Project1: Unix Shell using Multi-Processing

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Project1 Unix Shell using Multi-Processing

• Goals
• Descriptions
• Methodology
• Submission

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Goals

• Understand how a simple shell works.
• Understand process creation systems calls, such
  as fork, read, wait, execvp, and etc.
• Understand signal handling mechanisms

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Descriptions

• Demo
• commandgt ls
• commnadgt cat hello.cpp
• commandgt ctr-c
• commandgt ctr-d
• commandgt cat hello.cpp
• Input commands from keyboard
• Fork a child process to perform the command
• Store the past commands in a buffer
• Given a signal, display the most recent commands
  in the buffer
• Ctrl-c and ctrl-d terminate the shell

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Methodology

• How to get the command from the keyboard?
• Use system call read() with STDIN_FILENO or 0
• Implement a setup()
• void setup(char inputBuffer, char args,
  int background)
• setup() reads in the next command line,
  separating it into distinct tokens using
  whitespace as delimiters. setup() sets the args
  parameter as a null-terminated string. Also
  update background if is met
• If ctrl-d is met, just simply call exit(0)

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Methodology

• How to execute the command?
• while (1) / Program terminates normally inside
  setup /
• background 0
• printf(" MyShell-gt\n")
• setup(inputBuffer,args,background) / get
  next command /
• / the steps are
• (1) fork a child process using fork()
• (2) the child process will invoke execvp()
• (3) the parent waits or returns to the
  setup() function, depending on background /

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Methodology

• How to display recent commands?
• Use signal handler CTRL-C is the SIGINT signal
• / the signal handler function /
• void handle_SIGINT()
• write(STDOUT_FILENO,buffer,strlen(buffer))
• exit(0)
• int main(int argc, char argv)
• / set up the signal handler /
• struct sigaction handler
• handler.sa_handler handle_SIGINT
• sigaction(SIGINT, handler, NULL)
• strcpy(buffer,"Caught ltctrlgtltcgt\n")
• / wait for ltcontrolgt ltCgt /
• while (1)
• return 0

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Methodology

• How to keep track of past commands?
• Limited-size buffer, why not use circular buffer?
• Modify setup() to store the current command which
  may overwrite the oldest command in the buffer
• Implement SIGINT signal handler to display the 10
  most recent commands

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Suggested Steps

• Step 1 implement setup()
• Step 2 execute the command from setup()
• Step 3 add the history feature

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Submission

• All source files
• A readme file that describes each file, and how
  to run the file. If there is any problem running
  the file, please state it here as well.
• Makefile
• Tar package
• Electronic submission

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Particular Notes for execvp() and its Variants

• Replace the old program with a new one
• Never returns unless failures
• Often used after a fork() to load a new program
  for the current process!
• Other variants execl(),

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Questions

• Makefile?