envsubst implementation in pure bash

envsubst.sh

#!/bin/bash

# Pure Bash implementation of envsubst.
# Usage: ./envsubst.sh [SHELL-FORMAT]

# self-ref
# https://gist.githubusercontent.com/vroomfondel/7ef185d8d6d726fdf720ad0f4ef23619/raw/envsubst.sh

# per curl with cache-busting query-arg
# curl -L https://gist.githubusercontent.com/vroomfondel/7ef185d8d6d726fdf720ad0f4ef23619/raw/envsubst.sh?cache_bust=$(date +%s)

# per api (usage throttled)
# curl -s https://api.github.com/gists/7ef185d8d6d726fdf720ad0f4ef23619 | jq -r '.files["envsubst.sh"].content'


# If arguments were passed, extract the variable names to be replaced.
REPLACE_ONLY=""
if [ $# -gt 0 ]; then
    # Extracts variables like $VAR or ${VAR} from the argument string.
    REPLACE_ONLY=$(echo "$1" | grep -oE '\$([a-zA-Z_][a-zA-Z0-9_]*|\{[a-zA-Z_][a-zA-Z0-9_]*\})')
fi

# Function to process a single line.
process_line() {
    local line="$1"
    local result=""
    
    # Regex for $VAR or ${VAR}.
    # Matches $ followed by a valid shell variable name or {name}.
    local regex='\$([a-zA-Z_][a-zA-Z0-9_]*|\{[a-zA-Z_][a-zA-Z0-9_]*\})'
    
    local remaining="$line"
    while [[ "$remaining" =~ $regex ]]; do
        local full_match="${BASH_REMATCH[0]}"
        local var_name_raw="${BASH_REMATCH[1]}"
        
        # Split the remaining string: before, match, after.
        local before="${remaining%%"$full_match"*}"
        local after="${remaining#*"$full_match"}"
        
        # Clean the variable name (remove curly braces).
        local clean_var_name="${var_name_raw#\{}"
        clean_var_name="${clean_var_name%\}}"
        
        # Check if this variable should be replaced.
        local should_replace=true
        
        # Only replace if the variable is set in the environment.
        if ! [[ -v "$clean_var_name" ]]; then
            should_replace=false
        fi

        # If REPLACE_ONLY is set, only replace if the variable is in the list.
        if [ "$should_replace" = true ] && [ -n "$REPLACE_ONLY" ]; then
            if ! echo "$REPLACE_ONLY" | grep -qxFe "$full_match"; then
                should_replace=false
            fi
        fi
        
        if [ "$should_replace" = true ]; then
            # Read value from the environment.
            local var_value="${!clean_var_name}"
            result="${result}${before}${var_value}"
        else
            result="${result}${before}${full_match}"
        fi
        
        remaining="$after"
    done
    
    result="${result}${remaining}"
    printf "%s\n" "$result"
}

# Read from stdin line by line.
# IFS= preserves leading/trailing whitespace.
# -r prevents backslashes from being interpreted.
while IFS= read -r input_line || [ -n "$input_line" ]; do
    process_line "$input_line"
done