Homework d: Unix Shell and History Feature

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Homework d Unix Shell and History Feature
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Problem Description

• Implement a shell interface
• accept user commands
• execute each command in a separate process
• creating the child process and executing the
  command in the child
• modifying the shell to allow a history feature
• Modify the source in chapter 3
• http//www3.interscience.wiley.com8100/legacy/col
  lege/silberschatz/0471694665/source/source.zip
• Example
• cat /etc/fstab
• parent process waits for the child to exit
• cat /etc/fstab
• (child process to run in the background)
• ltControlgtltCgt
• List the recent 10 commands entered
• r x
• run the most recent command with r the first
  letter of that command.
• ltControlgtltDgt
• terminate the shell

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Motivation

• Try to learn the process concept using system
  calls
• fork()
• exec() family
• signal handling

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The simple shell

• setup()
• read the users next command up to 80 characters
• parse the command into separate tokens (and form
  an argument vector for the command)
• if end with , the command is in background
• example ls -l
• argv0 becomes ls and argv1 becomes -l
• main()
• print the prompt COMMAND-gt
• invoke setup()
• create a child process
• child process will invoke execvp()
• if background, parent wont wait.

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setup()

• read in the next command line
• separate it into distinct tokens using whitespace
  as delimiters
• modify the args parameter to hold array of
  pointers to
• current user command
• arguments
• NULL

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Creating a Child Process

• fork a child process using fork()
• after setup(), invoke
• execvp(char command, char params)
• execvp(args0, args)
• wait or not wait by checking if it is a
  background job

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Creating a History Feature

• Provide a history feature
• access up to the 10 most recently entered
  commands
• commands numbered starting at 1 to grow larger
  even past 10, say 35 commands, and the most
  recent are numbered 26 to 35.
• list the recent commands by pressing
  ltControlgtltCgt, the SIGINT signal.

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Signal Handling

• example events
• division by zero, illegal memory access, user
  entering ltControlgtltCgt,etc.)
• signal handling
• ignoring the signal
• using the default signal handler
• providing a separate signal-handling function

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User defined signal handling

• include /usr/include/sys/signal.h
• struct sigaction handler
• handler.sa_handler user_defined_haandler
• sigaction(SIGINT, handler, NULL)