Nash - Linux Shell in C

Introduction

This is an implementation of a Linux shell written in C language.

Run the shell

1. Clone this directory and cd into it.
2. Run the command make.
3. Run ./shell to get a prompt of the form username@system_name:path.
4. Run any command in the shell. It can entail as many number of tabs and spaces, the shell accounts for those.
5. In order to exit, run quit.

Features

Built-in commands

These commands have been defined by me and are contained within the shell itself.

1. pwd

   • Displays the name of the working directory.
   • Implemented in pwd.c

2. ls [-l -a] [directory]

   • Lists all the files and directories in the specified directory in alphabetical order.
   • Variations such as ls, ls . , ls .. also work.
   • Also handles multiple directories as arguments. eg. ls -l dir1 dir2 dir3.
   • Throws error if you try to ls on anything except a directory.
   • Also highlights directories in blue and executable files in green.
   • Implemented in ls.c

3. cd [file]

   • Changes directory to the directory specified, throws an error if the directory does not exist.
   • Implemented in cd.c

4. echo [arguments]

   • Displays whatever is specified in [arguments].
   • Accounts for double quotes as well.
   • Implemented in echo.c

5. quit

   • Exits the shell with return status as success, and adds updates history.txt.

6. setenv var[value] & unset var

   • Creates an enviornmental variable var if it doesn't already exist and assigns it the value given
   • unset var destroys that environmental variable

7. jobs

   • Prints a list of all currently running jobs along with their pid in order of their creation
   • Gives the state of the job – Running, Sleeping, Stopped or Defunct
   • Implemented in jobs.c

8. kjob <jobNumber> <signalNumber>

   • Takes the job id of a running job and sends a signal value to that process
   • Implemented in kjob.c

9. fg <jobNumber>

   • Brings a running or a stopped background job with given job number to foreground.
   • Implemented in fg.c

10. bg <jobNumber>

    • Changes a stopped background job to a running background job.
    • Implemented in bg.c

11. overkill

    • ​ Kills all background process at once.
    • Implemented in overkill.c

Foreground and Background Processes

• All other commands are treated as system commands like emacs, vim etc.
• To run a process in the background, follow the command with a '&' symbol. Eg. emacs &.
• Upon termination of a background process, the shell prints its PID and exit status.
• Handles & no matter where it is in the end. eg. emacs& , emacs &, ls -l&.
• Also returns number of background processes running currently.
• Implemented in run.c

Additional Commands

1. pinfo [PID]

   • Prints numerous details about the process such as its status, memory, and executable path.
   • Just pinfo with no arguments gives details of the shell.
   • Implemented in pinfo.c

2. history [num]

   • Lists the last [num] commands. If no arguments are specified, it displays the last 10 commands.
   • Retains the history even upon Shell exit - uses history.txt.
   • Implemented in history.c

3. nightswatch -n [seconds] [dirty/interrupt]

   • interrupt argument prints the number of times CPU has been interrupted by keyboard.
   • dirty argument prints size of the dirty part of memory.
   • Executes every n number of seconds as specified by user.
   • Exits when symbol q is pressed.
   • Error handling done for incorrect arguments.

4. cronjob -c [command] -t [time_period] -p [duration]

   • Runs the command every t seconds until p time gets over.
   • Cron runs in the background so you can continue using the shell whilst the command is executed periodically

Additional Features

1. ​CTRL-Z

   • Changes the status of currently running job to stop, and pushes it to the background.

2. CTRL-C

   • Sends SIGINT signal to the current foreground job of the shell​.
   • If there is no foreground job, then the signal does not have any effect.

3. Command Recall using ‘UP’ arrow key

   • ‘UP’ key with the ‘ENTER’ key causes a new prompt to be displayed with the previous command and then that command is executed. (similar to normal Ubuntu environment).
   • Pressing ‘UP’ key ‘K’ times consecutively leads to the K​th ​ previous command getting executed.

4. Input-Output Redirection & Piping

   • Handles <, >, >>, and | operators appropriately, wherever they are in the command
   • Throws error if syntax is incorrect

Coding style

The code is completely modular with different .c files for each command, and a main.c binding them all together. headers.h entails all the necessary header files, and global variables.

License

This project is licensed under the MIT License - see the LICENSE.md file for details.