Working with Strings and Numbers

# CHAPTER 4

Working with Strings and Numbers #

1. Introduction #

2. Learning Objectives #

• Concatenate multiple string variables into a single output.

• Calculate the character length of a string using ${#VAR}.

• Extract specific sub-strings (slicing) from a larger variable.

• Perform basic integer arithmetic (addition, subtraction, multiplication).

• Utilize the expr, let, and Double Parentheses (( )) mathematical syntax.

3. String Manipulation #

1. Concatenation (Joining Strings): Joining strings in Bash is incredibly easy. You just smash the variables together inside double quotes.

FIRST="Linux"
SECOND="Server"
COMBINED="$FIRST $SECOND"
echo $COMBINED  # Output: Linux Server

2. Finding String Length: If you need to validate that a user's password is at least 8 characters long, you must count the characters. Place a Hash (#) inside the curly braces.

PASSWORD="MySecretPassword"
echo "The password length is: ${#PASSWORD}"
# Output: The password length is: 16

3. Substring Extraction (Slicing): You can cut a specific chunk of text out of a variable. *Syntax:* ${VARIABLE:Starting_Position:Length} *(Note: Bash starts counting at 0, not 1).*

PHONE="555-867-5309"
# Extract 3 characters, starting at position 4
PREFIX=${PHONE:4:3}
echo $PREFIX  # Output: 867

4. Arithmetic Operations (Math in Bash) #

Method 1: The Modern Way (( )) This is the standard, cleanest way to do math in modern Bash. The double parentheses instruct the shell to treat everything inside as numbers.

#!/bin/bash
NUM1=10
NUM2=5

# Calculate and store the result
RESULT=$(( NUM1 + NUM2 ))
echo "Addition: $RESULT"

# Multiplication
RESULT=$(( NUM1 * NUM2 ))
echo "Multiplication: $RESULT"

Method 2: The let Command let is a built-in command that evaluates arithmetic expressions.

let "RESULT = 10 + 20"
echo $RESULT

*Crucially, let allows you to increment counters easily without the $ symbol:*

let COUNTER++  # Adds 1 to the counter

Method 3: The Ancient expr Command You will see expr in older, legacy scripts. It is an external program, so it is slower than the double parentheses. *Warning: expr requires strict spaces around the mathematical operators!*

# Correct
RESULT=$(expr 10 + 5)

# Incorrect (Will output the literal string "10+5")
RESULT=$(expr 10+5)

5. The Float Problem (Decimals) #

# Scale=2 tells bc to provide 2 decimal places
echo "scale=2; 10 / 3" | bc
# Output: 3.33

6. Diagrams/Visual Suggestions #

7. Best Practices #

• Use $(( )) exclusively: While expr and let are historically important, $(( )) is POSIX compliant, runs faster because it is built directly into the shell, and is significantly easier to read. Standardize your scripts by using double parentheses for all integer math.

8. Common Mistakes #

• Multiplication Syntax in expr: If you are forced to use expr for multiplication, typing expr 5 * 5 will crash the script. Why? Because Bash sees the asterisk (*) and assumes it is the "Wildcard" character used to list files in a directory! You must "escape" the asterisk with a backslash so Bash treats it as a math symbol: expr 5 \* 5. *(This is another reason to use $(( )) instead).*

9. Mini Project: The System Storage Calculator #

1. 1. nano quota.sh

1. 2. Write the code:

#!/bin/bash

# Define variables in Gigabytes
TOTAL_DRIVE=500
USED_SPACE=350

# Calculate remaining space
FREE_SPACE=$(( TOTAL_DRIVE - USED_SPACE ))

# Calculate basic percentage (Integer only!)
PERCENT_USED=$(( (USED_SPACE * 100) / TOTAL_DRIVE ))

echo "Storage Report:"
echo "----------------"
echo "Total Capacity: ${TOTAL_DRIVE}GB"
echo "Free Space: ${FREE_SPACE}GB"
echo "Usage: ${PERCENT_USED}% Full"

1. 3. chmod +x quota.sh

1. 4. Run it: ./quota.sh. You just built a custom mathematical monitoring tool!

10. Practice Exercises #

1. 1. Given the variable IP="192.168.1.100", write the exact Bash syntax to output the total number of characters in that string.

1. 2. Why does the command RESULT=100/5 fail to assign the number 20 to the variable? What does RESULT actually equal in this scenario?

11. MCQs with Answers #

12. Interview Questions #

• Q: You are auditing a legacy Bash script and see the command TOTAL=$(expr $A * $B). The script constantly crashes on this specific line. Explain the mechanical conflict between the multiplication operator and the Bash shell, and how you would fix the syntax.

• Q: Explain the paradigm difference between how a language like Python handles the assignment x=5 versus how the Bash shell handles the assignment x=5. How does this dictate the necessity of the $(( )) syntax?

• Q: A developer wants to extract the last 4 digits of a dynamically generated 16-character string variable named TOKEN. Walk me through the mathematical slicing syntax required to achieve this.

13. FAQs #

14. Summary #

15. Next Chapter Recommendation #