A bash equivalent of gettext's envsubst executable.

envsubst.sh

# envsubst invokes all shell substitutions in a file
# use by first sourcing this file, e.g. `source envsubst.sh` or `. envsubst.sh`, 
#   and then invoking `envsubst` on STDIN or a specific file
# it should only be invoked on trusted or scrubbed files to avoid injection attacks
function envsubst {
  local infile=${1}

  if [[ "${SUPPRESS_ENVSUBST_WARNING}" != "1" ]]; then
    >&2 echo "[WARNING] the envsubst function should only be used on trusted files because it is vulnerable to injection attacks"
  fi

  # ${infile:--} means:
  #   1. if infile is set use its value
  #   2. if infile is unset or null (":-"), read from stdin ("-")
  # sed replaces '"' with '\"' so that it's retained by echo
  # the part within `$()` is executed first, then eval is called
  eval "echo \"$(cat ${infile:--} | sed 's/\"/\\"/g')\""
}