Wally Shell

This is a shell built in C. This shell implements a command line interface similar to well-known shells such as bash.

Technologies

• C

GIF Walkthrough

How to Run

• Clone this repository.
• Open the directory in terminal.
• Run make to compile the project.
• Run ./WallyShell to run the shell.

Project Details

• This shell prints interactive input prompt and receives user input.
  • Before printing a prompt message, this shell checks for any unwaited-for background processes in the same process group ID as the shell itself and prints informative messages to stderr.
  • After printing the command prompt, this shell reads a line of input from stdin. If reading is interrupted by a signal, a newline is printed followed by a new command prompt.
• It then splits command line input into semantic tokens, performs expansion, and parses the words.
  • Input Splitting
    • The line of input is split into words, delimited by the characters in the IFS environment variable.
    • If the IFS environment variable is unset, " \t\n" will be the delimiter.
  • Expansion
    • Any occurrence of ~/ at the beginning of a word will be replaced with the value of the HOME environment variable.
    • Any occurrence of $$ within a word will be replaced with the process ID of the shell process.
    • Any occurrence of $? within a word will be replaced with the exit status of the last foreground process.
    • Any occurrence of $! within a word will be replaced with the process ID of the most recent background process.
    • If no information is available, $? defaults to "0" and $! defaults to "\0".
  • Parsing
    • The first occurrence of the word # and any additional words following it will be ignored as a comment.
    • If the last word is &, it indicates that the command is to be run in the background.
    • If the last word is immediately preceded by the word <, it is interpreted as the filename operand of the input redirection operator.
    • If the last word is immediately preceded by the word >, it is interpreted as the filename operand of the output redirection operator.
• There are two built-in commands, exit and cd that this shell will execute.
  • exit Command
    • The exit command takes one integer argument. If not provided, the argument is implied to be the expansion of $?, the exit status of the last foreground process.
    • The shell prints "\nexit\n" to stderr and exits with the specified (or implied) value.
    • All child processes will be sent a SIGINT signal before exiting.
  • cd Command
    • The cd command takes one argument. If not provided, the argument is implied to be the expansion of ~/, the value of the HOME environment variable.
    • The shell changes its own current working directory to the specified (or implied) path.
• Other commands are executed using appropriate exec(3) function.
  • The command with its arguments is executed in the new child process via fork().
  • In the child process:
    • If a filename was specified as the operand to the input or output redirection operator (< or >), the specified file will be opened for reading/writing on stdin/stdout.
    • If a file name was specified as the operand to the output redirection operator but the file does not exist, then the child process will create the file with permissions 0777.
    • If an error occurs in the child process, it will print an error message to stderr and exit with a non-zero exit status.

Who is Wally?

Wally, after whom this shell is named, is my companion animal and a connoisseur of socks.

Contact

Elliott Larsen

• Email [email protected]
• GitHub @elliottlarsen
• LinkedIn @elliottlarsen