Bash Basics

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This session builds upon basic shell operations by introducing the core concepts of writing executable Bash scripts.

Learning Objectives

  • Understand the structure and execution requirements of a shell script.
  • Learn how to use the shebang line, manage script permissions, and handle standard input/output streams.
  • Master the use of variables, both local and environment-specific.
  • Effectively utilize command substitution for capturing command output.

Topics Covered

1. The Anatomy of a Script

  • The Shebang Line (#!): Understanding #!/bin/bash and why it's essential for script execution context.
  • Script Permissions: Using chmod +x script_name.sh to make a script executable.
  • Comments: Using the # symbol for documenting script logic.

2. Variables and Data Types

  • Declaring Variables: MY_VAR="hello" (No spaces!). Use readonly MY_VAR to prevent modification.
  • Referencing Variables: Use ${MY_VAR} to avoid ambiguity (e.g., ${USER}_file).
  • Arithmetic Expansion: Use $(( ... )) for basic math.

bash count=5 echo "Next is $((count + 1))"

  • Parameter Expansion (Advanced):
  • ${VAR:-default}: Use default if VAR is unset or empty.
  • ${VAR:0:5}: Extract the first 5 characters.
  • ${#VAR}: Get the length of the string.
  • ${VAR/old/new}: Replace the first occurrence of old with new.

3. Input and Output

  • Reading User Input: Using the read command to prompt the user for data dynamically.
  • Command Substitution: Capturing the output of a command using backticks (`command`) or modern $() syntax (e.g., CURRENT_DATE=$(date)).

4. Script Arguments (Positional Parameters)

  • Accessing Arguments: $0 (script name), $1...$9. For arguments above 9, use ${10}.
  • Special Parameters:
  • $#: Number of arguments passed.
  • $@: All arguments as separate quoted strings (Recommended!).
  • $*: All arguments as a single string.
  • $$: Process ID (PID) of the current script.
  • $!: PID of the last background job.

Lab/Assessment Focus

Goal: Create an enhanced user_info.sh script.

  1. Argument Handling: Check if a username was provided as $1. Use parameter expansion ${1:-$DEFAULT_USER} to set a default if none is provided.
  2. Input: If no argument is provided, prompt for it with a 10-second timeout.
  3. Command Substitution: Capture the User ID (id -u $USER) and the number of currently logged-in users (who | wc -l).
  4. Math: Calculate how many more users can join if the limit is 100 using $((100 - user_count)).
  5. Output: Display a detailed summary including the script's own PID ($$).

Advanced Topic References

Scripting Intro Control Structures