Arrays demo

In this tutorial, we’ll build a Bash script that picks a random lunch spot from a list—without repeats—by combining indexed arrays and simple file-based persistence. This pattern is perfect for one-time draws, rotating duties, or any scenario where you want to consume items until the list is exhausted.

Imagine a team of 20 colleagues who vote on their favorite restaurants each Friday. Everyone adds their go-to spot to an array. The script then:

• Reads the list into memory
• Selects a random entry
• Displays it
• Removes it from both memory and disk
• Exits with an error if the list is missing or empty

Why In-Memory vs Persistent Storage?

1. Manipulating data in memory means changes disappear once the script ends.
   

2. Persistent storage (files, databases) retains data between runs.
   

Key Bash Features

• mapfile -t: Loads lines from a file (or stdin) into an array.
  

• $RANDOM: Yields a pseudo-random integer between 0 and 32767.
  

Exit Code Reference

1. Prepare the Data File

Create food_places.txt with initial entries:

cat <<EOF > food_places.txt
Ramen
Sushi
Tacos
Dal makhani
EOF

2. Initialize the Script

Create lunch_selector.sh:

#!/usr/bin/env bash
declare -a lunch_options

# Determine script directory
work_dir=$(dirname "$(readlink -f "${0}")")
food_places="${work_dir}/food_places.txt"

# Exit codes
readonly FILE_NOT_FOUND=150
readonly NO_OPTIONS_LEFT=180

terminate() {
  local msg="$1"
  local code="${2:-$FILE_NOT_FOUND}"
  echo "$msg" >&2
  exit "$code"
}

# Guard clause
if [[ ! -f "$food_places" ]]; then
  terminate "Error: food_places.txt file doesn't exist" "$FILE_NOT_FOUND"
fi

3. Load and Validate the Array

Populate lunch_options and ensure it’s not empty:

fillout_array() {
  mapfile -t lunch_options < "$food_places"
  if [[ ${#lunch_options[@]} -eq 0 ]]; then
    terminate "Error: No food options left. Please add options to food_places.txt" "$NO_OPTIONS_LEFT"
  fi
}

fillout_array

4. Select and Display a Random Option

Choose a random index, echo it, and remove from the in-memory list:

index=$(( RANDOM % ${#lunch_options[@]} ))
chosen="${lunch_options[$index]}"
echo "$chosen"

# Remove the selected element
unset 'lunch_options[index]'

5. Persist the Updated List

Write the remaining entries back to food_places.txt:

update_options() {
  if [[ ${#lunch_options[@]} -eq 0 ]]; then
    # Empty the file when no items remain
    : > "$food_places"
  else
    printf "%s\n" "${lunch_options[@]}" > "$food_places"
  fi
}

update_options

6. Complete Script

Here’s the full lunch_selector.sh. Don’t forget to make it executable:

#!/usr/bin/env bash
declare -a lunch_options

work_dir=$(dirname "$(readlink -f "${0}")")
food_places="${work_dir}/food_places.txt"
readonly FILE_NOT_FOUND=150
readonly NO_OPTIONS_LEFT=180

terminate() {
  local msg="$1"
  local code="${2:-$FILE_NOT_FOUND}"
  echo "$msg" >&2
  exit "$code"
}

if [[ ! -f "$food_places" ]]; then
  terminate "Error: food_places.txt file doesn't exist" "$FILE_NOT_FOUND"
fi

fillout_array() {
  mapfile -t lunch_options < "$food_places"
  if [[ ${#lunch_options[@]} -eq 0 ]]; then
    terminate "Error: No food options left. Please add options to food_places.txt" "$NO_OPTIONS_LEFT"
  fi
}

fillout_array

index=$(( RANDOM % ${#lunch_options[@]} ))
chosen="${lunch_options[$index]}"
echo "$chosen"
unset 'lunch_options[index]'

update_options() {
  if [[ ${#lunch_options[@]} -eq 0 ]]; then
    : > "$food_places"
  else
    printf "%s\n" "${lunch_options[@]}" > "$food_places"
  fi
}

update_options

Make it executable and run:

chmod +x lunch_selector.sh
./lunch_selector.sh

Each run prints a random, non-repeating lunch spot until the list is empty.

References

• GNU Bash Reference Manual
• Bash FAQ
• Using mapfile in Bash (StackOverflow)