What is the difference between internal command and external command in Linux?

What's the difference between the Linux command source and executing a command directly?

• I have a script that when I execute it, it doesn't do anything, however when I execute source (script) it works! In example - ./program # does not work source program # does work Why is this? The program only exports the PS1 env variable to change my terminal.

• Answer:

  It's likely because the file ./program is not an executable file. Depending on your http://en.wikipedia.org/wiki/Umask settings, files may or may not have the executable permission bits set upon creation. You need to set the program to be executable before you can run it. To test this, you can create a new file, for example, I created cmd_test in my home directory with the following contents: #!/bin/bash echo "It works!" Now if I run it like this, it's likely that it won't work. ./cmd_test. I got a permission denied error in stderr because it cannot be executed. To make it executable you need to do chmod +x ./cmd_test As to why this file is executed when I do source ./cmd_test is concerned, it is because bash reads the file and executes the contents of it without forking a new shell. So in order for source to work the file needs to be readable. Transcript from my command window: $ vim cmd_test # Create a new file with a simple echo statement $ ls -l cmd_test # look at it's permissions -rw-r--r-- 1 gopaladhikari staff 30B May 21 17:52 cmd_test # as you can see, it's not executable $ ./cmd_test zsh: permission denied: ./cmd_test # Permission error (no surprise here) $ source ./cmd_test It works! # Using source, it actually works. $ chmod +x ./cmd_test # Make the file executable $ ./cmd_test # Execute the file It works!