What is globbing?

Simply said, a glob pattern describes injecting a wildcard character into a file path operation. So, when you do a cp * /tmp/, you copy all files (not directories!) in the current working directory to the /tmp/ directory.

The * expands to all regular files inside the working directory, and then all of those are copied to /tmp/.

Here's a simple example:

reader@ubuntu:~/scripts/chapter_10$ ls -l
total 8
-rw-rw-r-- 1 reader reader  29 Oct 14 10:29 character-class.txt
-rw-rw-r-- 1 reader reader 219 Oct  8 19:22 grep-file.txt
reader@ubuntu:~/scripts/chapter_10$ cp * /tmp/
reader@ubuntu:~/scripts/chapter_10$ ls -l /tmp/
total 20
-rw-rw-r-- 1 reader reader   29 Oct 14 16:35 character-class.txt
-rw-rw-r-- 1 reader reader  219 Oct 14 16:35 grep-file.txt
<SNIPPED>

Instead of executing both cp grep-file.txt /tmp/ and cp character-class.txt /tmp/, we used * to select both of them. The same glob pattern can be used with rm:

reader@ubuntu:/tmp$ ls -l
total 16
-rw-rw-r-- 1 reader reader   29 Oct 14 16:37 character-class.txt
-rw-rw-r-- 1 reader reader  219 Oct 14 16:37 grep-file.txt
drwx------ 3 root root 4096 Oct 14 09:22 systemd-private-c34c8acb350...
drwx------ 3 root root 4096 Oct 14 09:22 systemd-private-c34c8acb350...
reader@ubuntu:/tmp$ rm *
rm: cannot remove 'systemd-private-c34c8acb350...': Is a directory
rm: cannot remove 'systemd-private-c34c8acb350...': Is a directory
reader@ubuntu:/tmp$ ls -l
total 8
drwx------ 3 root root 4096 Oct 14 09:22 systemd-private-c34c8acb350...
drwx------ 3 root root 4096 Oct 14 09:22 systemd-private-c34c8acb350...

By default, rm only deletes files and not directories (as you can see from the errors in the previous example). As stated in Chapter 6, File Manipulation, adding a -r will delete directories recursively too.

Again, do think about how destructive this is: without warning, you could delete every file within the current tree location (if you have the permissions, of course). The preceding example shows how powerful the * glob pattern is: it expands to every file it can find, whatever the type.