envsubst

https://www.gnu.org/software/gettext/manual/html_node/envsubst-Invocation.html

envsubst < cluster-config.yaml | eksctl create cluster -f - ^[1]

help[edit]

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.

Report bugs in the bug tracker at <https://savannah.gnu.org/projects/gettext>
or by email to <[email protected]>.