Bash Programming

Bash, a powerful programming language, is perfectly designed for command line and shell scripts. In this module, we’ll learn how to use Bash. Its features will help us solve our daily computer tasks and even automate some of them.

This module covers the general principles of computing operations and basic programming concepts in detail. It will also help us consolidate our new knowledge through exercises for each topic. Overall, this experience should be a great start to our Bash programming journey.

Basic Programming with Bash

## Using `read`

Let’s consider that `contacts.txt` contains the following content.

```txt
Alice=alice@gmail.com
Bob=(697) 955-5984
Eve=(245) 317-0117
Mallory=mallory@hotmail.com
```
Here is an example to read the first line of the `contacts.txt` file. The following command does it:
```txt
read -r contact < contacts.txt
```

This command writes the `Alice=alice@gmail.com` string to the `contact` variable.

We can write the name and contact information to two different variables. We need to define the equal sign as a delimiter to do that. Then, we will get the following `read` call:
```txt
IFS=$'=' read -r name contact < contacts.txt
```

Now, the `name` variable gets the “Alice” name. The e-mail address comes to the `contact` variable.

Let’s try the following `while` loop for reading the entire `contacts.txt` file:

```txt
while IFS=$'=' read -r name contact < "contacts.txt"
do
  echo "$name = $contact"
done
```

Unfortunately, this approach does not work because it'll accidentally create an infinite loop. This happens because the `read` command always reads only the first line of the file. Then, it returns the zero exit status. The zero status leads to another execution of the loop body, which then leads to another zero status, and so on in a continuing cycle. 

We should force the `while` loop to pass through all lines of the file. The following form of the loop does it:
```txt
while CONDITION
do
  ACTION
done < FILE
```

This form of the loop can handle keyboard input too. We need to specify the `/dev/tty` input file for doing that. Then, the loop will read keystrokes until we press Ctrl+D.

Here is the corrected `while` loop that reads the entire `contacts.txt` file:
```txt
while IFS=$'=' read -r name contact
do
  echo "$name = $contact"
done < "contacts.txt"
```

This loop prints all lines of the contacts file.

There is the last step left to finish our script. We should write the `name` and `contact` variables to the associative array on each iteration. The `name` variable is the key and `contact` is the value.

The file `while-contacts.sh` shows the final version of the script for reading the contacts from the file. We can execute the file on following commands:

```txt
./while-contacts.sh "Alice"
./while-contacts.sh "Bob"
```

# Using `read`

Let’s consider that `contacts.txt` contains the following content.

```txt
Alice=alice@gmail.com
Bob=(697) 955-5984
Eve=(245) 317-0117
Mallory=mallory@hotmail.com
```
Here is an example to read the first line of the `contacts.txt` file. The following command does it:
```txt
read -r contact < contacts.txt
```

This command writes the `Alice=alice@gmail.com` string to the `contact` variable.

We can write the name and contact information to two different variables. We need to define the equal sign as a delimiter to do that. Then, we will get the following `read` call:
```txt
IFS=$'=' read -r name contact < contacts.txt
```

Now, the `name` variable gets the “Alice” name. The e-mail address comes to the `contact` variable.

Let’s try the following `while` loop for reading the entire `contacts.txt` file:

```txt
while IFS=$'=' read -r name contact < "contacts.txt"
do
  echo "$name = $contact"
done
```

Unfortunately, this approach does not work because it'll accidentally create an infinite loop. This happens because the `read` command always reads only the first line of the file. Then, it returns the zero exit status. The zero status leads to another execution of the loop body, which then leads to another zero status, and so on in a continuing cycle. 

We should force the `while` loop to pass through all lines of the file. The following form of the loop does it:
```txt
while CONDITION
do
  ACTION
done < FILE
```

This form of the loop can handle keyboard input too. We need to specify the `/dev/tty` input file for doing that. Then, the loop will read keystrokes until we press Ctrl+D.

Here is the corrected `while` loop that reads the entire `contacts.txt` file:
```txt
while IFS=$'=' read -r name contact
do
  echo "$name = $contact"
done < "contacts.txt"
```

This loop prints all lines of the contacts file.

There is the last step left to finish our script. We should write the `name` and `contact` variables to the associative array on each iteration. The `name` variable is the key and `contact` is the value.

The file `while-contacts.sh` shows the final version of the script for reading the contacts from the file. We can execute the file on following commands:

```txt
./while-contacts.sh "Alice"
./while-contacts.sh "Bob"
```

Learn how the read command can process files.

Before We Begin

File System

Command Information

Bash Extra Features

Scripts Features

Variables and Parameters

Conditional Statements

Arithmetic Expressions and Operations

Bitwise Operations

Logical Operations and Increments

Loop Operators

Functions

Package Manager

Bash Programming from Scratch

Bash Scripts Tools

APPENDIX

Conclusion

Script for Managing the Contacts

Using `read`