Operating Systems and Networks - Assignment 3

Name: AKSHETT RAI JINDAL

Roll Number: 2019114001

Prerequisites

• You need to have the following programs installed:
  • gcc
  • make
• Your terminal should support colors

Assumptions

• The host name has maximum of 256 characters
• The commands entered are no more than 10000 characters long
• The path sizes are a max of PATH_MAX characters long (linux/limits.h)
• The arguments are tokenized on spaces and tabs
• The different commands are tokenized on the basis of ;
• The 6 month period for ls -l is calculated on the basis of day on which file was modified and does not take the hour or minutes or seconds of the day into account.
• The history file is not deleted during a shell session
• The history is stored in the file $HOME/.2019114001_ash_history - HERE THE $HOME IS THE USER HOME DIR AND NOT THE ONE FROM WHERE THE SHELL WAS EXECUTED

Bonus

• From the bonus part, the following is implemented:
  • replay command to run a command at fixed intervals for a fixed period of time
  • baywatch command to watch some things about the system:
    • interrupt - the number of interrupts given to i8042 with IRC id 1
    • dirty - the size of the dirty memory used by the system
    • newborn - the PID of the newly born processes in the system

Compilation

• Run make
• Then to run the shell, execute ./ash

Source Directory Structure

src
├── utils -- Utilities used throughout the code
│  ├── terminal_modes.h, terminal_modes.c -- toggling the raw mode of the terminal
│  ├── run_command.h, run_command.c -- forming pipes, handling redirection and start execution of the commands
│  ├── prompt.h, prompt.c -- printing the shell prompt on the terminal
│  ├── process_path.h, process_path.c -- processing any symbols present in paths
│  ├── parser.h, parser.c -- parse the user input and identify the various commands
│  ├── history.h, history.c -- managing the history file
│  ├── create_process.h, create_process.c -- forking for running non-internal commands
│  └── child_process_list.h, child_process_list.c -- managing the jobs started by the shell
├── main.c -- entry point of the shell, take input and pass to parser and command runner
├── headers.h -- the libraries used throughout the source code
├── globals.c -- some global variables to be used throughout the shell
├── consts.h -- some global constant values to be used throughout the shell
└── commands -- Inbuilt commands of the shell
   ├── sig.h, sig.c -- send signals to the background jobs
   ├── replay.h, replay.c -- replay a command at fixed intervals for a period of time
   ├── pwd.h, pwd.c -- print the present working directory
   ├── pinfo.h, pinfo.c -- print the information about the state of a process
   ├── ls.h, ls.c -- list files and directories
   ├── jobs.h, jobs.c -- display the jobs running by the shell
   ├── history.h, history.c -- print content from history file
   ├── fg.h, fg.c -- bring a background job to run in foreground
   ├── echo.h, echo.c -- print something to stdout
   ├── cd.h, cd.c -- change the present working directory
   ├── bg.h, bg.c -- start the background execution of a stopped background process
   └── baywatch.h, baywatch.c -- watch the interrupts, dirty memory and PID of newborn processes of the system

Features and Inbuilt Commands

Processes and Jobs

• Can create new processes for executing any command other than the inbuilt commands
• Can run the processes in BACKGROUND by adding a & at the end of the arguments:
  • Not Supported for inbuilt commands
  • The BG processes are stored as jobs in the shell
  • When a job exists, then feedback is given by user about the exit code

Signals

• Ctrl+C:
  • SIGINT signal is sent to the current foreground running process
  • Ignored if no command is running in the shell
• Ctrl+Z:
  • The current running foreground process is suspended by sending SIGSTSTP signal
  • Ignored if no command is running in the shell
• Ctrl+D:
  • Exits from the shell if the shell is currently asking for input from user
  • Ignored if the shell is executing some command

Piping & Redirection with files

• The | symbol can be used to create pipes between various commands. Multiple pipes can be created at the same time between various processes (command1 | command2 | command3 | ...)
• Redirection is also implemented with files:
  • <: Input Redirection
  • >: Output Redirection with truncation if file already exists
  • >>: Output Redirection and appended to the file if it already exists
• Piping and redirection can be used together but Redirection will take precedence over Piping (like bash) if the output/input is both redirected and piped

Job Management Commands

• jobs:
  • Print out the current jobs sorted alphabetically by the command run and show whether its stopped or running
  • Flags -r and -s can be used to print only the running or only the stopped jobs
• sig:
  • Send a signal to a job
• bg:
  • Resume the execution of a stopped job by sending SIGCONT
• fg:
  • Bring a background job to foreground and resume execution if stopped

replay

• This command allows the user to run a command at fixed intervals for a fixed period of time

baywatch

• This command is equivalent to the watch command but can be used for only 3 specific things:
  • interrupt - the number of interrupts given to i8042 with IRC id 1
  • dirty - the size of the dirty memory used by the system
  • newborn - the PID of the newly born processes in the system
• This keeps on running until the user presses q key on the keyboard

cd

• Changes the current working directory of the shell
• Both absolute and relative paths supported
• Support for following flags:
  • . - Current Working Directory
  • .. - Parent Working Directory
  • - - Previous Working Directory
  • ~ - Home Directory of the shell
• No arguments: Home directory

echo

• Outputs the arguments passed to the terminal
• Removes the extra spaces

exit

• Exits out of the shell

history

• Print out commands history
• By default tries to print the latest 10 commands
• It also supports history <n> where the latest n commands are printed from history
• A command does not get stored in history if it is exactly same as the last entry in the history file
• History is stored in $HOME/.2019114001_ash_history
• Supports managing history for multiple shell sessions at the same time

ls

• Supports -a and -l flags
• Supports multiple arguments and flags in any order
• Supports multiple-flag arguments like -la, -al, -alal
• The arguments can be both directory and file

pinfo

• Prints information about a process
• Can take one process id (PID) as argument
• If no arguments are passed, info is shown for current process
• Shows the following stats:
  • Process ID
  • Process Status
  • Virtual Memory
  • Executable Path

pwd

• Prints out the current working directory

repeat

• Can repeat a command for n number of times
• Syntax: repeat <n> <command> <command-args>