Insert multiple lines containing trailing slashes after a matching line in a file

insert-lines.sh

#!/usr/bin/env bash

set -euo pipefail

cat >file.txt << 'EOF'
  LINE 1 \
  LINE 2 \
  LINE 5
EOF

set +e
IFS= read -r -d '' missing_lines << 'EOF'
  LINE 3 \
  LINE 4 \
EOF
set -e

awk -v lines="$missing_lines" -i inplace '{print} /LINE 2/{printf lines}' file.txt


# OUTPUT:
#  LINE 1 \
#  LINE 2 \
#  LINE 3 \
#  LINE 4 \
#  LINE 5