---
name: bash-53-features
description: Bash 5.3 new features and modern patterns (2025)
---

## 🚨 CRITICAL GUIDELINES

### Windows File Path Requirements

**MANDATORY: Always Use Backslashes on Windows for File Paths**

When using Edit or Write tools on Windows, you MUST use backslashes (`\`) in file paths, NOT forward slashes (`/`).

**Examples:**
- ❌ WRONG: `D:/repos/project/file.tsx`
- ✅ CORRECT: `D:\repos\project\file.tsx`

This applies to:
- Edit tool file_path parameter
- Write tool file_path parameter
- All file operations on Windows systems


### Documentation Guidelines

**NEVER create new documentation files unless explicitly requested by the user.**

- **Priority**: Update existing README.md files rather than creating new documentation
- **Repository cleanliness**: Keep repository root clean - only README.md unless user requests otherwise
- **Style**: Documentation should be concise, direct, and professional - avoid AI-generated tone
- **User preference**: Only create additional .md files when user specifically asks for documentation


---

# Bash 5.3 Features (2025)

## Overview

Bash 5.3 (released July 2025) introduces significant new features that improve performance, readability, and functionality.

## Key New Features

### 1. In-Shell Command Substitution

**New: ${ command; } syntax** - Executes without forking a subshell (runs in current shell context):

```bash
# OLD way (Bash < 5.3) - Creates subshell
output=$(expensive_command)

# NEW way (Bash 5.3+) - Runs in current shell, faster
output=${ expensive_command; }
```

**Benefits:**
- No subshell overhead (faster)
- Preserves variable scope
- Better performance in loops

**Example:**
```bash
#!/usr/bin/env bash

# Traditional approach
count=0
for file in *.txt; do
    lines=$(wc -l < "$file")  # Subshell created
    ((count += lines))
done

# Bash 5.3 approach (faster)
count=0
for file in *.txt; do
    lines=${ wc -l < "$file"; }  # No subshell
    ((count += lines))
done
```

### 2. REPLY Variable Command Substitution

**New: ${| command; } syntax** - Stores result in REPLY variable:

```bash
# Runs command, result goes to $REPLY automatically
${| complex_calculation; }
echo "Result: $REPLY"

# Multiple operations
${|
    local_var="processing"
    echo "$local_var: $((42 * 2))"
}
echo "Got: $REPLY"
```

**Use Cases:**
- Avoid variable naming conflicts
- Clean syntax for temporary values
- Standardized result variable

### 3. Enhanced `read` Builtin

**New: -E option** - Uses readline with programmable completion:

```bash
# Interactive input with tab completion
read -E -p "Enter filename: " filename
# User can now tab-complete file paths!

# With custom completion
read -E -p "Select environment: " env
# Enables full readline features (history, editing)
```

**Benefits:**
- Better UX for interactive scripts
- Built-in path completion
- Command history support

### 4. Enhanced `source` Builtin

**New: -p PATH option** - Custom search path for sourcing:

```bash
# OLD way
source /opt/myapp/lib/helpers.sh

# NEW way - Search custom path
source -p /opt/myapp/lib:/usr/local/lib helpers.sh

# Respects CUSTOM_PATH instead of current directory
CUSTOM_PATH=/app/modules:/shared/lib
source -p "$CUSTOM_PATH" database.sh
```

**Benefits:**
- Modular library organization
- Avoid hard-coded paths
- Environment-specific sourcing

### 5. Enhanced `compgen` Builtin

**New: Variable output option** - Store completions in variable:

```bash
# OLD way - Output to stdout
completions=$(compgen -f)

# NEW way - Directly to variable
compgen -v completions_var -f
# Results now in $completions_var
```

**Benefits:**
- Cleaner completion handling
- No extra subshells
- Better performance

### 6. GLOBSORT Variable

**New: Control glob sorting behavior**:

```bash
# Default: alphabetical sort
echo *.txt

# Sort by modification time (newest first)
GLOBSORT="-mtime"
echo *.txt

# Sort by size
GLOBSORT="size"
echo *.txt

# Reverse alphabetical
GLOBSORT="reverse"
echo *.txt
```

**Options:**
- `name` - Alphabetical (default)
- `reverse` - Reverse alphabetical
- `size` - By file size
- `mtime` - By modification time
- `-mtime` - Reverse modification time

### 7. BASH_TRAPSIG Variable

**New: Signal number variable in traps**:

```bash
#!/usr/bin/env bash
set -euo pipefail

# BASH_TRAPSIG contains the signal number being handled
handle_signal() {
    echo "Caught signal: $BASH_TRAPSIG" >&2
    case "$BASH_TRAPSIG" in
        15) echo "SIGTERM (15) received, shutting down gracefully" ;;
        2)  echo "SIGINT (2) received, cleaning up" ;;
        *)  echo "Signal $BASH_TRAPSIG received" ;;
    esac
}

trap handle_signal SIGTERM SIGINT SIGHUP
```

**Benefits:**
- Reusable signal handlers
- Dynamic signal-specific behavior
- Better logging and debugging

### 8. Floating-Point Arithmetic

**New: `fltexpr` loadable builtin**:

```bash
# Enable floating-point support
enable -f /usr/lib/bash/fltexpr fltexpr

# Perform calculations
fltexpr result = 42.5 * 1.5
echo "$result"  # 63.75

# Complex expressions
fltexpr pi_area = 3.14159 * 5 * 5
echo "Area: $pi_area"
```

**Use Cases:**
- Scientific calculations
- Financial computations
- Avoid external tools (bc, awk)

## Performance Improvements

### Avoid Subshells

```bash
# ❌ OLD (Bash < 5.3) - Multiple subshells
for i in {1..1000}; do
    result=$(echo "$i * 2" | bc)
    process "$result"
done

# ✅ NEW (Bash 5.3+) - No subshells
for i in {1..1000}; do
    result=${ echo $((i * 2)); }
    process "$result"
done
```

**Performance Gain:** ~40% faster in benchmarks

### Efficient File Processing

```bash
#!/usr/bin/env bash

# Process large file efficiently
process_log() {
    local line_count=0
    local error_count=0

    while IFS= read -r line; do
        ((line_count++))

        # Bash 5.3: No subshell for grep
        if ${ grep -q "ERROR" <<< "$line"; }; then
            ((error_count++))
        fi
    done < "$1"

    echo "Processed $line_count lines, found $error_count errors"
}

process_log /var/log/app.log
```

## Migration Guide

### Check Bash Version

```bash
#!/usr/bin/env bash

# Require Bash 5.3+
if ((BASH_VERSINFO[0] < 5 || (BASH_VERSINFO[0] == 5 && BASH_VERSINFO[1] < 3))); then
    echo "Error: Bash 5.3+ required (found $BASH_VERSION)" >&2
    exit 1
fi
```

### Feature Detection

```bash
# Test for 5.3 features
has_bash_53_features() {
    # Try using ${ } syntax
    if eval 'test=${ echo "yes"; }' 2>/dev/null; then
        return 0
    else
        return 1
    fi
}

if has_bash_53_features; then
    echo "Bash 5.3 features available"
else
    echo "Using legacy mode"
fi
```

### Gradual Adoption

```bash
#!/usr/bin/env bash
set -euo pipefail

# Support both old and new bash
if ((BASH_VERSINFO[0] > 5 || (BASH_VERSINFO[0] == 5 && BASH_VERSINFO[1] >= 3))); then
    # Bash 5.3+ path
    result=${ compute_value; }
else
    # Legacy path
    result=$(compute_value)
fi
```

## Best Practices (2025)

1. **Use ${ } for performance-critical loops**
   ```bash
   for item in "${large_array[@]}"; do
       processed=${ transform "$item"; }
   done
   ```

2. **Use ${| } for clean temporary values**
   ```bash
   ${| calculate_hash "$file"; }
   if [[ "$REPLY" == "$expected_hash" ]]; then
       echo "Valid"
   fi
   ```

3. **Enable readline for interactive scripts**
   ```bash
   read -E -p "Config file: " config
   ```

4. **Use source -p for modular libraries**
   ```bash
   source -p "$LIB_PATH" database.sh logging.sh
   ```

5. **Document version requirements**
   ```bash
   # Requires: Bash 5.3+ for performance features
   ```

## Compatibility Notes

### Bash 5.3 Availability (2025)

**Note:** Bash 5.3 (released July 2025) is the latest stable version. There is no Bash 5.4 as of October 2025.

- **Linux**: Ubuntu 24.04+, Fedora 40+, Arch (current)
- **macOS**: Homebrew (`brew install bash`)
- **Windows**: WSL2 with Ubuntu 24.04+
- **Containers**: `bash:5.3` official image

### C23 Conformance

Bash 5.3 updated to C23 language standard. **Note:** K&R style C compilers are no longer supported.

### Fallback Pattern

```bash
#!/usr/bin/env bash
set -euo pipefail

# Detect bash version
readonly BASH_53_PLUS=$((BASH_VERSINFO[0] > 5 || (BASH_VERSINFO[0] == 5 && BASH_VERSINFO[1] >= 3)))

process_items() {
    local item
    for item in "$@"; do
        if ((BASH_53_PLUS)); then
            result=${ transform "$item"; }  # Fast path
        else
            result=$(transform "$item")      # Compatible path
        fi
        echo "$result"
    done
}
```

## Real-World Examples

### Fast Log Parser

```bash
#!/usr/bin/env bash
set -euo pipefail

# Parse log file (Bash 5.3 optimized)
parse_log() {
    local file="$1"
    local stats_errors=0
    local stats_warnings=0
    local stats_lines=0

    while IFS= read -r line; do
        ((stats_lines++))

        # Fast pattern matching (no subshell)
        ${| grep -q "ERROR" <<< "$line"; } && ((stats_errors++))
        ${| grep -q "WARN" <<< "$line"; }  && ((stats_warnings++))
    done < "$file"

    echo "Lines: $stats_lines, Errors: $stats_errors, Warnings: $stats_warnings"
}

parse_log /var/log/application.log
```

### Interactive Configuration

```bash
#!/usr/bin/env bash
set -euo pipefail

# Interactive setup with readline
setup_config() {
    echo "Configuration Setup"
    echo "==================="

    # Tab completion for paths
    read -E -p "Data directory: " data_dir
    read -E -p "Config file: " config_file

    # Validate and store
    ${|
        [[ -d "$data_dir" ]] && echo "valid" || echo "invalid"
    }

    if [[ "$REPLY" == "valid" ]]; then
        echo "DATA_DIR=$data_dir" > config.env
        echo "CONFIG_FILE=$config_file" >> config.env
        echo "✓ Configuration saved"
    else
        echo "✗ Invalid directory" >&2
        return 1
    fi
}

setup_config
```

## Resources

- [Bash 5.3 Release Notes](https://lists.gnu.org/archive/html/bash-announce/2025-07/msg00000.html)
- [Bash Manual - Command Substitution](https://www.gnu.org/software/bash/manual/html_node/Command-Substitution.html)
- [ShellCheck Bash 5.3 Support](https://github.com/koalaman/shellcheck/releases)

---

**Bash 5.3 provides significant performance and usability improvements. Adopt these features gradually while maintaining backwards compatibility for older systems.**