4.6b Conditionals: if/elif/else, test expressions, and compound conditions¶

🧭 Introduction¶

In AI infrastructure operations, your scripts often need to make decisions based on system states — like checking if a GPU is available, verifying disk space, or confirming that a model file exists. Conditionals are the decision-making tools that allow your Bash scripts to respond intelligently to different situations. This section covers the fundamental conditional structures: if/elif/else, test expressions, and compound conditions.

⚙️ The Basic if Statement¶

The simplest conditional checks a condition and runs commands only if that condition is true.

• The structure follows this pattern: if a condition is true, then execute these commands, and close with fi.
• The condition is placed inside square brackets with spaces around them.
• Example structure: if [ condition ]; then followed by commands on the next line, then fi to close.

For reference:

if [ -f "/models/llama2.bin" ]
then
    echo "Model file exists"
fi

🔀 Adding else and elif¶

When you need multiple decision paths, use else for the fallback and elif for additional conditions.

• else runs when the if condition is false.
• elif (else if) lets you check another condition before reaching else.
• You can chain multiple elif statements together.
• Always end the entire block with fi.

For reference:

gpu_count=4

if [ $gpu_count -ge 8 ]
then
    echo "High-performance GPU cluster"
elif [ $gpu_count -ge 4 ]
then
    echo "Standard GPU configuration"
else
    echo "Insufficient GPU resources"
fi

📊 Visual Representation: Conditional Branching Logic (if/elif/else)¶

This flowchart maps the decision-making branching logic of an if-elif-else conditional evaluation check for GPU cluster scaling resources.

🧪 Test Expressions¶

Test expressions are the conditions you place inside the square brackets. They check files, strings, and numbers.

📁 File Test Operators¶

These check properties of files and directories:

• -f file — checks if a file exists and is a regular file
• -d directory — checks if a directory exists
• -s file — checks if a file exists and is not empty
• -r file — checks if a file is readable
• -w file — checks if a file is writable
• -x file — checks if a file is executable

🔢 Numeric Comparison Operators¶

Use these to compare numbers:

🔤 String Comparison Operators¶

Use these to compare text:

For reference:

model_name="llama2"
model_size=7

if [ -z "$model_name" ]
then
    echo "No model specified"
elif [ $model_size -gt 10 ]
then
    echo "Large model detected"
else
    echo "Model: $model_name, Size: ${model_size}B"
fi

🔗 Compound Conditions with AND and OR¶

Sometimes you need to check multiple conditions at once. Use logical operators to combine them.

Using && (AND)¶

Both conditions must be true for the overall condition to be true.

• Syntax: [[ condition1 && condition2 ]]
• Example: Check if a file exists AND is readable.

For reference:

if [[ -f "/data/training_set.csv" && -r "/data/training_set.csv" ]]
then
    echo "Training data is available and readable"
fi

Using || (OR)¶

At least one condition must be true for the overall condition to be true.

• Syntax: [[ condition1 || condition2 ]]
• Example: Check if either a file or a backup file exists.

For reference:

if [[ -f "/models/current.pt" || -f "/models/backup.pt" ]]
then
    echo "Model checkpoint found"
fi

Combining AND and OR¶

You can nest these for complex logic using parentheses.

For reference:

if [[ ( $gpu_count -ge 4 && $memory_gb -ge 64 ) || $cluster_mode == "high_perf" ]]
then
    echo "Environment meets minimum requirements"
fi

🛠️ Practical AI Infrastructure Examples¶

Example 1: Checking GPU Availability¶

This script checks if NVIDIA GPUs are detected and if enough are available.

For reference:

gpu_count=$(nvidia-smi --query-gpu=count --format=csv,noheader 2>/dev/null)

if [ -z "$gpu_count" ]
then
    echo "No NVIDIA GPUs detected"
elif [ $gpu_count -eq 0 ]
then
    echo "GPUs present but unavailable"
elif [ $gpu_count -ge 4 ]
then
    echo "Sufficient GPUs for distributed training"
else
    echo "Only $gpu_count GPU(s) available"
fi

Example 2: Validating Model Directory Structure¶

Check that required directories exist before running a training job.

For reference:

if [[ -d "/models" && -d "/data" && -d "/logs" ]]
then
    echo "All required directories exist"
elif [ ! -d "/models" ]
then
    echo "Missing /models directory"
elif [ ! -d "/data" ]
then
    echo "Missing /data directory"
else
    echo "Missing /logs directory"
fi

🕵️ Common Pitfalls and Best Practices¶

❌ Common Mistakes¶

• Forgetting spaces inside brackets: if[$var -eq 1] is wrong — always use if [ $var -eq 1 ]
• Using single brackets for compound conditions: Use double brackets [[ ]] for && and || operators
• Not quoting string variables: Always use "$variable" to handle empty strings safely
• Mixing numeric and string operators: Use -eq for numbers, = for strings

✅ Best Practices¶

• Always test your conditions with both true and false scenarios
• Use elif instead of nested if statements for cleaner code
• Add comments to explain complex compound conditions
• Test for error conditions first (e.g., missing files) before proceeding
• Use exit 1 inside conditionals to stop scripts on critical failures

📊 Quick Reference Table¶

✅ Summary¶

Conditionals are the backbone of intelligent scripting in AI infrastructure. With if/elif/else, you can create decision trees that respond to system states. Test expressions let you check files, numbers, and strings. Compound conditions with && and || allow for sophisticated logic in a single line. Practice writing conditionals that handle real scenarios like GPU checks, disk space monitoring, and model validation — these skills will be essential as you automate AI infrastructure operations.