sh conditionally pass an option to command - bash

Question

I want to do something like:
#!/bin/sh
[ -f "/tmp/nodes" ]
[[ $? -eq 0 ]] && VAL=$? ||
geth --datadir /root/.ethereum \
${VAL+"--nodekey \"/root/nodekey.txt\""} \
--networkid 1999 \
--rpc \
--rpcaddr "0.0.0.0" \
I want the option --nodekey "/root/nodekey.txt" to be passed if the file /tmp/nodes exists. How can that be done more elegantly than an if with two nearly identical commands?
--EDIT--
This is the best I've been able to get working so far:
if [ $VAL -eq 0 ]; then
/geth --datadir /root/.ethereum \
--nodekey "/root/nodekey.txt" \
# No dice
# Would be nice if this worked so I didn't need the if
# ${VAL+ --nodekey "/root/nodekey.txt" } \
--networkid 1999 \
--rpc \
--rpcaddr "0.0.0.0"
else
/geth --datadir /root/.ethereum \
--networkid 1999 \
--rpc \
--rpcaddr "0.0.0.0" \
fi
This is another line in the file and works fine:
ENODE_URL=$(/geth --datadir /root/.ethereum ${VAL+ --nodekey "/root/nodekey.txt"} --exec "${JS}" console 2>/dev/null | sed -e 's/^"\(.*\)"$/\1/')

Answer 1

There's a bashism here, but it's [[ $? -eq 0 ]], as [[ is a ksh extension adopted by bash. There's no point to using $? at all here, since you can just directly perform your assignment based on whether the test -f succeeds:
touch /tmp/nodes # set us up for the truthy path
if test -f /tmp/nodes; then tmp_nodes_exists=1; else unset tmp_nodes_exists; fi
printf '%s\n' /tmp/nodes ${tmp_nodes_exists+"REALLY EXISTS" "(yes, really)"}
...properly emits as output (as run with dash, perhaps the most common minimal /bin/sh interpreter):
/tmp/nodes
REALLY EXISTS
(yes, really)
By contrast, to demonstrate that the other path fails as it should:
rm -f -- /tmp/nodes # set us up for the falsey path
if test -f /tmp/nodes; then tmp_nodes_exists=1; else unset tmp_nodes_exists; fi
printf '%s\n' /tmp/nodes ${tmp_nodes_exists+"REALLY EXISTS" "(yes, really)"}
emits as output only:
/tmp/nodes