envsubst is a Unix command-line utility that substitutes environment variables with their corresponding values in a given text or file. Here are the key points about envsubst:

• It takes a template file or text as input, and replaces placeholders like $VARIABLE or ${VARIABLE} with the actual values of those environment variables.

• The basic usage is envsubst < input_file which reads the input file, substitutes variables, and prints the result to stdout.

• You can redirect the output to a file using envsubst < input_file > output_file.

• The -v or --variables option lists all the variables referenced in the input, without performing substitution.

• You can specify a subset of variables to substitute using the SHELL-FORMAT argument, e.g. envsubst '$VAR1 $VAR2' < input_file.

• It is particularly useful for templating configuration files, scripts, or any text that needs to be dynamically populated with environment-specific values.

In summary, envsubst is a simple yet powerful tool for variable substitution, making it easier to manage and deploy applications or configurations across different environments.