criptors 27 3.6.9 Opening File Descriptors for Reading and Writing 28 3.7 Executing Commands 28 3.7.1 Simple Command Expansion 28 3.7.2 Command Search and Execution 29 3.7.3 Command Execution Environment 29 3.7.4 Environment 30 3.7.5 Exit Status 31 3.7.6 Signals 31 3.8 Shell Scripts 32 4 Shell B