Shell Scripting / Bash Tricks

Mastering Safe Variable Expansion with Parameter Indirection

🧩 The Challenge

You often need to access the value of a variable whose name is stored inside another variable, but using eval is a security risk. Standard syntax fails because the shell does not perform a second level of expansion on the variable name by default.

💡 The Fix

Use bash parameter indirection syntax to dynamically reference variable contents safely without invoking dangerous shell evaluation.

var_name="target"
target="production_db"
echo ${!var_name}

⚙️ Why It Works

The exclamation mark prefix tells the bash shell to interpret the content of the variable as a reference to another variable name, performing an indirect lookup. This avoids the shell injection vulnerabilities associated with eval or arbitrary command execution.

🚀 Pro-Tip: Use the ${!prefix*} syntax to list all variable names currently defined that begin with a specific prefix string.

Linux Tips & Tricks | © ngelinux.com | 7/7/2026