Control Structures

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This session introduces the essential building blocks for creating dynamic and decision-making shell scripts: conditional logic and exit code handling.

Learning Objectives

  • Implement conditional logic using if, elif, and else structures.
  • Understand and utilize various test operators for file, string, and numeric comparisons.
  • Master the use of the case statement for multi-way branching.
  • Learn to check and utilize command exit codes ($?) for flow control.

Topics Covered

1. Conditional Execution with if Statements

  • Structure: The standard if ... then ... elif ... else ... fi block.
  • The Test Command:
  • [ ... ]: Standard POSIX test. Needs quoting for variables.
  • [[ ... ]]: Modern Bash test. Handles empty variables better and supports pattern matching.
  • Pattern Matching (Advanced):

bash if [[ "$filename" == *.jpg ]]; then echo "It is a JPEG image." fi

  • Regex Matching (Advanced):

bash if [[ "$email" =~ ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}$ ]]; then echo "Valid email format." fi

2. Comparison Operators

  • File Tests: Checking for existence, type (file/directory), and permissions (-f, -d, -r, -x).
  • String Comparisons: Equality (=), inequality (!=), and emptiness (-z, -n).
  • Numeric Comparisons: Using -eq, -ne, -gt, -lt (must use these for numbers, not standard operators like >, <).

3. The case Statement

  • Multi-way Branching: Much cleaner than multiple if statements when checking one variable.
  • Pattern Matching & Fall-through:

bash case "$extension" in jpg|jpeg|png) echo "Image file" ;; mp3|wav|flac) echo "Audio file" ;; *) echo "Unknown format" ;; esac

4. Exit Statuses and Short-Circuiting

  • Boolean Logic:
  • cmd1 && cmd2: Run cmd2 only if cmd1 succeeded.
  • cmd1 || cmd2: Run cmd2 only if cmd1 failed.
  • The ! operator: Negating a condition.

bash if ! grep -q "secret" config.txt; then echo "Security check passed." fi

Lab/Assessment Focus

Goal: Create a robust file_checker.sh.

  1. Validation: Use [[ -z "$1" ]] to check if an argument was provided.
  2. Logic: Check if the path exists (-e) and if it's a directory (-d).
  3. Permissions: Check if it's readable (-r) and writable (-w).
  4. Case Statement: Use a case statement to handle different file extensions if the user passes a file instead of a directory.
  5. Output: Use && to print success only if the previous checks passed.

Advanced Topic References

Bash Basics Loops