Function to conditionally set a variable read-only

If I had a script which sets variables read-only to some odd values, and sets errexit because of other unsafe operations: #!/bin/bash set -e declare -r NOTIFY=$(case "$OS" in (macosx) echo macos_notify ;; (linux) echo linux_notify ;; (*) echo : ;; esac) declare -r SAY=_say # _say is a function declare -r VERSION=0.99 set +e And I source it to get the definitions, the second time because it's in development: $ . s.bash $ . s.bash bash: declare: NOTIFY: readonly variable Exited Normally declare -r EXISTING_VAR would neither stop the script nor remove the old, working definition of EXISTING_VAR. But with errexit, assigning to an existing variable is understandably a failure. The easy options are to remove -r or use set +e for that part of the script. Barring those, **is it possible to write a Bash function to take the place of declare -r but not re-assign if the name already exists**? I tried: # arg #1: var name, #2: value set_var_once () { # test whether the variable with the # name stored in $1 exists if [[ -z "${!1}" ]] then # if it doesn't, set it declare -r $1=$2 fi } I also tried things along the lines of eval "declare -r $1=$(eval $2)", it feels like eval is required somewhere here but I'm not sure where. All of the versions of set_var_once result in not setting the variable they should.