Usage: envsubst [OPTION] [SHELL-FORMAT]
Substitutes the values of environment variables.
Operation mode:
  -v, --variables             output the variables occurring in SHELL-FORMAT
Informative output:
  -h, --help                  display this help and exit
  -V, --version               output version information and exit
In normal operation mode, standard input is copied to standard output,
with references to environment variables of the form $VARIABLE or ${VARIABLE}
being replaced with the corresponding values.  If a SHELL-FORMAT is given,
only those environment variables that are referenced in SHELL-FORMAT are
substituted; otherwise all environment variables references occurring in
standard input are substituted.
When --variables is used, standard input is ignored, and the output consists
of the environment variables that are referenced in SHELL-FORMAT, one per line.

$ cat envsubst-template.yaml
apiVersion: v1
kind: Deploy
metadata:
  name: $name
  namespace: $ns

$ name='hello' ns='world' envsubst < envsubst-template.yaml > envsubst-1.yaml
$ cat envsubst-1.yaml
apiVersion: v1
kind: Deploy
metadata:
  name: hello
  namespace: world

$ export name='hello' 
$ export ns='world' 
$ envsubst < envsubst-template.yaml > envsubst-1.yaml

$ name='hello' ns='world' envsubst '$name'< envsubst-template.yaml > envsubst-2.yaml
$ cat envsubst-2.yaml 
apiVersion: v1
kind: Deploy
metadata:
  name: hello
  namespace: $ns

$ name='hello' ns='world' envsubst --variables '$name'< envsubst-template.yaml > envsubst-3.yaml
$ cat envsubst-3.yaml 
name

$ name='hello' ns='world' envsubst --variables '$name,$ns'< envsubst-template.yaml > envsubst-4.yaml
$ cat envsubst-4.yaml 
name
ns