Declaring a variable name (not value) in bash with string substitution [duplicate] - bash

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Dynamic variable names in Bash
(19 answers)
Closed 1 year ago.
I have a set of variables whose values I need to modify in a loop, if a match is found in an array which I am reading later. The array elements are substrings of the variable names, so I thought of writing a generic for loop, which would modify the variable values on the fly, attempting string substitution in the variable names, but that doesn't work.
Here is the code:
CUT_APPLE=false
CUT_GUAVA=false
CUT_MANGO=false
AVAILABLE_FRUITS=("APPLE" "GUAVA")
for i in "${AVAILABLE_FRUITS[#]}"; do
CUT_$i=true
done
**CUT_APPLE=true: command not found**
**CUT_GUAVA=true: command not found**
Is there any way using which I can get interpolation in variable name declaration to work?

Answer 1

If your Bash version is greater than 4, it supports associative arrays which make it easy to check whether a boolean key exists:
#!/usr/bin/env bash
# Associative array to store keys of available fruits
declare -A cut_fruit=()
declare -a available_fruits=('APPLE' 'GUAVA' 'RED BANANA')
for fruit in "${available_fruits[#]}"; do
# Create key for fruit in Associative array if available
# shellcheck disable=SC2034 # cut_fruit is indeed used
cut_fruit[$fruit]=
done
# Check if cut fruit is available by checking key existence
for fruit in 'APPLE' 'BANANA' 'RED BANANA' 'MANGO'; do
if [[ -v cut_fruit[$fruit] ]]; then
printf 'Cut %s is available.\n' "$fruit"
else
printf 'There is no cut %s.\n' "$fruit"
fi
done
Sample output:
Cut APPLE is available.
There is no cut BANANA.
Cut RED BANANA is available.
There is no cut MANGO.
Here is a little game to play with associative arrays keys and boolean checks:
#!/usr/bin/env bash
print_basket() {
# Print remaining keys (content of basket)
for fruit in "${!basket[#]}"; do
printf '%d %s\n' "${basket[$fruit]}" "${fruit,,}"
done
}
cat <<'EOF'
--:{# Guess the fruits in my basket #}:--
EOF
declare -a all_fruits=(
'apple' 'banana' 'cherry' 'lemon' 'mango'
'orange' 'peach' 'plum' 'red banana' 'sweat chestnut')
# Number of fruits in the basked
selection_size=3
declare -a fruits_selection
mapfile -t fruits_selection < <(
# Shuffle a selection of fruits
printf '%s\n' "${all_fruits[#]}" | shuf -n "$selection_size"
)
declare -A basket
# Set selected fruits into basked
for fruit in "${fruits_selection[#]}"; do
# Uppercase the key for case insensitive match later
basket[${fruit^^}]=$((RANDOM % 9 + 1))
done
# Cheating
declare -p basket
max_tries=3
tries_left=$max_tries
guessed=0
while [ $guessed -lt $selection_size ] && [ $tries_left -gt 0 ]; do
printf 'Enter the name of a fruit: '
read -r fruit
# Uppercase fruit for case insensitive match
fruit="${fruit^^}"
if [[ -v basket[$fruit] ]]; then
printf 'Yes, there is %d %s in my basket!\n' \
"${basket[$fruit]}" "${fruit,,}"
guessed=$((guessed + 1))
# Remove fruit from basket
unset "basket[$fruit]"
else
tries_left=$((tries_left - 1))
printf 'No, there is no %s in my basket!\nTries left: %d\n' \
"${fruit,,}" "$tries_left"
fi
done
case $guessed in
"$selection_size")
printf '* * * Congratulations, you guessed all fruilts! * * *\n'
;;
0)
printf 'Sorry you could not guess any of the fruits!\nBasket contained:\n'
print_basket
;;
*)
printf 'Nice! You guessed %d fruits out of %s!\nUnguessed fruits:\n' \
"$guessed" "$selection_size"
print_basket
;;
esac