Terraform Variable Precedence Explained: Which Source Wins When Values Conflict

Terraform allows variables to be set through multiple sources, including default values, environment variables, .tfvars files, and command-line flags, which can cause confusion when the same variable is defined in several places. The order in which Terraform resolves these conflicts is called variable precedence, where higher-priority sources override lower-priority ones. Default values defined inside variable blocks carry the least weight, while command-line flags passed via -var or -var-file hold the highest priority. Between these two extremes, Terraform processes environment variables, then terraform.tfvars, then terraform.tfvars.json, and finally any .auto.tfvars files in alphabetical order. Understanding this fixed priority order helps developers avoid unexpected behavior when managing infrastructure configurations across different environments.