Enables a file to use template placeholders to be replaced by variables.

define.sh

#!/bin/bash

# **Example**

# tree="orange"
# live="house"
# sport="football"
# vehicle="car"
# drive="true"
#
# I live in a {{ live }}.
# My tree is an {{ tree }} tree.
# I like {{ sport }}.
# I drive a {{ vehicle }}.
# Driving enabled: {{ drive }}.
# Number of pets: {{ pets }}.
#
# ./define.sh test.txt \
#   live s house \
#   tree s orange \
#   sport s baseball \
#   vehicle s car \
#   drive b true \
#   pets n 3
#
# <selector> <type> <value>

# Check file path provided
if [ $# -lt 1 ]; then
  echo "Error: Missing file path argument." >&2
  echo "Usage: $0 filepath [selector type value]..." >&2
  exit 1
fi

filepath="$1"
shift

# Check file exists
if [ ! -f "$filepath" ]; then
  echo "Error: File not found: $filepath" >&2
  exit 2
fi

# Check args count divisible by 3 (selector, type, value)
if (( $# % 3 != 0 )); then
  echo "Error: Arguments after file path must be in triples: selector type value" >&2
  echo "You provided $# extra arguments, which is not divisible by 3." >&2
  exit 4
fi

content=$(cat "$filepath")
error_count=0

while [ $# -gt 2 ]; do
  selector="$1"
  type="$2"
  value="$3"

  # Validate type & value
  case "$type" in
    n)
      if ! [[ "$value" =~ ^-?[0-9]+(\.[0-9]+)?$ ]]; then
        echo "Error: Selector '$selector' expects a number but got '$value'" >&2
        error_count=$((error_count + 1))
      fi
      ;;
    s)
      # Strings: accept anything (no spaces expected)
      ;;
    b)
      if ! [[ "$value" =~ ^(true|false|1|0|yes|no)$ ]]; then
        echo "Error: Selector '$selector' expects a boolean (true|false|1|0|yes|no) but got '$value'" >&2
        error_count=$((error_count + 1))
      fi
      ;;
    *)
      echo "Error: Unknown type '$type' for selector '$selector'" >&2
      error_count=$((error_count + 1))
      ;;
  esac

  # If no error, replace all occurrences of {{ selector }} in content
  if [ $error_count -eq 0 ]; then
    esc_value=$(printf '%s\n' "$value" | sed 's/[&/\]/\\&/g')
    content=$(echo "$content" | sed -E "s/\{\{\s*$selector\s*\}\}/$esc_value/g")
  fi

  shift 3
done

if [ $error_count -gt 0 ]; then
  echo "Aborting due to $error_count error(s)." >&2
  exit 3
fi

echo "$content" > "$filepath"