Function Scope

Intermediate~15 min read

Bash variables are global by default. Use local within functions to contain changes and prevent accidental side effects.

Local vs Global

Declare a variable with local inside a function to keep it scoped. Without local, assignments modify the global variable.

Output

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Note: export makes variables available to child processes. It does not make them local.

Pro Tip: Use a consistent prefix for globals (e.g., APP_) and always declare temporaries with local inside functions.

Caution: Sourcing files (. file.sh) executes in the current shell and can override globals; keep sourced files tidy and namespaced.

Common Mistakes

1) Accidental global mutation

# Wrong
count=0
inc() { count=$((count+1)); }
inc; echo "$count"  # modified globally

# Correct
inc() { local count=0; count=$((count+1)); echo "$count"; }
echo "local: $(inc)"; echo "global: $count"

2) Assuming `export` makes a variable local

# Wrong
foo=bar; export foo  # Still global in current shell

# Correct
local foo=bar  # scoped to function only
export APP_TOKEN  # only for children

Exercise: Safe Increment

Task: Implement safe_inc that increments a number passed by value and echoes the result without changing any global counter variable.

Do not modify globals.
Return non-zero if the input is not an integer.

Output

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Show Solution

safe_inc() {
  local n=$1
  [[ $n =~ ^-?[0-9]+$ ]] || return 1
  echo $(( n + 1 ))
}

counter=41
safe_inc "$counter"  # echoes 42
echo "$counter"      # unchanged

Summary

Variables are global unless declared local in a function.
Shadowing lets you reuse names safely with local.
Use export to pass values to child processes, not for scoping.

What's Next?

Finally, build recursive functions for problems like factorial and directory traversal in Recursive Functions.

Previous Return Values

Next Recursive Functions