Shell conditional in makefile

Mastering Shell Conditionals in Makefiles

Unlock the power of conditional logic in your Makefiles using shell commands. This guide covers common patterns, pitfalls, and best practices for robust build automation.

Makefiles are powerful tools for automating build processes, but sometimes you need more than just simple command execution. Integrating shell conditionals directly into your Makefile rules allows for dynamic behavior based on various factors like file existence, variable values, or command output. This article will guide you through effectively using if, else, and endif constructs within your Makefile, leveraging the shell's capabilities to create more flexible and intelligent build scripts.

Understanding Shell vs. Makefile Conditionals

It's crucial to distinguish between Makefile's own conditional directives (ifeq, ifneq, ifdef, ifndef) and shell conditionals (if, test, [, [[). Makefile conditionals are evaluated before any shell commands are executed, during the parsing phase of the Makefile. Shell conditionals, on the other hand, are part of the commands themselves and are executed by the shell invoked for a specific rule. This distinction dictates where and how you can use each type of conditional.

flowchart TD
    A[Makefile Parsing Phase] --> B{Makefile Conditional?}
    B -- Yes --> C[Evaluate `ifeq`/`ifdef`]
    C -- True --> D[Include/Exclude Makefile lines]
    B -- No --> E[Rule Execution Phase]
    E --> F[Shell Invoked for Rule]
    F --> G{Shell Conditional?}
    G -- Yes --> H[Execute `if`/`test`/`[`]
    H -- True --> I[Execute Shell Commands]
    H -- False --> J[Skip Shell Commands]
    G -- No --> K[Execute All Shell Commands]
    D --> L[Continue Parsing]
    I --> M[Rule Complete]
    J --> M
    K --> M

Flowchart illustrating the difference between Makefile and Shell conditional evaluation.

Basic Shell Conditionals in Makefile Rules

To embed shell conditionals, you typically use the shell keyword or directly include them within a rule's command block. Remember that each line in a Makefile rule is executed in a separate shell instance by default, unless you explicitly chain commands or use the .ONESHELL special target. For multi-line shell conditionals, it's often best to use a single logical line by escaping newlines with backslashes or by wrapping the entire block in a bash -c '...' or similar construct.

all:
	@echo "Checking for a file..."
	@if [ -f "myfile.txt" ]; then \
		echo "myfile.txt exists!"; \
	else \
		echo "myfile.txt does not exist."; \
	fi

clean:
	rm -f myfile.txt

A basic shell if/else conditional checking for file existence within a Makefile rule.

Advanced Conditional Patterns and Best Practices

Beyond simple if/else, you can use more advanced shell features. For instance, checking the exit status of a command, performing string comparisons, or using logical operators (&&, ||). For complex logic, consider encapsulating it in a shell script and calling that script from your Makefile, or using the .ONESHELL directive for a specific target to ensure all commands run in the same shell instance.

.ONESHELL:

build:
	@echo "Starting build..."
	@if command -v gcc >/dev/null 2>&1; then
		echo "GCC found. Compiling...";
		gcc main.c -o app
		if [ $$? -eq 0 ]; then
			echo "Compilation successful.";
		else
			echo "Compilation failed.";
			exit 1;
		fi
	else
		echo "GCC not found. Please install GCC.";
		exit 1;
	fi

clean:
	rm -f app

Using .ONESHELL for a target to execute multi-line shell conditionals and check command exit status.