Bash code injection into bash script

inject_bash_code.sh

#!/bin/bash
# Bash code injection into bash script
# Copyright (c) Alexey Baikov <sysboss [at] mail.ru>
#
# This script will inject multiline code from 'injection.sh' file
# before or after a certain line in 'script.sh' file
#
# target_script    - Script to inject into
# injection_script - Code lines to inject
# patern           - Line to inject the code (before/after)

# Configuration
target_script="script.sh"
injection_script="injection.sh"
patern="Doing stuff"

# Let's initialize the files,
# just for demonstration
cat << EOF > script.sh
#!/bin/bash

echo "Starting the original script"
echo "Doing stuff..."
echo "Exiting"
EOF

cat << EOF > injection.sh
echo "Yay! It works!"
EOF

# create output file
output_file="${target_script}_injected.sh"
echo > ${output_file}

while IFS= read -r line
do
    if [[ "${line}" =~ ${patern} ]]; then
        # uncomment to past before the line
        #cat ${injection_script} >> ${output_file}
        echo "${line}" >> ${output_file}

        # past after the line
        cat ${injection_script} >> ${output_file}
    else
        echo "${line}" >> ${output_file}
    fi
done < "${target_script}"

It's ugly, but it works 🥇