3. Command options

Let's see some options of the command ls.

List only some files:
```
ls /bin
```
```
ls /bin/b* /bin/c*
```
We are listing only those files that start with b and those that start with c, on the directory /bin.
Long listing:
```
ls -l /bin/b* /bin/c*
```
The option -l stands for long listing, where each file is printed on its own line, with more details.
Long and short options.

Notice that the middle column shows the size of the file (in bytes). To make the size more readable we can use the option --human-readable:
```
ls -l --human-readable /bin/b* /bin/c*
```
Instead of this long option we can use its short equivalent -h, which is more convenient to write:
```
ls -l -h /bin/b* /bin/c*
```
Tip: In order to modify the previous command, you can use the up-arrow key on the keyboard to display the previous command, use left-arrow and right-arrow keys to locate the cursor, modify the command, and then press [Enter].
Merging short options.

We can also merge the short options like this:
```
ls -lh /bin/b* /bin/c*
```
By default files are listed alphabetically, but we can sort them by modification time, using the option -t:
```
ls -lht /bin/b* /bin/c*
```

With the option --reverse or -r we can reverse the order of display:

ls -lt --reverse /bin/b* /bin/c*

ls -lh --reverse /bin/b* /bin/c*

ls -ltr /bin/b* /bin/c*

ls -lhr /bin/b* /bin/c*

Usually the options have a long version (like --reverse or --human-readable) and a short one (like -r or -h). But not all of them. For example the options -l or -t don't have a long version.

It seems like the short options are more convenient when writing commands. In your opinion, why do we have long options as well? Why they might be useful?

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