it into account when you are typing it.

You can escape the dollar

./dd.sh "sample\$name.mp4"

or just type it with single quotes

./dd.sh 'sample$name.mp4'

To check if there is a dollar sign in a variable, do

[[ $variable == *\$* ]] && echo 'I HAZ A DOLAR!!!' || echo 'MEH'

One option:

# Replace occurrences of $ with \$ to prevent variable substitution:
filename="${filename//$/\\$}"

I just realized my prompt was showing foo rather than foo$bar$baz as the name of the current branch. foo$bar$baz was getting assigned to PS1 and $bar and $baz were then expanded. Escaping the dollar signs before including the branch name in PS1 prevents unwanted expansions.

Your issue is not with the echo but with the assignment to $filename.

You say

filename="sample$name.mp4"

This will interpolate the string, which means expanding the variable $name. This will result in $filename having the value sample.mp4 (since $name is presumably undefined, which means it expands to an empty string)

Instead, use single quotes in the assignment:

filename='sample$name.mp4'

echo "$filename" will now result in the expected sample$name.mp4. Obviously, echo '$filename' will still just print $filename because of the single quotes.