qtShell: Linux Shell built in C

OSN Assignment 2, Hariharan Kalimuthu - 2020115015

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Execution Instructions

• run make to create an executable qtShell
• run command ./qtShell in the terminal , and there the shell runs.

Introduction

This is a Linux C-Shell implemented by me.

• To exit the shell :
  1. Press Ctrl + D , saves the current session history , and exits the terminal
  2. enter exit command in terminal.
  3. Assumptions :

     1. note that : max_length of command names are assumed to be of max-length name_len defined in header.h as 1024

     2. history command displays the 10 last commands entered , with latest command on the top.

Shell Features

1. The shell supports the following commands being implemented by me : bg , baywatch , cd , echo , fg , history , jobs , ls , pinfo , pwd , replay , repeat , sig , and other external shell commands .

2. It with support for redirection operations [ > , < , >> ] and piping [ | ] .

3. Error Handling

4. Signal Handling for signals like SIGINT , SIGTSTP , SIGCHLD and external keyboard interrupts like Ctrl +C , Ctrl + Z.

5. Running processes in foreground & background environments along with

6. Command history implemented, storing the latest commands , that remains saved across shell-sessions.

7. Multiple commands separated by ; handled, However there's no support for handling " or ' ( quotes ).

The command implemented are as follows :

• bg : used to continue the execution of stopped background processes in the background environment.

• baywatch : used to execute the given commands from the list of options newborn,dirty,interrupt after a fixed interval as given input by the user, until q is pressed. Handles errors and invalid argument kind. enter baywatch help for correct syntax information.

• cd : To change the current working directory of the shell , it also supports multiple flags like : - , . , ~ , .. etc. along with error messages.

• echo : To output the input string onto the terminal ,supports redirection and piping

• fg : To continue the execution of a stopped background process [ stored in the jobs list ] in the foreground.

• history : Implemented linked list kind structure to store the 20 latest commands entered in the shell . History remains saved across the sessions.

  history : displays latest entered 10 commands.

  history <number> : displays the latest number history commands.

• jobs : A linked list data structure , implemented to keep a track of processes invoked in the background of the shell ( either by the user directly ( using the & flag)) or when a process running in foreground is stopped due to recieving of signal or interrupt ( like Ctrl + Z or any other signal).

  stores the process sorted lexographically in a linked list structure.

  jobs prints the [job_number] process_status process_name pid for processes stored in the data structure. Enter jobs help for getting correct syntax and list of supported options.

  [-options] : -r and -s flags for specifically printing processes with status Running or Stopped respectively.

• ls : executes the listing command like normal shell , along with proper handling of flags [-al] in any order and any number of times.

  supports multiple files and directories , and flags in any order.

  Implemented colors for different kind of files - directory,files,executable etc. to make them visually distinguishable.

• pinfo : outputs the status information of the processes.

  pinfo : displays the process information for the shell .

  pinfo <proc_pid> : displays the process information for the process with given pid and displays appropriate error messages, in case of failures or wrong input format.

• pwd : outputs the present working directory , handles errors.

• replay : executes the given command after fixed interval for a given period of time. Enter replay help for more command syntax information.

  Usage :

  replay -command <command> -interval <interval> -period <period> , that makes the command provided by the command flag , to replay itself ( or execute ) at fixed intervals provided by interval flag <interval> for a given <period>.

  • Handles errors in case of wrong syntax or missing arguments.
  • Flags can be used in any order
  • Multiple flags of same kind , not allowed ; considered as wrong syntax and an error message is displayed.

• repeat :

  Usage:

  repeat <number_of_repetitions> <command> : repeats the command given , <number_of_repetitions> times.

  • Handles errors , in case of wrong syntax or incorrect arguments provided.

• sig : used to communicate with processes running in background of the shell ( stored in jobs data structure) by sending them signals to processes indicated by their job numbers .

  sig <job_num> <sig_num> : sends the signal indicated by <sig_num> to the process indicated by <job_num>, handles errors in case of wrong syntax or failure in sending the signals.

• other shell commands : runs the other shell commands by creating a child process using forking , and executes the commands provided using execvp sys call , displays apt error message in case of any failure.

The above implemented commands commands have been organised several headers and implementation files as described by their file_name.

1. main.c

   • Initializes the shell variables.
   • Contains the main shell loop.

2. bg.c

   • Implements the bg command
   • handles error in case of an invalid job_number is given or invalid syntax is used.
   • continues execution of a stopped background process in background by using.

3. cd.c

   • Implements the cd function in the shell using syscalls of chdir , throws apt error msgs in case of any error.
   • Changes directory to the directory given in input, gives an error if an invalid directory is given.

4. echo.c

   • Implements the standard echo command for the shell, takes into account of multiple spaces and tabs in the given argument.
   • Doesn't account for quotes currently.

5. execute.c

   • contains the main code for parsing the given input command string along with the apt arguments , into it's arguments
   • calls the apt execute_command function from amongst the various functions implemented using if & else and other string compare functions.
   • handles repeat command.
   • contains code for repeat implemented in it, handles nested repeats as well :)

6. header.h

   • Includes all the standard C-libraries used all throughout implementing the C-Shell.
   • defined useful macros used all across the shell.

7. fg.c

   • Implements the fg command
   • Does error handling and handles permissions and other signal .

8. history.c

   • Implements the functionality of storing the 20 most recent commands as an array implemented structure.
   • Provides functionality like printing the entire history or printing n most recent commands as specified by the user
   • Error handling for invalid argument.
   • Provides functionality to store the history of the session into a file ( a .txt file) , and init history function that saves the history of commands across the sessions of the shell.
   • Provides method to store back history into file ( saving the history upon exiting the current shell session )

9. input.c

   • contains implemented getInput command that gets the user entered command and returns the char* string it reads from the terminal.
   • Ctrl + D - exits the shell .

10. ls.c

    • Handles the shell command ls with flags -[al] , can be given in any order.
    • Handles error , and input in any given order.

11. jobs.c

    • Implements the jobs command , and associated utility functions for implementing the job data structure , updating it , etc .

12. pinfo.c

    • implements the pinfo command .

13. prompt.c

    • ​Implements the shell prompt as <USERNAME@HOSTNAME : curr_dir >

14. pwd.c

    • implements the shell pwd command, prints to the teminal the current working directory , or displays apt error in case it fails to do so.

15. pipe.c

    • Implements the piping for shell.

16. redirection.c

    • Implements the redirection commands.

17. replay.c

    • Implements the replay command.

18. signals.c

• SIGINT_HANDLER : ctrl + C , terminates the foreground process , does nothing for shell.

• SIGCHLD_HANDLER : Implements the apt message of the process ( with process name and pid ) on terminal once a background process ( a child process of the terminal ) stops it's execution or finishes or is stopped by a signal, also updates the jobs data structure in case child process terminates it's execution or is killed ( normally or abnormally or on recieving a signal ).

• SIGTSTP_HANDLER : ctrl + Z , stops the foreground process and pushes it to stop state in background ( gets stored in the jobs data structure ). Does nothing for the shell ( doesn't ) exits it.

• implements the sig command .

19. system_command.c

    • contains code for implementing the system commands ( that arent implemented by me ) , using fork and execvp functionality.
    • throws apt error in case of failures
    • executes system commands in background or in foreground as specified by the user.

20. tokenize.c

    • contains basic code for tokenizing the commands taken from user into the constituent commands
    • handles extra spaces and tabs and ";"
    • returns the list of commands (char ** ) from the given input command string ( char *)

21. helper.c

    • Contains helper, sub-routine functions for other files , such as a function that returns the path relative to the home_dir ( our pseudo home dir).

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