Creating bash case patterns from a space-delimited string

With bash 4 or 5 (bash 3 doesn't have associative arrays), you could use a dispatch table:

FRUITS="apple orange pear banana cherry kiwi" 

declare -A dispatch

# set default action
# note: will expand pathnames, etc (consider FRUITS="mango /*")
for action in $FRUITS; do
    dispatch[$action]=default_action
done

# add overrides
dispatch[apple]=action_1
dispatch[cherry]=action_2

# define actions
action_1(){ echo action_1; }
action_2(){ echo action_2; }
default_action(){ echo default_action; }
invalid_action(){ echo "Invalid fruit"; exit 1; }

# loop over arguments
for fruit in "$@"; do
     ${dispatch[$fruit]:-invalid_action}
done

Use another loop to match against the patterns like so:

for fruit in $FRUITS; do
  case $fruit in
  apple)
    action_1 ;;
  cherry)
    action_2 ;;
  *)
    for pat in $FRUITS; do
      case $fruit in "$pat")
        default_action
        continue 2
      esac
    done
    echo Invalid fruit
    exit 1
  esac
done

I ended up using sed, shopt -s extglob and a new variable. It was the easiest way to get what I desired without modifying the script too much:

fruits="apple orange pear banana cherry kiwi"
fruits_pattern=$(echo "$fruits" | sed 's/[[:space:]]/|/g')
shopt -s extglob
if ! [ "$1" = "" ]; then fruits=$*; fi
for fruit in $fruits; do
  case "$fruit" in
    apple)
      action_1 ;;
    cherry)
      action_2 ;;
    $fruits_pattern)
      default_action ;;
    *)
      echo "Invalid fruit"
      exit 1 ;;
  esac
done

Thanks to @oguzismail for continue 2. I did not know about it... VERY useful in the future.

Thanks to @EdMorton for a better future method using bash arrays.

Your script could never print Invalid fruit if you want to use $FRUITS before the * case since you're looping on the contents of $FRUITS. Maybe you want to get rid of if ! [ "$1" = "" ]; then FRUITS=$*; fi and then change for fruit in $FRUITS; do to for fruit in "$@"; do? Assuming that's the case, the following would do what you want:

$ cat tst.sh
#!/usr/bin/env bash

declare -A fruits
for fruit in apple orange pear banana cherry kiwi; do
    fruits["$fruit"]=1
done

for fruit in "$@"; do
    case "$fruit" in
    apple)
        echo action_1 ;;
    cherry)
        echo action_2 ;;
    *)
        if [[ -v "fruits[$fruit]" ]]; then
            echo default_action
        else
            echo "Invalid fruit"
            exit 1
        fi
        ;;
    esac
done

but in reality I'd reorganize it to test for valid fruit before the case statement which would then just test the valid values:

#!/usr/bin/env bash

declare -A fruits
for fruit in apple orange pear banana cherry kiwi; do
    fruits["$fruit"]=1
done

for fruit in "$@"; do
    if [[ -v "fruits[$fruit]" ]]; then
        case "$fruit" in
        apple )
            echo action_1 ;;
        cherry )
            echo action_2 ;;
        * )
            echo default_action ;;
        esac
    else
        echo "Invalid fruit"
        exit 1
    fi
done

I'm using lower case letters in the variable name fruits since it's not an environment variable (see Correct Bash and shell script variable capitalization) and an associative array instead of a scalar so I can quote its expansion (see https://mywiki.wooledge.org/Quotes). I made fruits an associative instead of indexed array so I could test for a given fruit existing as an index using -v.

If you really did want your code to start by possibly populating fruits from arguments, you could add an initial scalar array:

if (( $# == 0 )); then
    vals=( apple orange pear banana cherry kiwi )
else
    vals=( "$@" )
fi

declare -A fruits
for fruit in "${vals[@]}"; do
    fruits["$fruit"]=1
done