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

1. Execute the following in the directory that contains the source files:

   $ make
   $ ./run
   

• On executing the code, a shell prompt of the following form appears along with it. [exit code] <username@system_name:curr_dir> where [exit code] is :') if last command exited successfully and :'( if the exit was unsuccessful.
• The directory from which the ​shell is invoked will be the home directory of the shell and is indicated by ~.
• If the user executes cd i.e., change dir, then the corresponding change is reflected in the shell as well.
• If the current working directory is the directory from which your shell is invoked, then on executing command cd .., the shell displays the absolute path of the current directory from the root.

• Modular
• The commands are implemented in separate .c files.
• The shell is called from the main function in main.c.
• Common header files are included, functions and global variables are declared in the headers.h file.

• Builtin commands are contained within the shell itself. When the name of a builtin command is used as the first word of a simple command, the shell executes the command directly without invoking another program.
• Builtin commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities.
• Built-in commands have been defined and implemented.

1. pwd

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

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

   • Implemented in ls.c
   • Lists all the files and directories in the specified directory in alphabetical order.
   • Variations such as ls, ls . , ls .. also work.
   • Accepts multiple directories as arguments. eg. ls -la dir1 dir2 dir3.
   • Highlights directories in blue and executable files in green.

3. cd [file]

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

4. echo [arguments]

   • Implemented in echo.c
   • Displays whatever follows echo.

5. exit and quit

   • Exits the shell successfully, and updates hist.txt.

6. setenv var[value] & unset var

   • Implemented in set_unset_env.c
   • Creates an environment variable var if it doesn't already exist and assigns it the value given
   • unset var destroys that environment variable

7. jobs

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

8. kjob <jobNumber> <signalNumber>

   • Implemented in kjob.c
   • Takes the job number of a running job and sends the signal corresponding to ​signal number​ to that process.

9. fg <jobNumber>

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

10. bg <jobNumber>

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

11. overkill

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

• To run a process in the background, enter the command followed by & symbol. Eg. emacs &.
• Upon termination of a background process, the shell prints its PID and exit status.

1. pinfo [PID]

   • Implemented in pinfo.c
   • Prints process status, memory and executable path.
   • pinfo with no arguments gives details of the shell itself.

2. history [n]

   • Implemented in history.c
   • Lists the last [n] commands. If n is not specified, it displays the last 10 commands.
   • Retains the history even upon exit.
   • Stores last 20 commands in hist.txt.

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

   • Implemented in nightswatch.c
   • night_interrupt function prints the number of times CPU has been interrupted by the keyboard. The number of interrupts has been taken to be on line 2 of /proc/interrupts.
   • night_newborn function prints the PID of the process that is most recently created on the system.
   • n specifies the number of seconds after which the interrupt or newborn process is watched, as applicable. Here, n is taken as an input from the user.
   • Takes input only in the specified format.
   • Terminates when q is pressed.

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​.
   • If there is no foreground job, then the signal does not have any effect on the shell.

3. Input-Output Redirection & Piping

   • Handles <, >, >>, and | operators appropriately, wherever they are in the command.