Linux-Shell

  • Functionality

    • Prompt: <username@system_name:curr_dir> 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” change directory then the corresponding change must be reflected in the shell as well.
    • Builtin commands (cd , pwd and echo) 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.
    • Foreground processes: For example, executing a “vi” command in the foreground implies that your shell will wait for this process ​to complete and regain control when this process exits.
    • Background processes: Any command invoked with “&” is treated as background command. This implies that your shell will spawn that process and doesn’t wait for the process to exit. It will keep taking user commands.
    • E.g:
      • <Name@UBUNTU:~> ls & This command when finished, will print its result.
      • <Name@UBUNTU:~> emacs &
      • <Name@UBUNTU:~> ls -l -a
      • <Name@UBUNTU:~> echo hello
    • If the background process exits then the shell display the appropriate message to the user.
    • pinfo : prints the process related info of your shell program.
    • Output of running one (or more) commands redirected to a file. Similarly, a command might be prompted to read input data from a file and asked to write output to another file. Appropriate error handling is done (like if the input file does not exist etc.)
      • E.g: Output redirection
        • <NAME@UBUNTU:~>diff file1.txt file2.txt > output.txt
      • E.g: Input redirection
        • <NAME@UBUNTU:~>sort < lines.txt
      • E.g: Input-Output redirection
        • <NAME@UBUNTU:~>sort < lines.txt > sorted-lines.txt
        • Note: There is another clause for output direction ‘»’, is implemented too.
    • A pipe is identified by ” ”. One or more commands can be piped as the following examples shown, supports any number of pipes.
      • E.g: Two Commands
        • <NAME@UBUNTU:~> more file.txt | wc
      • E.g: Three commands
        • <NAME@UBUNTU:~> grep "new" temp.txt | cat somefile.txt | wc
    • listjobs: prints a list of all currently running jobs along with their pid, particularly background jobs, in order of their creation.
      • E.g:
        • <NAME@UBUNTU:~>jobs [1] emacs Assign.txt [231] [2] firefox [234] [3] vim [5678] Here [3] i.e vim is most recent background job, and the oldest one is emacs.
    • sendsig  : takes the job id of a running job and send given signal value to that process
      • E.g: <NAME@UBUNTU:~> kjob 2 9 It sends sigkill to the process firefox, and as a result it is terminated. Here 9 represents the signal number, which is sigkill. For further info, lookup man 7 Signal.
        • fg : brings background job with given job number to foreground.
      • E.g: <NAME@UBUNTU:~> fg 3 Either brings the 3rd job which is vim to foreground or returns error if no such background number exists.
        • killallbg: kills all background process at once
        • quit: exits the shell. Your shell should exit only if the user types this “quit” command. It should ignore any other signal from user like : CTRL-D, CTRL-C, SIGINT, SIGCHLD etc.
        • CTRL-Z : It should change the status of currently running job to stop, and push it in Background process.
        • CTRL+D : Exit the shell.
        • CTRL+C : Match the functionality of bash shell.

  • Code Structure

    • shell.h has all the declarations of the methods and structures used in shell.c.
    • shell.c has all the definitions of the methods of declarations in shell.h.
    • main.c has the runnable code and main method in it.
    • pinfo, cd, pwd, echo commands are implemented in process method.
    • foreground, background running of process are implemented using waitpid(), fork() methods.
    • commands seperated with ”;”(semicolon seperated) are executed sequentially.
    • when a process is terminated shell shows its exit messages and pid of that process.
    • run process in backround “&” should be appended at last.   - strtok_r method is used to parse the input with ”;” (semicolon separated), ” “ (space seperated) tokens.

  • Execute

    • run make in cloned directory.
    • execute ./main to start shell.