Update cheatsheets

envsubst

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# envsubst
+
+> Substitutes environment variables with their value in shell format strings.
+> Variables to be replaced should be in either `${var}` or `$var` format.
+> More information: <https://www.gnu.org/software/gettext/manual/html_node/envsubst-Invocation.html>.
+
+- 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}}`