Linux Shell in C

Running Shell

• Go to the directory
• Type the command make on the terminal
• Run ./a.out on the terminal
• The shell starts with prompt on the terminal

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Exit Shell

To exit the shell you should type exit . You can also use Ctrl + C to exit the shell.

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Important Features

• This shell is colorful (prompts ,errors,success etc are displayed in various colors )
• Spaces and tabs are handled for all commands
• You can enter multiple commands separated by ;

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NOTE

Some of the .h files also contain header files specific to their corresponding .c file

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Files

• headers.h : It contains the common header files used across all the C files
• main.c : It contains the shell loop and also checks for which command to execute.
• pwd.h : It contains the declaration of functions used in pwd.c
• pwd.c : It contains the implementation of pwd command.
  • You cannot enter argument along with pwd command (only spaces and tabs is allowed)
• pinfo.h : It contains the declaration of functions used in pinfo.c
• pinfo.c : It contains the implementation of pinfo command(user defined).
  • You cannot enter more than one pid as an argument. If enterred then error is displayed.
• ls.h : contains the declaration of functions used in ls.c
• ls.c : It contains the implementation of ls command (along with the flags)
  • In case of long listing format the normal file is displayed with different colour, directory with other and executable file with other (If directory has execute permissions for user then also directory is displayed with color of directory)
  • You can have repetitions in flag such as : [-la -la], [ -llalal ] etc.
• changedir.h : contains the declaration of functions used in changedir.c
• changedir.c : It contains the implementation of cd command
  • If just cd (i.e no argument to cd) is typed it works same as cd ~
  • Error is displayed if directory does not exist
• echo.h : contains the declaration of functions used in echo.c
• echo.c : It contains the implementation of echo command
• run.h : contains the declaration of functions used in run.c
• run.c : It contains the code to manage system commands (i.e NOT built-in commands).
  • It checks if there is & at the end of string (of form "&" or "xyz&") , if it is there then the command is run in background else foreground .If no such command then error is displayed.
  • The pid of a background process is printed on the stdout if succesfully created along with the count of background process (if suppose 2 background process are already running then for new process count=3). The display format is : [count] pid
  • On termination of a background process appropriate message is displayed.
• prompt.h : contains the declaration of functions used in prompt.c
• prompt.c : It contains the implementation of the prompt on the stdout (<username@system_name:curr_dir>)
  • if there is any error getting any of the values required for prompt then error is displayed.
• nightswatch.h : contains the declaration of functions used in nightswatch.c
• nightswatch.c : It contains the implementation of the nightswatch command.
  • Implemented for interrupt command
  • If you run this command as nightswatch interrupt (i.e without specifying seconds ) then the default time after which it displays interrupts is 5 seconds.
  • To exit you need to type q followed by Enter . It will exit instead of writing the interrupts next time.
  • If instead of q you type any string and press Enter it won't affect the nightswatch command (but \n should be there only at the end of the string)
• history_store.txt : stores the history of the last 20 commands . Commands are written in it according to the algo described in history.c
  • Do not alter this file after running a.out
• makefile : It is the makefile that is used to compile all the c programs together (just need to type make in the terminal)
• history.h : contains the declaration of functions used in history.c
• history.c : It contains the implementation of history command
  • The new command enterred is added at the end of the history_store.txt file (if already 20 then first is deleted)
  • When you type a command and press enter then first it gets updated in th history_store.txt and then the command is executed.
  • If the string typed is exactly same as the last one it is not added in the history_store.txt file.
  • The string is as it is written(that is tabs and spaces are left as it is) in the history_store.txt.
  • If there are multiple commands separated by ; then each of them is stored in history as a separate command one after the other in the order they appear.

Files added in Assignment-3

• manage_jobs.h : contains the declaration of functions used in manage_jobs.c
• manage_jobs.c : contains the implementation of jobs and kjob commands . In jobs command if there is no background process that is displayed .
• manage_signal.h : contains the declaration of functions used in manage_signal.c
• manage_signal.c : contains the signal handlers for signals SIGINT SIGTSTP SIGCHLD
• overkill.h : contains the declaration of functions used in overkill.c
• overkill.c : contains the implementation of overkill command . If there is no background process already then it is displayed .
• pipe.h : contains the declaration of functions used in pipe.c
• pipe.c : contains functions to implement piping . It is checked in the main.c file if the command contains | , if present then piping is executed by using functions in pipe.c
• logical_chain.h : contains the declaration of functions used in logical_chain.c
• logical_chain.c : contains functions to implement logical chaining of commands using operators @ (logical AND) and $ (logical OR) . Error is displayed if the number of operators is not 1 less than the number of commands in the chain . Chain is executed from left to right along with short circuit if the next command exit status is not required .
• io_redirection.h : contains the declaration of functions used in io_redirection.c
• io_redirection.c : contains functions to implement redirection of input output . It is checked in the main.c file if the command contains > >> < , if present then IO redirection is executed by using functions in io_redirection.c
• handle_env.h : contains the declaration of functions used in handle_env.c
• handle_env.c : contains implementation of setenv and unsetenv commands that are used to set and unset environment variables for the shell .
• fg.h : contains the declaration of functions used in fg.c
• fg.c : contains the implementation of fg command
• bg.h : contains the declaration of functions used in bg.c
• bg.h : contains the implementation of bg command

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