INTERACTIVE USER DEFINED SHELL

GOAL:

The goal of the assignment is to create a user-defined interactive shell program that can create and manage new processes. Implemented a shell that supports a semi-colon separated list of commands (using strtok) Also, supports '&' operator which lets a program run in the background after printing the process id of the newly created process.Handles background , foreground process ,signals sent to them and i/o redirection and command pipelines. code is written in modular fashion.

TO COMPILE : make

RUN: ./a.out

PROMPT:

When you execute your code, a shell prompts like this; <username@system_name:curr_dir>

COMMANDS THAT ARE CUSTOM IMPLEMENTED:

1)echo 2)quit 3)cd { cd cd. cd.. cd ~ and cd -{bonus}) 4)pwd 5) ls { all variations } 6)pinfo 7)setenv 8)unsetenv 9)overkill 10)History 11)jobs 12)fg 13)bg IMPLEMENTATION:

commands will be passed onto a for loop one by one , check if its custom commands else use fork execute execvp in child , if command has & and ask parent to wait , else continue.

custom commands are called from main () , each command's code is written in separate file . ECHO: using echo you can print on terminal example; echo hai gives hai as output

PWD : gives path of current working directory

PINFO [pid]: gives info about process pid , status , vm size (using contents in /proc/pid/status file)and executable path.

LS: works along with flags -l , -a , -al and directories shows directory permissions and list all directories and files uses stat to check permissions opendir and readdir are used to read files in given directory

CD: opens the specified directory

ls and cd both work with absolute and relative paths

Background and Foreground processes : Any command invoked with "&" is treated as background process.builtin commands are executed by execvp ,and signal is used for knowing the status of background process.

HISTORY: displays last 10 commands used (least recent to most)

SETENV var [value]: sets the environment variable var to value.

UNSETENV var: destroys the environment variable var ,if it exists.

OVERKILL: kills all backprocesses at once

JOBS: displays all currently running bg processes. os version used: ubuntu 18.04

QUIT: terminates the shell program

I/O REDIRECTION: implemented < (used for input), > (for output) , >> (for appending into output) ,which are usually used to redirect input /output or both .If output file doesnt exists it creates one. Both input and output redirection works simultaneously.

COMMAND PIPELINES: pipe redirects output of left command as input to right,this shell can support multiple pipes.

I/O redirection with COMMAND PIPELINES: this shell program can handle io redirection within command pipelines

SIGNALS: CTRL +C: interrupts currently running foreground job by sending SIGINT signal.

CTRL +Z: pushes any currently running foreground job into background ,changes its state from running to stopped.

FG: brings background processes to fg and changes state from running to stopped

BG: starts the stopped processes in bg and no affect on running processes

some assumptions/demerits: 1)commands are of size less than or equal to 1024 works 2)in proc/pid/status file hardcodes numbers to get desired output (it wasnt mentioned in manpage that file structure changes through diff. versions of linux ) 3)max of 20 ; separated commands can be passed at once 4)error handling for all syscalls havent been done 5)need to pass valid arugments ( for custom commands especially)