builtin

Understanding the builtin Command

The builtin command ensures that a specific command is executed as a shell built-in, rather than searching for an external executable with the same name. This is particularly useful when you have a command with the same name as an executable file in your PATH. Using builtin guarantees the shell’s version is used, preventing potential conflicts or unintended consequences.

Syntax:

builtin <command> [arguments]

Where <command> is the name of the shell built-in and [arguments] are any arguments passed to that command.

Code Examples: Illustrating builtin’s Power

Let’s look at many practical examples showcasing builtin’s functionality:

Example 1: Avoiding External Command Conflicts

Imagine you have a script named cd in your current directory. If you try to use the cd command, the shell might execute your script instead of the built-in cd command. builtin resolves this:

#!/bin/bash


touch cd
chmod +x cd


echo "Attempting to change directory without builtin:"
cd /tmp  # Might execute the script instead!


rm cd


echo "Attempting to change directory with builtin:"
builtin cd /tmp

This example highlights the critical role of builtin in ensuring the correct execution of built-in commands, even in the presence of similarly named files.

Example 2: Explicitly Using Built-in echo

While less important for echo, this example demonstrates builtin’s use for any built-in:

#!/bin/bash


echo "hello" > echo
chmod +x echo


builtin echo "This is the shell's echo."


rm echo

Example 3: Using builtin within functions

The power of builtin truly shines within functions to ensure consistent behavior regardless of the user’s environment:

#!/bin/bash

my_function() {
  builtin cd "$1"
  echo "Current directory: $(pwd)"
}

my_function /home

This function uses builtin cd to reliably change directories regardless of any user-defined cd commands.

Example 4: Handling Complex Scenarios with eval and builtin

In more complex scenarios, combining eval and builtin can enable dynamic command execution while maintaining control over built-in commands:

#!/bin/bash

command_to_run="echo"
arguments="Hello from eval and builtin!"

eval "builtin $command_to_run \"$arguments\""

This example dynamically constructs a command using eval and ensures it’s executed as a built-in using builtin. Note that using eval requires caution, as it can introduce security risks if not handled carefully.

These examples demonstrate the practical applications of the builtin command in various shell scripting contexts. By understanding and utilizing builtin appropriately, you can write more robust, reliable, and predictable shell scripts.