Heredoc Templating (Generating a Config File from Variables)

#!/bin/bash
host="db.example.com"
port=5432
user="app_user"

cat > db_config.ini <<EOF
[database]
host=$host
port=$port
user=$user
EOF

cat db_config.ini
rm -f db_config.ini

#!/bin/bash
version="1.2.0"

cat > release_notes.txt <<EOF
Release: $version
Generated year: $(date +%Y)
EOF

cat release_notes.txt
rm -f release_notes.txt

#!/bin/bash
host="localhost"
port=""

if [[ -z $host || -z $port ]]; then
    echo "missing required template variable(s), skipping generation" >&2
else
    cat > out.ini <<EOF
host=$host
port=$port
EOF
fi
echo "validation check complete"

1. Quoting the heredoc delimiter (<<EOF) when you actually wanted variable substitution, which produces literal $var text instead of the expanded value.
2. Forgetting that command substitution and arithmetic expansion also happen inside an unquoted heredoc, not just simple variable expansion.
3. Not validating that all required variables are set before generating a config file, which can silently produce a broken file with empty values.

• An unquoted heredoc delimiter allows variables, command substitution, and arithmetic expansion inside the block.
• Redirecting a heredoc into a file with > filename is a simple, readable way to generate config files from a template.
• Combine heredoc templating with set -u (or explicit checks) so a missing variable causes a clear error rather than an empty gap in the generated file.
• Nested double quotes inside an unquoted heredoc don't need escaping the way they would inside a quoted string.