How do external commands use PATH?

External commands in Linux utilize the PATH environment variable to locate executable files. The PATH variable is a colon-separated list of directories that the shell searches through when you enter a command. Here's how it works:

How PATH Works

Command Input: When you type a command in the terminal, the shell checks if it’s a built-in command first. If not, it proceeds to search for an external command.
Directory Search: The shell looks for the command in the directories listed in the PATH variable, in the order they are specified. For example, if your PATH is set to:
```
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
```
The shell will first check /usr/local/sbin, then /usr/local/bin, and so on.
Execution: Once the shell finds the executable file that matches the command name, it runs that file. If the command is not found in any of the directories listed in PATH, you will receive a "command not found" error.

Viewing and Modifying PATH

View PATH: You can view the current PATH variable by running:
```
echo $PATH
```
Modify PATH: You can temporarily add a directory to your PATH for the current session using:
```
export PATH=$PATH:/new/directory/path
```
To make this change permanent, you would typically add the export command to your shell's configuration file (like ~/.bashrc or ~/.bash_profile).

Example

Suppose you have an executable script named my_script located in /usr/local/bin. If /usr/local/bin is included in your PATH, you can run the script simply by typing:

my_script

If it were not in your PATH, you would need to specify the full path:

/usr/local/bin/my_script

Summary

The PATH environment variable is essential for the shell to locate and execute external commands efficiently. By managing your PATH, you can control which directories are searched for executables, making it easier to run your scripts and programs.

If you have any further questions or need more examples, feel free to ask!