Read Inputs

In this lesson, we explore different methods for obtaining user input in a shell script. Previously, we demonstrated how to pass an input to a script using a command-line argument (with the positional parameter $1). For example, consider the following script that expects the mission name to be provided when executing the script:

#!/bin/bash
mission_name=$1

mkdir "$mission_name"

rocket-add "$mission_name"
rocket-start-power "$mission_name"
rocket-internal-power "$mission_name"
rocket-start-sequence "$mission_name"
rocket-start-engine "$mission_name"
rocket-lift-off "$mission_name"

rocket_status=$(rocket-status "$mission_name")
echo "Status of launch: $rocket_status"

In this version, the mission name is passed directly on the command line as follows:

bash
$ create-and-launch-rocket saturn-mission

If you prefer the script to prompt the user for input when it starts, you can use the read command. Initially, a basic use of read might look like this:

#!/bin/bash
read mission_name

In this simple example, the script waits for the user to type a value, but it does not indicate what is expected. To provide guidance to the user, you can include a prompt using the -p option:

#!/bin/bash
read -p "Enter mission name: " mission_name

mkdir "$mission_name"

rocket-add "$mission_name"
rocket-start-power "$mission_name"
rocket-internal-power "$mission_name"
rocket-start-sequence "$mission_name"
rocket-start-engine "$mission_name"
rocket-lift-off "$mission_name"

rocket_status=$(rocket-status "$mission_name")
echo "Status of launch: $rocket_status"

With this approach, when the script reaches the read statement, it displays the message "Enter mission name: " and waits for the user to input the mission name. The entered value is stored in the mission_name variable, and the script continues with that input.

When to Use Command-Line Arguments vs. Prompts

Alternatively, you can combine both methods. For example, you might first check if the mission name is provided as a command-line argument. If not, the script can prompt the user for input. An enhanced version of the script that incorporates both methods is shown below:

#!/bin/bash
if [ -z "$1" ]; then
    read -p "Enter mission name: " mission_name
else
    mission_name=$1
fi

mkdir "$mission_name"

rocket-add "$mission_name"
rocket-start-power "$mission_name"
rocket-internal-power "$mission_name"
rocket-start-sequence "$mission_name"
rocket-start-engine "$mission_name"
rocket-lift-off "$mission_name"

rocket_status=$(rocket-status "$mission_name")
echo "Status of launch: $rocket_status"

This script checks if the first argument is empty. If it is, the script prompts the user for the mission name, otherwise, it uses the command-line argument.

Now, let's move on to some practical exercises to work with both command-line arguments and read input statements.