When it comes to gaining absolute control over your Linux system, then nothing comes close to the command line interface (CLI). In order to become a Linux power user, one must understand the [different types of shell commands][1] and the appropriate ways of using them from the terminal.
In Linux, there are several types of commands, and for a new Linux user, knowing the meaning of different commands enables for efficient and precise usage. Therefore, in this article, we shall walk through the various classifications of shell commands inLinux.
在 Linux 下，有一些不同种类的命令，对于一个 Linux 新手来说，知道不同命令的意思才能够高效和准确的使用它们。因此，在这篇文章里，我们将会分享 Linux 下不同分类的 Shell 命令。
One important thing to note is that the command line interface is different from the shell, it only provides a means for you to access the shell. The shell, which is also programmable then makes it possible to communicate with the kernel using commands.
When you run a command, Linux searches through the directories stored in the $PATH environmental variable from left to right for the executable of that specific command.
In the above order, the directory /home/aaronkilik/bin will be searched first followed by /usr/local/sbin and so on, the order is significant in the search process.
These are user defined commands, they are created using the alias shell built-in command, and contain other shell commands with some options and arguments. The ideas is to basically use new and short names for lengthy commands.
However, the aliases we have created above only work temporarily, when the system is restarted, they will not work after the next boot. You can set permanent aliases in your `.bashrc` file as shown below.
In shell programming, words such as if, then, fi, for, while, case, esac, else, until and many others are shell reserved words. As the description implies, they have specialized meaning to the shell.
A shell function is a group of commands that are executed collectively within the current shell. Functions help to carry out a specific task in a shell script. The conventional form of writing shell functions in a script is:
#write a shell function to update and upgrade installed packages
upgrade_system(){
sudo apt update;
sudo apt dist-upgrade;
}
#execute function
upgrade_system
```
Instead of executing the two commands: sudo apt update and sudo apt dist-upgrade from the command line, we have written a simple shell function to execute the two commands as a single command, upgrade_system within a script.
These are Linux commands that built into the shell, thus you cannot find them within the file system. They include pwd, cd, bg, alias, history, type, source, read, exit and many others.
As a Linux user, it is always important to know the type of command you are running. I believe, with the precise and simple-to-understand explanation above including a few relevant illustrations, you probably have a good understanding of the [various categories of Linux commands][5].
