BASH functions inside echo strings - string

Question

I feel like an idiot. I want a BASH function that alternates values every time it's called. The script itself is very simple, and it works if I call the function directly. But it doesn't work the same when called inside a string. Here's the code:
odd_or_even()
{
if [ $ODDEVEN -eq 1 ]; then
echo "odd"
let "ODDEVEN+=1"
else
echo "even"
let "ODDEVEN-=1"
fi
}
ODDEVEN=1
odd_or_even # Prints "odd"
odd_or_even # Prints "even"
echo "<td class=\"`odd_or_even`\">Test</td>" # Prints class=odd
echo "<td class=\"`odd_or_even`\">Test</td>" # Prints class=odd
Does BASH have restrictions about calling functions inside strings? It seems to work because it's outputting something, but it's not performing the mathematical operation.

Answer 1

The back quotes create sub-shells and the environment is reset in each sub-shell so you don't actually modify the same variable ODDEVEN.
You can use a file:
odd_or_even()
{
ODDEVEN=$(cat oddfile)
if [ $ODDEVEN -eq 1 ]; then
echo "odd"
let "ODDEVEN=0"
else
echo "even"
let "ODDEVEN=1"
fi
echo $ODDEVEN > oddfile
}
Or let the function do the string manipulation:
odd_or_even()
{
prefix=$1
suffix=$2
if [ $ODDEVEN -eq 1 ]; then
out="odd"
let "ODDEVEN=0"
else
out="even"
let "ODDEVEN=1"
fi
echo $prefix$out$suffix
}
ODDEVEN=1
odd_or_even "<td class=\"" "\">Test</td>"
odd_or_even "<td class=\"" "\">Test</td>"

Answer 2

Perhaps not the most elegant solution around, but you could use file descriptors since they get inherited by child processes (such as subshells).
As already pointed out, variable assignments (such as your let "ODDEVEN+=1" or let "ODDEVEN-=1") in a (backticked) subshell (child process) are not visible to the parent shell (parent process).
odd_or_even()
{
if [ $ODDEVEN -eq 1 ]; then
#echo "odd"
exec 3<&-
exec 3<<<"odd"
let "ODDEVEN+=1"
else
#echo "even"
exec 3<&-
exec 3<<<"even"
let "ODDEVEN-=1"
fi
}
export -f odd_or_even
{
ODDEVEN=1
odd_or_even && cat <&3 3<&- # Prints "odd"
odd_or_even && cat <&3 3<&- # Prints "even"
odd_or_even
echo "<td class=\"`cat <&3 3<&-`\">Test</td>" # Prints class=odd
odd_or_even
echo "<td class=\"`cat <&3 3<&-`\">Test</td>" # Prints class=odd
}
# output:
# odd
# even
# <td class="odd">Test</td>
# <td class="even">Test</td>

Answer 3

The goal in cases like this is to get the function out of the subshell.
function odd_or_even {
case $oddeven in
?([01]))
typeset -a strings=(even odd)
printf %s "${strings[oddeven^=1]}"
;;
*) return 1
esac
}
odd_or_even > >(echo "${prefix}$(</dev/fd/0)${suffix}") || exit
Since you're assigning to a nonlocal variable anyway there isn't really any point in not just using it directly instead of worring about I/O. This keeps both halves out of a subshell.
oddeven=
typeset -a strings=(even odd)
echo "${prefix}${strings[oddeven^=1]}${suffix}"
Your original solution is only possible in ksh93t and mksh R41 or greater using a special command substitution form that doesn't create a subshell.
function odd_or_even {
...
}
print -r -- "${prefix}${ odd_or_even;}${suffix}"
As an aside, Stop using backticks!