bash use cat in sed command
# Basic syntax:
sed 's/<search_text>/'"`cat replacement.txt`"'/' your_file
# Where:
# - The replacement.txt file contains a single line that will be used
# to replace the search_text, where found in your_file.
# Note, this isn't very useful unless combined with other functions like
# find exec, e.g.:
find *.txt -exec sed -i 's/search_text/'"`cat replacement.txt`"'/' {} \;