envsubst: add page (#2064)

pages/linux/envsubst.md

@@ -0,0 +1,20 @@
+# envsubst
+
+> Substitutes shell format strings with environment variables in text.
+> Strings 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}}`
+
+- Replace environment variables in an input file and output to a file:
+
+`envsubst < {{path/to/input}} > {{path/to/output}}`
+
+- Replace environment variables in input from a space-separated list:
+
+`envsubst {{variables}} < {{path/to/input}}`