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Defining Functions

Intermediate~25 min read

Functions package reusable logic in Bash scripts. You can declare functions using either the function name() or name() style, then call them like commands.

Two Declaration Styles

Both styles are widely used. Pick one and be consistent:

  • With keyword: function foo() { ... }
  • Without keyword: foo() { ... }
Output 
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Tip: Functions inherit the parent shell environment. To avoid accidental global changes, use local variables inside functions.
Pro Tip: Keep function names lowercase with hyphens or underscores (e.g., print_summary) and group related helpers together for readability.
Caution: A function that runs set -e or unguarded commands can terminate your whole script. Handle errors explicitly or return status codes.

Local vs Global (Preview)

By default, variables are global in Bash. Use local inside functions to limit scope:

Output 
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Common Mistakes

1) Forgetting local in helpers

# Wrong: overwrites global var
name="Global"
set_name() { name="$1"; }
set_name "Bob"
echo "$name"  # Global changed unintentionally

# Correct: use local inside function
name="Global"
set_name() { local name="$1"; echo "local=$name"; }
set_name "Bob"
echo "$name"  # Still "Global"

2) Relying on return for data

# Wrong: return is only 0..255
add() { return $(( $1 + $2 )); }
add 200 100  # Overflow risk

# Correct: echo and capture output
add() { echo $(( $1 + $2 )); }
sum=$(add 200 100)
echo "$sum"

Exercise: Greeting Function

Task: Write a function greet that prints "Hello, NAME". If no argument is provided, default to "World".

  • Support zero or one argument.
  • Do not modify any global variables.
Output 
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Show Solution 
greet() {
  local name=${1:-World}
  echo "Hello, $name"
}

greet
greet "Alice"

Summary

  • Declare functions with either function name() or name().
  • Call functions like regular commands: my_func arg1 arg2.
  • Use local to prevent side effects on global variables.

What's Next?

Next, pass data into functions and iterate arguments in Function Parameters.

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Next Function Parameters