LPI Linux Essentials Exam 010-160 - Topic 2.1 - Command Types - Internal vs External

In the Linux operating system, commands can be categorized into two primary types: internal commands and external commands. The distinction between these two types is essential to comprehend how the shell processes commands.

Internal Commands

Internal commands, also known as built-in commands, are an integral part of the shell itself. These commands are directly executed by the shell without the need for invoking an external program. This inherent integration makes internal commands more efficient, as they do not require the overhead of creating a new process. Common examples of internal commands include cd for changing directories, echo for printing text, pwd for displaying the current working directory, and alias for creating custom command shortcuts.

External Commands

External commands, in contrast, are standalone programs or scripts residing in directories specified in the system's PATH environment variable. When you issue an external command, it is executed by the shell as a separate process. External commands can include a wide range of utilities, either provided by the Linux distribution or third-party software. Popular external commands comprise ls for listing directory contents, cp for copying files, and grep for searching text. Additionally, custom scripts or programs you create and install are also considered external commands.

Using the `type` Command

To distinguish between internal and external commands, the type command in Linux serves as a helpful tool. It reveals the nature of a given command by specifying whether it's internal or external. The command will return either command is a shell builtin for internal commands, or it will return something else such as the full command path, or whether it is an alias for an external command. The following exercise will get you using the type command to see the differences:

Exercise: Using the type command, identify whether a command is an internal or external command:

Check what type of command cd is:

type cd

The output will resemble, the following, indicating it is an internal command:

cd is a shell builtin

Check what type of command echo is:

type echo

The output will resemble, the following, indicating it is an internal command:

echo is a shell builtin

Check what type of command ls is:

type ls

The output will resemble, the following, indicating it is an external command:

ls is aliased to `ls --color=auto'

Check what type of command cp is:

type cp

The output will resemble, the following, indicating it is an external command:

cp is /usr/bin/cp

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