tldr/pages/common/envsubst.md

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# 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}}`