This is a shell built in C. This shell implements a command line interface similar to well-known shells such as bash.
- C
- Clone this repository.
- Open the directory in terminal.
- Run
maketo compile the project. - Run
./WallyShellto run the shell.
- 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.
- 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
- 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
IFSenvironment variable. - If the
IFSenvironment variable is unset," \t\n"will be the delimiter.
- The line of input is split into words, delimited by the characters in the
- Expansion
- Any occurrence of
~/at the beginning of a word will be replaced with the value of theHOMEenvironment 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".
- Any occurrence of
- 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.
- The first occurrence of the word
- Input Splitting
- There are two built-in commands,
exitandcdthat this shell will execute.exitCommand- The
exitcommand 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"tostderrand exits with the specified (or implied) value. - All child processes will be sent a
SIGINTsignal before exiting.
- The
cdCommand- The
cdcommand takes one argument. If not provided, the argument is implied to be the expansion of~/, the value of theHOMEenvironment variable. - The shell changes its own current working directory to the specified (or implied) path.
- The
- 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 onstdin/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
stderrand exit with a non-zero exit status.
- If a filename was specified as the operand to the input or output redirection operator (
- The command with its arguments is executed in the new child process via
Wally, after whom this shell is named, is my companion animal and a connoisseur of socks.
Elliott Larsen
- Email elliottlrsn@gmail.com
- GitHub @elliottlarsen
- LinkedIn @elliottlarsen