Heredoc Multi Line String

Heredoc Syntax

Heredoc (<<DELIMITER) allows multi-line string input in shell:

• Content between delimiters is passed as input to the command
• Variables are expanded by default
  • Use single quotes around first delimiter usage (<<'DELIMITER') to prevent expansion.
• Delimiter can be any word (commonly EOF, END, etc.)
• ${VAR:?} expands variable or exits with error if unset

Common pattern: cat > file.txt <<EOF to write multi-line content to a file.

Example

cat > ./.tmp/bucket-policy.json <<__EOF__
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "PublicReadGetObject",
      "Effect": "Allow",
      "Principal": "*",
      "Action": "s3:GetObject",
      "Resource": "arn:aws:s3:::${BUCKET_NAME:?}/*"
    }
  ]
}
__EOF__