envsubst 命令

envsubst substitutes environment variable references in shell format strings. It reads text from stdin, replaces $VAR and ${VAR} patterns with their values from the environment, and writes the result to stdout. This tool is essential for generating configuration files from templates in deployment pipelines and containerized environments. A template might contain ${DATABASE_HOST} which gets replaced with the actual value at deployment time. When called without a variable list, envsubst replaces all environment variable references. To substitute only specific variables (leaving others as literal text), provide a SHELL-FORMAT argument listing the desired variables. The tool handles both $VAR and ${VAR} syntax. Missing variables are replaced with empty strings. The ${VAR:-default} syntax for defaults is NOT supported; envsubst performs simple substitution only. Common uses include Docker entrypoint scripts that configure services at runtime, CI/CD pipelines generating configs, and any scenario where configuration needs to adapt to the runtime environment.

Run `envsubst < [template.txt]` in a terminal, then adjust file names, paths, flags, or remote targets for your system.

Output the variables occurring in SHELL-FORMAT or stdin.