Dash

The $- variable in Bash shows the current shell’s option flags. These flags differ when you run commands interactively versus inside a script. In this guide, we’ll:

• Inspect $- interactively
• Compare interactive vs. non-interactive outputs
• Demonstrate how set -e affects $-
• Review common $- flags in a summary table

Checking $- Interactively

Run the following command in your terminal to see which flags are active in your interactive shell:

echo $-

Example output:

himBHs

Each letter (such as h, i, m, B, H, s) represents a specific shell option. These may vary between systems and user configurations.

Comparing with a Non‐Interactive Script

Create a file named special-dash.sh:

#!/usr/bin/env bash
echo $-

Make it executable and run it:

chmod +x special-dash.sh
./special-dash.sh

Typical output:

hB

You’ll notice the i flag (interactive mode) is missing because scripts run non-interactively by default.

Enabling “Exit on Error” with set -e

The set -e option causes Bash to exit immediately if any command returns a non-zero status. Let’s enable it in our script:

#!/usr/bin/env bash
set -e
echo $-

Run the updated script:

./special-dash.sh

Now you should see:

ehB

The leading e indicates that “exit on error” is active.

Common $- Flags

Further Reading

• Bash Reference Manual – Invocations
• Bash Scripting Guide

By understanding and inspecting the $- variable, you can better control your shell’s behavior in both interactive sessions and automated scripts.