Home Free Linux Course Linux Free Course Module 2 Chapter 3 BUILT-IN & EXTERNAL COMMANDS

Linux Free Course Module 2 Chapter 3 BUILT-IN & EXTERNAL COMMANDS

January 14, 2020

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Table of Contents

BASH SHELL BUILT-IN & EXTERNAL COMMANDS

We can classify all of the Linux commands into two types – Internal And External.

Internal commands are the shell built-in commands.

Commands like pwd, cd, echo comes under the category of the internal commands

External commands are files present in the $PATH. (Remember we treat everything in the Linux as a file).

Commands ls, cp, etc.. comes in the external category.

To check whether the command is internal or external we will use the type utility (type itself is internal one).

type cd
cd is a shell builtin

type type
type is a shell builtin

type cp
cp is /bin/cp

• If you got a message like “cd is a shell builtin” after the execution of the command it is an internal command.

• If you got something else it comes under the category of external commands (like cp in this case).

Internal Commands

•Internal commands don’t need files for their execution. The set of internal commands is stored in a shell by default.

External Commands

• For the external commands, execution is possible, if the command file is present in the locations mentioned in the $PATH.

• If the command file is available, but the path to that file is not included in the PATH variable, it will show you an error.

• To execute the external file as a command that is not included in the PATH variable :

Use the ABSOLUTE PATH or RELATIVE PATH

ABSOLUTE PATH: To give the complete pathname of the file

RELATIVE PATH: Executing using dot “.”

./external-executable-file

Ambiguity

•Sometimes a command with the same name exists in both the categories.

•Example – echo (external one lies in the /bin/echo).

•In this case, the shell will give preference to the internal echo command first.