The first candidate for inclusion in our own private function library is correctly setting the current working directory. This is a pretty simple function, so we'll add it without too much explanation:

reader@ubuntu:~/scripts/chapter_13$ vim ~/bash-function-library.sh 
reader@ubuntu:~/scripts/chapter_13$ cat ~/bash-function-library.sh 
#!/bin/bash
#####################################
# Author: Sebastiaan Tammer
# Version: v1.1.0
# Date: 2018-11-17
# Description: Bash function library.
# Usage: source ~/bash-function-library.sh
#####################################
<SNIPPED>
# Set the current working directory to the script location.
set_cwd() {
  cd $(dirname $0)
}

Because a function library is something that is potentially updated quite frequently, correctly updating the information in the header is very important. Preferably (and most likely in an enterprise environment), you will commit new versions of your function library to a version control system. Using proper Semantic Version in the header will help you keep a clean history. Particularly, if you combine this with configuration management tooling such as Chef.io, Puppet, and Ansible, you will keep a good overview of what you've changed and deployed to where.

Now, we'll update our script from the previous chapter, redirect-to-file.sh, with our library inclusion and function call. The end result should be the following:

reader@ubuntu:~/scripts/chapter_13$ cp ../chapter_12/redirect-to-file.sh library-redirect-to-file.sh
reader@ubuntu:~/scripts/chapter_13$ vim library-redirect-to-file.sh 
reader@ubuntu:~/scripts/chapter_13$ cat library-redirect-to-file.sh 
#!/bin/bash

#####################################
# Author: Sebastiaan Tammer
# Version: v1.0.0
# Date: 2018-11-17
# Description: Redirect user input to file.
# Usage: ./library-redirect-to-file.sh
#####################################

# Load our Bash function library.
source ~/bash-function-library.sh

# Since we're dealing with paths, set current working directory.
set_cwd

# Capture the users' input.
read -p "Type anything you like: " user_input

# Save the users' input to a file. > for overwrite, >> for append.
echo ${user_input} >> redirect-to-file.txt

For teaching purposes, we've copied the file to the directory of the current chapter; normally, we would just update our original file. We've only added the inclusion of our function library and replaced the magical cd $(dirname $0) with our set_cwd function call. Let's run it from a location where the script is not and see if the directory is correctly set:

reader@ubuntu:/tmp$ bash ~/scripts/chapter_13/library-redirect-to-file.sh
Type anything you like: I like ice cream, I guess
reader@ubuntu:/tmp$ ls -l
drwx------ 3 root root 4096 Nov 17 11:20 systemd-private-af82e37c...
drwx------ 3 root root 4096 Nov 17 11:20 systemd-private-af82e37c...
reader@ubuntu:/tmp$ cd ~/scripts/chapter_13
reader@ubuntu:~/scripts/chapter_13$ ls -l
<SNIPPED>
-rw-rw-r-- 1 reader reader 567 Nov 17 19:32 library-redirect-to-file.sh
-rw-rw-r-- 1 reader reader 26 Nov 17 19:35 redirect-to-file.txt
-rw-rw-r-- 1 reader reader 933 Nov 17 15:18 reverser.sh
reader@ubuntu:~/scripts/chapter_13$ cat redirect-to-file.txt 
I like ice cream, I guess

So, even though we used the $0 syntax (which, as you remember, prints the fully qualified path of the script), we see here that it refers to the path of library-redirect-to-file.sh and not, as you might have reasonably assumed, to the location of the bash-function-library.sh script. This should confirm our explanation that only function definitions are included, and when the functions are called at runtime they take on the environment of the script that includes them.