bash-53-features

Bash 5.3 Features and Improvements

Released in July 2025, Bash 5.3 introduces significant enhancements including revolutionary command substitution syntax, new variables, loadable builtins, and improved C standard conformance.

Revolutionary Command Substitution

Efficient In-Shell Execution: ${ command; }

Execute commands without forking, dramatically improving performance:

# Traditional command substitution (creates subshell)
result=$(echo "Hello, World")

# NEW: In-shell command substitution (no fork!)
# Note: a space (or tab/newline/|) is required after the opening '{'
result=${ echo "Hello, World"; }

# Practical example: Fast variable assignment
config_value=${ grep "^timeout=" config.txt | cut -d= -f2; }

# Performance comparison function
benchmark_substitution() {
    local i
    local start end

    echo "Testing traditional substitution..."
    start="${EPOCHREALTIME}"
    for ((i = 0; i < 1000; i++)); do
        result=$(echo "${i}")
    done
    end="${EPOCHREALTIME}"
    printf 'Traditional: %.4f seconds\n' \
        "$(awk "BEGIN {print ${end} - ${start}}")"

    echo "Testing new in-shell substitution..."
    start="${EPOCHREALTIME}"
    for ((i = 0; i < 1000; i++)); do
        result=${ echo "${i}"; }
    done
    end="${EPOCHREALTIME}"
    printf 'In-shell: %.4f seconds\n' \
        "$(awk "BEGIN {print ${end} - ${start}}")"
}

benchmark_substitution

Benefits:

• No subprocess creation overhead
• Significantly faster for simple commands
• Reduced resource usage in loops
• Same syntax familiarity as command substitution

REPLY Variable Capture: ${| command; }

Execute commands and automatically store output in REPLY. Note: REPLY is local to the substitution — its value is restored after completion, so capture it immediately:

Code examples

Benefits:

• Cleaner code without intermediate variables
• Consistent capture mechanism
• Reduced visual clutter
• Perfect for rapid prototyping