Contrary to common assumption, Busybox doesn't default to GNU bash. Instead, it implements ash (Almquist Shell) as its default shell, which is a lightweight POSIX-compliant shell designed for embedded systems.

You can check the current shell in Busybox with:

echo $SHELL
# Or alternatively:
ls -l /bin/sh

Typical output will show:

/bin/sh -> busybox

• No command history by default
• Simpler syntax highlighting
• Fewer built-in commands
• Smaller memory footprint (~100KB vs bash's ~1MB)

While ash is the default, you can compile Busybox with different shell options:

make menuconfig
# Then navigate to:
# Shells -> Choose your default shell

Available options typically include:

• ash (default)
• hush (simpler alternative)
• msh (minimal shell)
• lsh (larger shell)

When writing scripts for Busybox, use POSIX-compliant syntax:

#!/bin/sh
# This works in both ash and bash
for i in 1 2 3; do
    echo "Count: $i"
done

Avoid bashisms like:

#!/bin/bash
# This might fail in Busybox's ash
echo {1..10}

You can temporarily switch shells in a running Busybox system:

busybox ash
# or
busybox hush

Contrary to common assumption, Busybox does not default to Bash. The default shell is actually ash, a lightweight Bourne-compatible shell that's compiled directly into the Busybox binary. This design choice reflects Busybox's philosophy of minimalism for embedded systems.

Busybox prioritizes small footprint over feature richness:

• Ash typically requires just 60-100KB compared to Bash's 1MB+
• No dynamic library dependencies (pure static linking)
• Sufficient for most recovery/initramfs operations

Run these commands to verify:

# Method 1: Check symlinks
ls -l /bin/sh

# Method 2: Busybox shell detection
busybox --list | grep -E 'ash|sh|bash'

# Method 3: Shell self-report
echo $0

To override the default, modify Busybox compilation:

# .config file modification example
CONFIG_ASH=y
CONFIG_FEATURE_SH_MATH=y
CONFIG_HUSH=n

When porting Bash scripts to Busybox:

# Problematic Bash-ism:
array=(item1 item2)

# Busybox-compatible alternative:
i=0
array="item1 item2"
for item in $array; do
    echo "Item $((i+=1)): $item"
done

In speed-critical scenarios:

• Ash executes simple scripts 20-30% faster than Bash
• Significantly lower memory overhead during init sequences
• No shell initialization files (.bashrc overhead)