Bash Functions - Definition, Arguments, Return Values, Scope

Functions in Bash

Functions encapsulate reusable blocks of code. They accept arguments, can return values, and support local variables for proper scoping.

Function Definition

# Standard syntax
greet() {
  echo "Hello, $1!"
}

# Alternative syntax
function greet {
  echo "Hello, $1!"
}

# Call the function
greet "Alice"    # Hello, Alice!
greet "Bob"      # Hello, Bob!

Arguments

Functions access arguments using $1, $2, etc. (same as script arguments):

create_user() {
  local name="$1"
  local email="$2"
  local role="${3:-user}"    # default to "user"

  echo "Creating $role account for $name ($email)"
}

create_user "Alice" "alice@example.com" "admin"
create_user "Bob" "bob@example.com"

Return Values

Functions return exit codes (0-255). Use echo to output values:

# Return exit code
is_installed() {
  command -v "$1" > /dev/null 2>&1
}

if is_installed "docker"; then
  echo "Docker is available"
fi

# Return a value via echo
get_timestamp() {
  date +%s
}

TIMESTAMP=$(get_timestamp)

Local Variables

Use local to prevent variables from leaking into the global scope:

process_file() {
  local filename="$1"
  local line_count=0

  while IFS= read -r line; do
    line_count=$((line_count + 1))
  done < "$filename"

  echo "$line_count"
}

Error Handling in Functions

safe_delete() {
  local target="$1"

  if [ -z "$target" ]; then
    echo "Error: No target specified" >&2
    return 1
  fi

  if [ ! -e "$target" ]; then
    echo "Error: $target does not exist" >&2
    return 2
  fi

  rm -rf "$target"
  return 0
}

# Usage with error checking
if ! safe_delete "/tmp/cache"; then
  echo "Cleanup failed"
fi

Practical Patterns

# Logging function
log() {
  local level="$1"
  shift
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] [$level] $*" | tee -a app.log
}

log "INFO" "Application starting"
log "ERROR" "Failed to connect to database"

# Cleanup trap
cleanup() {
  echo "Cleaning up temporary files..."
  rm -rf "$TEMP_DIR"
}
trap cleanup EXIT

TEMP_DIR=$(mktemp -d)
# ... rest of script