Chapter 6. Neat Tricks with Shell Scripting

In the last chapter, you learned about shell and environment variables. You also learned about how to export environment variables, read-only variables, command-line arguments, and create/handle arrays.

In this chapter, we will cover following topics:

Interactive Shell scripts and reading from the keyboard
Using the here operator (<<) and here string (<<<)
File handling
Enabling debugging
Syntax checking
Shell tracing

Interactive Shell scripts – reading user input

The read command is a shell built-in command for reading data from a file or keyboard.

The read command receives the input from the keyboard or a file until it receives a newline character. Then, it converts the newline character into a null character:

Read a value and store it in the variable, shown as follows:
```
read variable
echo $variable
```
This will receive text from the keyboard. The received text will be stored in the variable.
Command read with prompt. Whenever we need to display the prompt with certain text, we use the –p option. The option -p displays the text that is placed after –p on the screen:
```
#!/bin/bash
# following line will print "Enter value: " and then read data
# The received text will be stored in variable value
read -p "Enter value :  " value
```
Output:
```
Enter value : abcd
```
If the variable name is not supplied next to the read command, then the received data or text will be stored in a special built-in variable called REPLY. Let's write a simple script read_01.sh, shown as follows:
```
#!/bin/bash
echo "Where do you stay ?"
read     # we have not supplied any option or variable
echo "You stay in $REPLY"
```
Save the file, give the permission to execute, and run the script as follows:
```
$ chmod u+x read_01.sh
$ ./read_01.sh
```
Output:
```
"Where do you stay?"
Mumbai
"You stay at Mumbai"
```
We will write the script read_02.sh. This script prompts the user to enter their first and last name to greet the user with their full name:
```
#!/bin/bash
echo "Enter first Name"
read FIRSTNAME
echo "Enter Last Name"
read LASTNAME
NAME="$FIRSTNAME $LASTNAME"
echo "Name is $NAME"
```

For reading text and storing in multiple variables, the syntax is as follows:

$ read value1 value2 value3

Let's write Shell script read_03.sh, shown as follows:

#!/bin/bash
echo "What is your name?"
read fname mname lname
echo "Your first name is : $fname"
echo "Your middle name is : $mname"
echo "Your last name is : $lname"

Save the file, give the permission to execute, and run the script as follows:

What is your name?
Ganesh Sanjiv Naik
"Your first name is : Ganesh"
"Your middle name is : Sanjiv"
"Your last name is : Naik"

Let's learn about reading a list of words and storing them in an array:
```
#!/bin/bash
echo -n "Name few cities? "
read -a cities
echo "Name of city is ${cities[2]}."
```
Save the file, give the permission to execute, and run the script as follows:
```
Name few cities? Delhi London Washington Tokyo
Name of city is Washington.
```
In this case, the list of cities is stored in the array of cities. The elements in the array are here:
```
cities[0] = Delhi
cities[1] = London
cities[2] = Washington
cities[3] = Tokyo
```
The index of the array starts with 0, and in this case, it ends at 3. In this case, four elements are added in the cities[] array.
If we want the user to press the Enter key, then we can use the read command along with one unused variable, shown as follows:
```
Echo "Please press enter to proceed further "
read temp
echo "Now backup operation will be started ! "
```

Summarizing the read command with options

The following table summarizes various read command-related options that you learned in the previous sections:

Format	Meaning
read	This command will read text from a keyboard and store the received text in a built-in variable `REPLY`.
read value	This reads a text from a keyboard or standard input and stores it into the variable value.
read first last	This will read the first word in variable first and the remaining text of the line in variable last. The first word is separated by white space from the remaining words in the line.
read –e	This is used in interactive shells for command-line editing. If vi editor is used, then vi commands can be used.
read –a array_name	This will store a list of words received in to an array.
read –r line	The text with backslash can be received here.
read –p prompt	This will print the prompt and wait for the user input. The received text will be stored in the variable REPLY.

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Summary

Next Chapter

The here document and the << operator

Table of Contents for Learning Linux Shell Scripting

Chapter 6. Neat Tricks with Shell Scripting

Interactive Shell scripts – reading user input

Summarizing the read command with options

Table of Contents for
Learning Linux Shell Scripting