What is eval?

The eval command takes string arguments and executes them as commands. It’s super useful when you need to manipulate commands at the string level before executing them.

Think of it as building commands dynamically and then running them!

Basic Usage

# Simple example
command="ls -la"
eval $command

# Same as running: ls -la

Practical Examples

1. Dynamic Variable Names

# Create variables dynamically
for i in {1..3}; do
    eval "var$i=value$i"
done

# Now you have var1=value1, var2=value2, var3=value3

# Access them dynamically
for i in {1..3}; do
    eval "echo \$var$i"
done

2. Building Complex Commands

# Build a command string
search_term="*.txt"
options="-name"
action="-exec cat {} \;"

find_command="find . $options '$search_term' $action"
eval $find_command

3. Configuration Processing

# Process config-like strings
config="USER=john PASSWORD=secret123 HOST=localhost"
eval $config

echo "Connecting to $HOST as $USER"

When to Use eval

✅ Good use cases:

• Dynamic variable creation
• Building commands from user input (with proper validation!)
• Processing configuration strings
• Metaprogramming in shell scripts

❌ Avoid when:

• You can use arrays or other safer alternatives
• Processing untrusted input without validation
• Simple variable assignments (use direct assignment instead)

Security Warning

⚠️ BE CAREFUL! eval executes whatever you give it, which can be dangerous:

# DANGEROUS - never do this with user input
user_input="rm -rf /"
eval $user_input  # This would delete everything!

Always validate and sanitize input before using eval!

Safer Alternatives

Sometimes you don’t need eval:

# Instead of eval for simple cases
# Bad:
eval "ls $options"

# Good:
ls $options

# For arrays:
# Instead of dynamic variables, use arrays
declare -a values=("value1" "value2" "value3")

Pro Tips

1. Quote properly: Always quote variables to prevent word splitting
2. Validate input: Never eval untrusted data
3. Consider alternatives: Arrays, parameter expansion, etc.
4. Debug first: Echo the command before eval to see what you’re executing

# Debug pattern
command="ls -la /tmp"
echo "About to execute: $command"
eval $command

References

• General: Bash Beginners Guide

Use eval wisely - with great power comes great responsibility! 🚀