The final thing we find noteworthy to describe in regards to exclamation marks is interacting with the history. As you learned just a few pages ago, the history saves your commands. With the exclamation mark, you can quickly run commands from your history: either by supplying the number of the command (for example, !100) or by entering part of the command (for example: !ls). In our experience, these functions are not used as much as the reverse search we'll explain shortly, but it is still good to be aware of this functionality.

Let's take a look at how this looks in practice:

reader@ubuntu:~$ history | grep 100
 1100  date
 2033  history | grep 100
reader@ubuntu:~$ !1100
date
Sat Dec 22 19:27:55 UTC 2018
reader@ubuntu:~$ !ls
ls -al
total 152
drwxr-xr-x  7 reader reader  4096 Dec 22 19:20 .
drwxr-xr-x  3 root   root    4096 Nov 10 14:35 ..
-rw-rw-r--  1 reader reader  1530 Nov 17 20:47 bash-function-library.sh
<SNIPPED>

By supplying the number, !1100 ran the command date again. You should realize that the history, once it reaches its maximum, will change. The command that is today equal to !1100 might next week be a different command altogether. In practice, this is considered a risky move and is often best avoided, because you do not get a confirmation: you see what is being executed, while it is running (or probably, it is done by the time you have seen what you ran). You can only be sure if you check the history first, and in that case you're not saving any time, only using extra.

What is interesting, though, is repeating a command based on the command itself, such as !ls shows. It is still somewhat risky, especially if used in combination with destructive commands such as rm, but if you're sure what the last command was that matches your exclamation mark query, you should be relatively safe (especially for nondestructive commands such as cat or ls). Again, before you start incorporating this practice into your daily life, be sure to keep reading until we've explained reverse searching. At that point, we expect/hope that those are much more interesting to you, and then you can file away the information here as good to know.