sed replace string with file contents code example

Example 1: sed replace in file

sed -i 's/foo/bar/g' hello.txt

Example 2: sed replace with variable

Just concatenate var '"$var"' to replace values string in sed command:
$ sed -i 's/string_to_replace/'"$var"'/g' file_to_replace.txt

Example 3: replace text with sed

sed -i 's/old-text/new-text/g' input.txt

Example 4: bash find and replace all files with specifc name with another file

find . -name "boom.txt" -exec cp ~/replace.txt {} \;