Working with Operators

# CHAPTER 4

Working with Operators #

1. Introduction #

2. Learning Objectives #

• Perform basic integer math using Arithmetic Operators and the expr command.

• Compare numeric values using Relational Operators (-eq, -gt, -lt).

• Compare text variables using String Operators (=, !=, -z).

• Interrogate the filesystem logically using File Test Operators (-f, -d, -r).

• Combine logic using Boolean AND/OR Operators (-a, -o).

3. Arithmetic Operators (Math) #

#!/bin/sh
A=10
B=5

# Addition
SUM=$(expr $A + $B)
echo "Addition: $SUM"

# Subtraction
SUB=$(expr $A - $B)

# Multiplication (Requires escaping the asterisk with a backslash!)
# If you don't use \*, the shell thinks it means "all files in this directory".
MULT=$(expr $A \* $B)
echo "Multiplication: $MULT"

4. Relational Operators (Numeric Comparison) #

• -eq : Equal to

• -ne : Not Equal to

• -gt : Greater Than

• -ge : Greater Than or Equal To

• -lt : Less Than

• -le : Less Than or Equal To

#!/bin/sh
CPU_LOAD=85

# Check if CPU is greater than 80
[ $CPU_LOAD -gt 80 ]
# (In the next chapter, we will wrap this inside an 'if' statement to make decisions!)

5. String Operators (Text Comparison) #

• = : Strings are exactly identical. (Note: standard sh uses a single =, while bash allows ==).

• != : Strings are not identical.

• -z : String is empty (Zero length).

• -n : String is NOT empty (Non-zero length).

#!/bin/sh
USER_ROLE="admin"

# Check if role exactly equals "admin"
[ "$USER_ROLE" = "admin" ]

6. File Test Operators #

• -f : True if the path is a normal File.

• -d : True if the path is a Directory (folder).

• -r : True if the file has Read permission.

• -w : True if the file has Write permission.

• -x : True if the file is Executable.

• -s : True if the file size is greater than 0 bytes (not empty).

#!/bin/sh
LOG="/var/log/syslog"

# Check if the log file exists and is a regular file
[ -f "$LOG" ]

7. Boolean Operators (AND / OR) #

• -a (Logical AND): Both conditions must be True.

• -o (Logical OR): At least one condition must be True.

#!/bin/sh
AGE=25
CITIZEN="yes"

# Check if age > 18 AND citizen = "yes"
[ $AGE -gt 18 -a "$CITIZEN" = "yes" ]

8. Diagrams/Visual Suggestions #

• == (Math) -> -eq (Shell Numeric) -> = (Shell String)

• > (Math) -> -gt (Shell Numeric)

9. Best Practices #

• Quote Strings in Comparisons: When using [ "$VAR" = "Text" ], ALWAYS wrap the variable in double quotes. If the variable is empty or contains a space, failing to use quotes will break the bracket syntax and crash the script with a "too many arguments" error.

10. Common Mistakes #

• Using < or > for numbers: If you type [ 10 > 5 ] in a shell script, the shell sees the > symbol and assumes you want to perform output redirection (Chapter 9). It will literally create a text file named 5 in your current directory! Always use -gt and -lt for numbers.

11. Mini Project: The Calculator and Verifier #

1. 1. nano verify.sh

1. 2. Write the code:

#!/bin/sh

FILE="data.txt"

# Create a dummy file so the script works
touch "$FILE"

echo "1. Performing Math Operation..."
VAL1=50
VAL2=25
RESULT=$(expr $VAL1 / $VAL2)
echo "$VAL1 divided by $VAL2 is: $RESULT"

echo "2. Testing numeric relation..."
# Test if result is equal to 2 (Silent test)
[ $RESULT -eq 2 ] && echo "Math verification passed!"

echo "3. Testing File Operators..."
# Check if file exists AND has write permission
[ -f "$FILE" -a -w "$FILE" ] && echo "File $FILE is ready for data injection."

# Cleanup
rm -f "$FILE"

1. 3. chmod +x verify.sh and run it. You have successfully utilized Arithmetic, Relational, Boolean, and File operators in a single workflow.

12. Practice Exercises #

1. 1. Differentiate the operational use of the = operator versus the -eq operator in POSIX shell scripting.

1. 2. Explain the catastrophic failure that occurs when attempting to use the > symbol to determine if integer A is greater than integer B within a script.

13. MCQs with Answers #

14. Interview Questions #

• Q: A junior engineer writes the syntax [ $AGE>18 ] to verify if a user is an adult. Explain why this syntax is invalid in standard sh scripting, and describe the exact file-system anomaly this typo will silently generate.

• Q: Walk me through the structural requirement of the -z String Operator. In what specific security or validation scenario is testing for a "Zero-length string" absolutely mandatory?

• Q: Explain the necessity of wrapping variables in double quotes during string comparisons (e.g., [ "$INPUT" = "deploy" ]). What specific error is prevented by this formatting?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #