When you type a command into a Linux terminal, what’s really happening is that a program is being executed. Normally, to execute a custom program or script, we need to use its full path, such as /path/to/script.sh or just ./script.sh if we’re already in its residing directory. Alternatively, we can execute a lot of commands without specifying paths, like uptime or date, etc.
When you type a command in Linux, a program is executed with a full path. Learn how to permanently add a directory to shell PATH here.