CS131 Week 5 and Week 6

File Descriptors in UNIX

• Standard input is represented by file descriptor 0.
• Standard output is represented by file descriptor 1.
• Standard error is represented by file descriptor 2.
• File descriptor 2 (standard error) is used for sending error messages and diagnostics.

UNIX Shells

• Not all command-line interfaces qualify as UNIX shells; examples of non-UNIX shells should be recognized.

Command Functions in UNIX

• The 'ps aux' command displays information about all running processes, including user, PID, CPU, and memory usage.
• An exit code of 0 typically indicates successful completion of a command or program.
• The 'top' command provides a dynamic, real-time view of system processes, including resource usage metrics.

Process Identifications

• The program with a process ID of 0 is usually the scheduler (also known as the idle process).
• The 'login' program's purpose is to authenticate users and start a user session.

Programs for Terminal Management

• The 'getty' program manages terminal sessions, providing login prompts.
• The 'sshd' program allows secure remote access through the SSH protocol.

Named Pipes and Inter-process Communication

• The command used to create a named pipe in UNIX is mkfifo.
• Sockets facilitate inter-process communication (IPC) by enabling data exchange between processes, often over a network.
• Sockets and pipes are both mechanisms for IPC in the UNIX kernel, allowing processes to communicate data.

Filter Commands and Interprocess Communication

• An example of a filter command in UNIX includes commands like grep or awk, which process data streams.
• The /dev/null device in UNIX acts as a data sink that discards any data written to it.

IPC and Its Functions

• Interprocess communication (IPC) allows processes to share data, synchronize their actions, and manage concurrent execution.
• Pipes are used in UNIX to facilitate data flow between processes, allowing the output of one command to serve as input for another.

File and Line Counting

• The command wc is used to count lines, words, and characters in files and directories.

Command Pipelining

• Pipes in a command pipeline enable efficient data transfer between commands without the need for temporary files, saving both time and system resources.
• The tee command in a command pipeline duplicates input, allowing the data to be sent to both a file and the standard output simultaneously.