Introduction to Bash Scripting: Display User Messages and Catch User Input

By: Daniel Farina   |   Updated: 2018-12-05   |   Comments (2)   |   Related: > SQL Server on Linux

Problem

You are a SQL Server Database Administrator and you are starting to use SQL Server on Linux. As you have probably found out, there are several administrative tasks that you need to do differently on Linux versus Windows. In this tip I will show you the different ways you can use Bash scripting to display messages and also how to catch user input.

Solution

When you want to build a Bash script (or any other kind of script) the first thing you need to know is how the script is going to be used. Without too much thinking we can define two categories of scripts: those that run in the background like Cron jobs and those that require user input, like an installation script.

If your goal is to create a script that runs in the background, you can't catch any user input and must rely on getting everything for your script to work from a configuration file. We will see how to read from files in another tip. Also, you should probably re-think about writing to the console for these background scripts and consider writing to a text file for logging instead.

If your goal is to write an interactive script that can be able to display information and receive user input, then I will show how this can be done in this tip.

Working with Interactive Bash Scripts

When we aim to give some informative message to the console, Bash gives us two tools we can take advantage of. If you have been reading the tips in this series you already know one, which is the echo command. The other one is the printf function. If you ever worked with C or C++ languages you will find this easy. But if you haven't, after reading this tip you will see it is easy to do.

The Bash echo Command

If you have been reading my previous tips in this series you already saw how to use the echo command to display basic output to the console. But this command allows more complex usage depending on which parameter you pass to it.

In the next table I have copied the parameters available from the echo command page.

Let's consider the following script:

#!/bin/bash

Variable1="Hello"
Variable2="World"
echo $Variable1
echo $Variable2

echo -n $Variable1
echo -n $Variable2

On the next screen capture you will see the output of that script execution. Notice that the addition of the –n parameter suppressed the new line.

The –e parameter of the echo command allows us with the possibility to change the way we write our messages to the console by using escape sequences. In the next table I wrote a list of each escape sequence and its meaning. All the ASCII codes are expressed in decimal notation.

Echo Examples

Now I will give you a few examples using an escape sequence.

In the next script I show you how the \c escape sequence suppresses the output of the text that follows and additionally how to write a vertical tab with the \v escape sequence.

#!/bin/bash

echo -e "Hello \c World!"
echo
echo 
echo -e "Hello\v World!"

In the next example, I will show you how to write characters by using hexadecimal and octal notation.  You can use the following link to get an ASCII table in decimal, hexadecimal and octal numeric systems: http://www.asciitable.com/.

#!/bin/bash

#H 48 110
#e 65 145
#l 6C 154
#l 6C 154
#o 6F 157
echo -e "\x48\x65\x6C\x6C\x6F World!" 
echo -e "\0110\0145\0154\0154\0157 World!"

The Bash printf Command

This command is an improved version of echo, because it provides a method for printing formatted text like the C/C++ printf function does.

The syntax of this command is as follows:

printf [FORMAT] [ARGUMENT_1] …[ARGUMENT_n]

Basically FORMAT contains the static text and variables or constants you want to print to screen formatted with any of the available format codes and modifiers. In case you want to write variables or constants you should not include them in the format section. Instead you have to specify the format which you want to be used to print each of those. After setting the format you have to include as arguments the variables or constants you like to print in the same order you specified their format. As a side note, notice that both format and arguments are not separated by commas.

In the following script you can take a look at some examples on the usage of the format wildcard.

Printf Escape codes

Just like the echo command, printf also has escape codes. In order to maintain compatibility, printf keeps the same escape codes than the echo command, but adds a few more that I will enumerate next.

Printf Examples

I will give a few examples showing the usage of printf.

Take a look at the next script when I show you how to format numbers. Notice that I use \n to start the next line in a new line.

#!/bin/bash
MyVar=-112450000000000
printf "MyVar as signed decimal number: %d\n" $MyVar
printf "MyVar as floating point number: %f\n" $MyVar
printf "MyVar as double precision number: %e\n" $MyVar
printf "MyVar as double precision number: %E\n" $MyVar
printf "MyVar as double precision number: %g\n" $MyVar
printf "MyVar as double precision number: %G\n" $MyVar
printf "MyVar as unsigned decimal number: %u\n" $MyVar
printf "MyVar as C99 hexadecimal floating-point number: %u\n" $MyVar
printf "MyVar as a character: %c\n" $MyVar
printf "MyVar as a string: %s\n" $MyVar
printf "Here is a percentage sign: %%\n"

On the next image you can see the execution of the previous script.

When to use printf instead of echo?

Printf is ideal when you need to display formatted text or a statement composed with many variables.

Using the Read Command to Catch User Input

In order to write an interactive script, the scripting language must give you the possibility to grab user input. Bash is not an exception. It provides us with the read command. This command reads one line from the console (the standard input) or any file descriptor you pass as a parameter. I will cover file descriptors in another tip, but for now think that everything in Linux is a file. Yes, even the console as file which is /dev/tty. As homework, try to write something to that file for example by running the following command.

ls > /dev/tty

The read command has the following syntax.

read [options] VAR1 VAR2 ... VARN

The options are described in the table below. VAR1 … VARN are the destination variables for each word separated by a space character of the line. In case you only use one variable, it will get all the words.

Read Command Examples

The next script prompts the user for his name and saves it in a variable.

#!/bin/bash
read -p "Enter your Name:" Name
echo "Your name is " $Name

On the next screen capture you will see the execution of the previous script.

In the next example I use the –d option to change the end of line character to ";" instead of the default CRLF.

#!/bin/bash

echo "Enter the following personal info. Finish with semicolon (;)"
echo "Name"
echo "Surname"
echo "Address"

read -d ";" Info
printf "\nYou entered:"
printf "%s\n" $Info 

The next screen capture shows the results of the execution of the previous script.

As you may guess, this is not very useful. It would be awesome if we could parse each data element in an array. The good thing is that we can do that! Take a look at the following script.

#!/bin/bash

declare -a MyArray
echo "Enter the following personal info. Finish with semicolon (;)"
echo "Name"
echo "Surname"
echo "Phone"

read -a MyArray -d ";"
printf "\nYou entered:"
printf "%s\n%s\n%s\n" ${MyArray[0]} ${MyArray[1]} ${MyArray[2]}

Next Steps

• Stay tuned for the next tips in this series.
• Are you new on SQL Server running on Linux? You must read this tip: Getting Started with SQL Server on Linux.
• Are you a SQL Server DBA new to Linux? Here you will find 7 Things Every SQL Server DBA Should Know About Linux.
• On the next tip you will find the Top 10 Linux Commands for SQL Server DBAs.
• Take a look at SQL Server on Linux Tips Category.

About the author

Daniel Farina was born in Buenos Aires, Argentina. Self-educated, since childhood he showed a passion for learning.

This author pledges the content of this article is based on professional experience and not AI generated.

View all my tips