envsubst

Substitutes environment variables with their value in shell format strings.

Variables to be replaced should be in either `${var}` or `$var` format.

• Replace environment variables in stdin and output to stdout:

echo '{$HOME}' | envsubst

• Replace environment variables in an input file and output to stdout:

envsubst < {path/to/input_file}

• Replace environment variables in an input file and output to a file:

envsubst < {path/to/input_file} > {path/to/output_file}

• Replace environment variables in an input file from a space-separated list:

envsubst '{$USER $SHELL $HOME}' < {path/to/input_file}

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