envsubst

Substitute environment variables in a file

$ envsubst < [template.txt]

$ envsubst < [template.txt] > [output.txt]

$ envsubst '$USER $HOME' < [template.txt]

$ echo 'Hello $USER' | envsubst

$ envsubst -v < [template.txt]

$ envsubst --variables < [template.txt]

$ cat [nginx.conf.template] | envsubst '$SERVER_NAME $PORT' > [nginx.conf]

envsubst [-v] [-V] [--variables] [SHELL-FORMAT] [< input]

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.

-v, --variables

Output the variables occurring in SHELL-FORMAT or stdin.

Display version information.

Display help information.

Space-separated list of variables to substitute. If specified, only these variables are replaced; others remain as literal text.

Does not support shell parameter expansion features like ${VAR:-default} or ${VAR:+value}. All variables must exist in the environment; missing ones become empty strings. Dollar signs that should remain literal need escaping as $$. Part of gettext package.

envsubst is part of GNU gettext, the internationalization toolkit. While gettext is primarily for software translation, envsubst became widely adopted as a general-purpose template tool, particularly in the DevOps community for configuration management. It's available on virtually all Unix-like systems as part of the gettext-base package.

gettext(1), sed(1), awk(1), env(1), export(1)