Project1: Unix Shell with History Feature

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Project1 Unix Shell with History Feature

• Goals
• Descriptions
• Methodology
• Submission

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Goals

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

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Descriptions

• Demo
• commandgt ls
• commnadgt cat proj1.c
• commandgt ctr-c
• commandgt ctr-d
• 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 terminates the shell

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Methodology

• How to get the command from the keyboard?
• Use system call read() with STDIN_FILENO
• 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 set
  background 1 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(" COMMAND-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) if background 1, the parent will
  wait,
• otherwise returns to the setup()
  function. /

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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 SININT 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

• Email to zhuy_at_seattleu.edu
• All source files
• A readme file that describes each file, how to
  compile the file(s), and how to run the file. If
  there is any problem running the file, please
  state it here as well.
• Makefile is preferred!

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Questions