What is the order of command search in Linux?

Understanding the Command Search Order in Linux

In the Linux operating system, when you execute a command, the shell (the command-line interface) follows a specific order to search for the executable file associated with that command. This order is known as the "command search order" or the "shell search path." Understanding this order is crucial for effectively using and troubleshooting commands in the Linux environment.

The Command Search Order

The command search order in Linux follows these steps:

Aliases: The shell first checks if the command is an alias, which is a shorthand name for a longer command or a sequence of commands. If the command matches an existing alias, the shell executes the corresponding command.
Functions: If the command is not an alias, the shell checks if it is a shell function, which is a user-defined command stored in the shell's memory.
Builtin Commands: If the command is not an alias or a function, the shell checks if it is a builtin command, which is a command that is part of the shell's internal functionality.
**Executable Files in the $PATH**: If the command is not an alias, function, or builtin, the shell searches for an executable file in the directories specified by the `$ PATHenvironment variable. The$PATH` variable contains a colon-separated list of directories where the shell will look for the command.
Current Directory: If the command is not found in the $PATH directories, the shell will search for an executable file in the current working directory.
Absolute or Relative Paths: If the command is not found in the current directory, the shell will attempt to execute the command using an absolute or relative path provided by the user.

Here's a Mermaid diagram that visually represents the command search order:

graph TD A[Alias] --> B[Function] B --> C[Builtin Command] C --> D[$PATH] D --> E[Current Directory] E --> F[Absolute/Relative Path]

Examples

Let's consider some examples to better understand the command search order:

Alias: Suppose you have defined an alias for the ls command, like this: alias ls='ls -l'. When you type ls in the terminal, the shell will execute the ls -l command instead of the default ls command.
Function: You can define a custom function in your shell's configuration file (e.g., .bashrc for Bash) and use it as a command. For example:
```
my_command() {
  echo "This is my custom command."
}
```
When you type my_command in the terminal, the shell will execute the function.
Builtin Command: The cd command is a builtin command in the shell, so it doesn't need to be searched for in the $PATH or the current directory.
** $PATH**: Suppose the `$ PATHvariable contains the following directories:/usr/local/bin:/usr/bin:/bin`. When you type a command, the shell will search for the executable file in these directories, in the order they are listed.
Current Directory: If you have an executable file named my_script.sh in the current directory, and you type ./my_script.sh, the shell will execute the script.
Absolute or Relative Path: If you have an executable file named my_script.sh in the /home/user/bin directory, you can execute it by typing /home/user/bin/my_script.sh or ../bin/my_script.sh (if you're in a different directory).

Understanding the command search order is essential for troubleshooting issues with command execution, ensuring that your custom commands or scripts are properly recognized, and optimizing the performance of your shell environment.