Including source files

In addition to functions, we can also create multiple scripts and include them such that we can utilize any shared variables of functions.

Let's say we have a library or utility script that contains a number of functions useful for creating files. This script by itself could be useful or reusable for a number of scripting tasks, so we make it program neutral. Then, we have another script, but this one is dedicated to a single task: performing useless file system operations (IO). In this case, we would have two files:

1. io_maker.sh (which includes library.sh and uses library.sh functions)
2. library.sh (which contains declared functions, but does not execute them)

The io_maker.sh script simply imports or includes the library.sh script and inherits knowledge of any global variables, functions, and other inclusions. In this manner, io_maker.sh effectively thinks that these other available functions are its own and can execute them as if they were contained within it.

Including/importing a library script and using external functions

To prepare for this example, create the following two files and open both:

• io_maker.sh
• library.sh

Inside library.sh, add the following:

#!/bin/bash

function create_file() {
    local FNAME=$1
    touch "${FNAME}"
    ls "${FNAME}" # If output doesn't return a value - file is missing
}

function delete_file() {
    local FNAME=$1
    rm "${FNAME}"
    ls "${FNAME}" # If output doesn't return a value - file is missing
}

Inside io_maker.sh, add the following:

#!/bin/bash

source library.sh # You may need to include the path as it is relative
FNAME="my_test_file.txt"
create_file "${FNAME}"
delete_file "${FNAME}"

exit 0

When you run the script, you should get the same output:

$ bash io_maker.sh
my_test_file.txt
ls: cannot access 'my_test_file.txt': No such file or directory

Although not obvious, we can see that both functions are executed. The first line of output is the ls command, successfully finding my_test_file.txt after creating the file in create_file(). In the second line, we can see that ls returns an error when we delete the file passed in as a parameter. 

Unfortunately, up until now, we have only been able to create and call functions, and execute commands. The next step, discussed in the next section, is to retrieve commands and function return codes or strings.