Set the value of myval to $1 if it is not empty. If it is empty, set value of myval to val. Is this syntax correct in bash? 
#!/bin/bash
val=5
myval=""
if [ "$val" != "" ]; then
    myval=("$1" == "" ? $val : $1)
fi
echo $myval
Set the value of myval to $1 if it is not empty. If it is empty, set value of myval to val. Is this syntax correct in bash? 
#!/bin/bash
val=5
myval=""
if [ "$val" != "" ]; then
    myval=("$1" == "" ? $val : $1)
fi
echo $myval
The syntax is incorrect, but there is a parameter expansion operator to do what you want.
myval=${1:-$val}
