Bash & Linux Reference

Shell Execution Model

When you type a command and press Enter, the shell follows a specific lookup order to find and run it. Understanding this model explains why aliases override builtins and why $PATH order matters:

User types: ls -la
     │
     ▼
┌─ Shell checks ─────────────────────┐
│ 1. Is it an alias?     → expand    │
│ 2. Is it a function?   → execute   │
│ 3. Is it a builtin?    → execute   │
│    (cd, echo, export, source...)   │
│ 4. Search $PATH dirs   → fork+exec│
│    /usr/local/bin                  │
│    /usr/bin                        │
│    /bin                            │
└────────────────────────────────────┘
     │
     ▼
  fork() → child process → exec(ls)
     │
  parent waits → child exits → prompt returns