Variables and User Input

# CHAPTER 3

Variables and User Input #

1. Introduction #

2. Learning Objectives #

• Declare and initialize string and numeric variables strictly.

• Call (expand) variable data using the Dollar Sign ($).

• Pause script execution to capture keyboard input using the read command.

• Utilize built-in Unix Environment Variables (e.g., $USER, $HOME).

• Store the standard output of terminal commands into variables using Command Substitution.

3. Declaring and Calling Variables #

1. Creating a Variable: To create a container, you write the name, an equals sign, and the data. *ABSOLUTE RULE:* You cannot have any spaces around the equals sign!

#!/bin/sh

# Correct Syntax
CITY="Tokyo"
PORT=8080

# FATAL SYNTAX ERROR (Will crash the script)
# CITY = "Tokyo"

2. Calling a Variable: To pull the data out of the container to use it, prepend the Dollar Sign ($) to the variable name.

echo "The target server is located in $CITY."

4. Reading User Input (read) #

#!/bin/sh

echo "Enter the target IP address to deploy:"
# The script pauses here and waits for typing
read TARGET_IP

echo "Deployment sequence initiated for $TARGET_IP."

*Note: In advanced shells like Bash, read supports a -p flag for prompts. In strict POSIX sh scripting, you must use an echo statement before the read command, as shown above.*

5. Environment Variables #

• $USER : The username of the person executing the script.

• $PWD : Print Working Directory (where the script is running from).

• $HOME : The path to the user's home folder.

• $HOSTNAME : The network name of the physical server.

#!/bin/sh
echo "Audit Log: Script executed by $USER on server $HOSTNAME."

6. Command Substitution (The Superpower) #

#!/bin/sh

# The shell runs the 'date' command and stores the result
CURRENT_TIME=$(date)

# The shell runs the 'whoami' command and stores the result
ADMIN=$(whoami)

echo "At $CURRENT_TIME, the user $ADMIN modified the system."

*(Legacy Syntax Note: You may see older scripts using backticks ` date instead of $(). Both work, but $() is the modern, readable standard).*

• Double Quotes Around Variables: Always place double quotes around your variables when printing them (e.g., echo "Hello $NAME"). If you use single quotes (echo 'Hello $NAME'), the shell will take it literally and print the exact text "$NAME" instead of the person's actual name.

• Variable Isolation (The Underscore Trap): If you define FILE="report" and want to print reportbackup.txt, typing echo "$FILEbackup.txt" will fail. The shell looks for a variable literally named FILEbackup, finds nothing, and prints a blank space. You must isolate the variable using curly braces: echo "${FILE}backup.txt".

1. 1. nano greeter.sh`

1. 2. Write the code:

#!/bin/sh

# Gather dynamic system data using Command Substitution
KERNEL_VER=$(uname -r)
UPTIME_STATS=$(uptime -p)

# Clear the screen for a clean UI
clear

echo "======================================"
echo "    SYSTEM DIAGNOSTIC INTERFACE       "
echo "======================================"

echo "Welcome, $USER."
echo "Please enter your Department ID:"
read DEPT_ID

echo ""
echo "Authenticating $USER for Department: $DEPT_ID..."
echo "Server Hostname : $HOSTNAME"
echo "Kernel Version  : $KERNEL_VER"
echo "System Uptime   : $UPTIME_STATS"
echo "======================================"

1. 3. chmod +x greeter.sh

1. 4. Run ./greeter.sh. You have built a fully interactive, dynamic terminal application!

11. Practice Exercises #

1. 1. Analyze the following variable declaration: IPADDR = "10.0.0.1". Why will this completely crash a shell script, and what is the exact fix?

• Q: A legacy shell script contains the line DISKUSAGE=\df -h / | awk '{print $5}'\`. Explain what this syntax accomplishes, and describe the modern, POSIX-compliant alternative syntax that is universally preferred for nested substitution.

• Q: Walk me through the exact scenario where the lack of spaces around the equals sign (=) in a variable assignment prevents the shell interpreter from misidentifying the variable name as an executable terminal command.

16. Next Chapter Recommendation #