Understanding bash vs dash — What Every DevOps Engineer Should Know

When writing shell scripts or running automation tools like Ansible, you’ll often see /bin/sh, /bin/bash, or even errors like:

set: Illegal option -o pipefail

This confusion stems from differences between bash and dash — two popular Unix shells. Let’s explore what they are, how they differ, and when it matters.

What is bash ?

Bash stands for Bourne Again Shell. It’s:

• A full-featured, powerful scripting shell
• The default on most Linux distributions (like CentOS, Red Hat, Amazon Linux)
• Widely used for scripting because of its extended features

Key Features of Bash:

• Arrays
• Arithmetic expressions
• [[ ]] test syntax
• Process substitution <(...)
• Advanced string manipulation: ${var//a/b}
• set -o pipefail
• The source command

What is dash ?

dash stands for Debian Almquist Shell. It’s:

• Lightweight, fast, and strictly POSIX-compliant
• The default /bin/sh on Debian and Ubuntu
• Ideal for system scripts and fast boot processes

What dash lacks:

• No arrays
• No [[ ]] syntax
• No brace expansion {1..5}
• No process substitution
• No set -o pipefail

bash vs dash: A Side-by-Side

Example That Works in Bash but Fails in Dash

#!/bin/sh
set -o pipefail
echo "Hello"

This will fail in dash (/bin/sh on Ubuntu) with:

/bin/sh: 1: set: Illegal option -o pipefail

It works fine in bash.

Why This Matters in DevOps & Ansible ?

In tools like Ansible, the shell module runs commands via /bin/sh by default. On Ubuntu/Debian systems, /bin/sh → dash, which means:

• Bash-specific scripts will fail unexpectedly
• You must either write portable, POSIX-compliant shell scripts
• Or explicitly use bash with:

shell: |
  set -o pipefail
  command
args:
  executable: /bin/bash

How to Switch /bin/sh to Bash (if needed)

echo "dash dash/sh boolean false" | sudo debconf-set-selections
sudo dpkg-reconfigure -f noninteractive dash

This will update /bin/sh → /bin/bash.

Best Practices

1. Use #!/bin/bash when using bash features
2. Use #!/bin/sh only for strictly POSIX-compliant scripts
3. Always test scripts on the actual shell being used
4. For Ansible, use args: executable: /bin/bash when needed

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