We have built a shell in C++ called Roosh. Beginners face a lot of problems learning Linux and its commands. Roosh will be helpful to beginners. We are adding tutorials in Roosh, which will teach commands and give practice exercises, and those exercises will be graded. So these tutorials can help beginners in learning the Linux commands. All the basic functionalities of a shell are wrapped together in a single place, so users need not go to different platforms to learn different functionalities. Following are the features provided in the Roosh.
- Google using shell without a web browser
- Tab auto-complete and history access through arrow keys
- Color coding
- Aliases and Shell arithmetic
- Interactive shell along with the batch mode
- Pipes and Redirections
- Display Git Branch Status
- Play and learn through Tutorials
Like Bash shell, Roosh shell shows several details like username, hostname, current working directory in the input prompt. As a distinct feature, Roosh Input Prompt shows the rupee-symbol "₹" in place of "$" in the bash shell. Color coding of these details is also followed for a better aesthetic feel and ease of distinction. We also tried a new feature especially useful for developers, Roosh Input Prompt also shows the name of the current branch if the user is working in a git directory. It is a handy feature for project developers using git as VCS.
void print_input_format()
{
char hostname[50];
struct passwd *p = getpwuid(getuid());
// Get the host name and user name of current user
if (gethostname(hostname, sizeof(hostname)) == 0 && p)
{
cout << BLUE << p->pw_name << "@";
cout << hostname << RESET;
}
// Print the current working directory of user
char cwd[100];
cout << ":" << GREEN;
if (getcwd(cwd, sizeof(cwd)) != NULL)
cout << cwd;
else
cout << ".";
cout << RESET;
// Get git branch if current directory is a git directory
print_git_branch();
cout << "₹ " << flush;
}Git branch is obtained by running the git branch --show-current command using popen system call and redirecting errors to /dev/null file if the current directory is not a git directory. This method is used because git doesn't have any system call implemented in C/C++ directly for this purpose.
void print_git_branch()
{
FILE *fp;
char branch_name[400];
// Get current branch and save its name in a file.
fp = popen("git branch --show-current 2>/dev/null", "r");
if (fp == NULL)
{
pclose(fp);
return;
}
// Read branch name and print current branch name
fgets(branch_name, 200, fp);
branch_name[strcspn(branch_name, "\n")] = 0;
// Should check status if any error occurs
pclose(fp);
if (branch_name && strcmp(branch_name, "") == 0)
return;
// print current branch name in red enclosed in round braces
cout << RED << " (" << branch_name << ")" << RESET;
}Roosh Loop repeatedly displays the input prompt and accept the command that the user types to execute. For that, we first need to turn off the canonical mode of the terminal. This will help in parsing each character pressed by the user and take action accordingly. Following code snippet describes the changes to be done in terminal settings.
static struct termios old, current;
tcgetattr(0, &old); /* store the current terminal setting */
current = old;
current.c_lflag &= ~ICANON; /* allow auto submit as soon as charachter is entered */
current.c_lflag &= ~ECHO; /* turn of the automatic display of input */
current.c_cc[VMIN] = 1;
current.c_cc[VTIME] = 0;
tcsetattr(0, TCSANOW, ¤t); /* set the new settings */We will store the input using a struct Buffer which will contain an int position that will store the next position to be filled, and a string buffer to store the actual command.
After that, we need to identify each character being pressed by the user and act accordingly.
int historyCount = 0; // The total number of commands that the user entered
int historyIndex = -1;
InputBuffer keyboard;
while (1)
{
keyword = getchar();
.......
}If the pressed key is a left-right arrow key, then we need to move the cursor accordingly. If it is an up-down arrow key, then we should access the history.
if (keyword == 27)
{
// If the key is Esc
keyword = getchar();
switch (getchar())
{
case 'A': // Up-arrow
store_current_command();
historyIndex++;
set_history_index();
case 'B': // Down-arrow
historyIndex--;
set_history_index();
case 'C': // Right arrow
moveforward(); // move cursor forward
keyboard.position++;
continue;
case 'D': // Left arrow
keyboard.position--;
movebackward();
continue;
default:
continue;
}
// Clear output in console
clear_console(keyboard);
// Replaces the string in the buffer with what the user typed in
if (historyIndex == -1)
{
keyboard.buffer = temp;
}
// Replaces the string in the buffer with a previous command using the arrow key
else
{
keyboard.buffer = history[historyIndex];
}
keyboard.position = keyboard.buffer.length();
cout << keyboard.buffer;
}If the back arrow key is pressed, then we need to delete the character and change the buffer accordingly.
else if (keyword == 0x7f) // Backspace
{
if (!keyboard.position)
{
continue;
}
clear_console(keyboard);
// remove the keyword from appropiate position
keyboard.buffer.erase(keyboard.position - 1, 1);
keyboard.position--;
recover_console(keyboard);
}If Enter is pressed, then that command needs to be executed with the help of roosh_launch.
else if (keyword == '\n')
{
cout << "\n";
string line = keyboard.buffer;
// empty command i.e 'enter key'
if (line.empty())
{
print_input_format();
continue;
}
// pushing 'line' into the history
push_command(line);
// execute all the commands specified in the line
tcsetattr(0, TCSANOW, &old); // restore original terminal settings
bool status = roosh_launch(line);
tcsetattr(0, TCSANOW, ¤t); // set the new settings
historyCount++;
// status equals false when
// exit command is run by the user
if (status == false)
{
tcsetattr(0, TCSANOW, &old); // restore original terminal settings
exit(EXIT_SUCCESS);
}
keyboard.buffer = "";
keyboard.position = 0;
historyIndex = -1;
print_input_format();
}If the tab key is pressed, then we need to call the complete method, which would return the list of files with given initials. If only one option is possible, it gets completed; otherwise, the list of files with the given initials would be displayed.
else if (keyword == '\t')
{
// If tab key is pressed
vector<string> files = complete(keyboard.buffer);
if (files.size() != 0)
{
if (files.size() == 1)
{
string temp1;
int index = keyboard.buffer.find_last_of("/");
if (index == -1)
{
index = keyboard.buffer.find_last_of(" ");
if(index!=-1){
temp1 = keyboard.buffer.substr(0, index+1);
}
else{
temp1 = "";
}
}
else{
temp1 = keyboard.buffer.substr(0, index+1);
}
clear_console(keyboard);
keyboard.buffer = temp1 + files[0];
keyboard.position = keyboard.buffer.length();
cout << keyboard.buffer;
}
If the pressed key is none from the characters checked above, it is simply added to the buffer, and the position is updated. In this way, the input of the user is handled. And when the user exits the shell, the terminal should be brought back to its original settings, therefore, we should add
tcsetattr(0, TCSANOW, &old); in the end.
Roosh parser is used when we have to run a single command, and the command is in the form of string. So we have to convert the command from string cmd to char** cmd.
Following code describes how roosh_parse works
roosh_parse(string command)
1. Create a list_args to store each argument of the command
2. Create an iterator for iterating over the command
3. Use the above iterator and add each argument to the list_args
4. Convert the list_args to array of cstrings (char **)
When the launcher function described below has to run, it uses roosh_tokenizer to divide the commands into several pieces. For example, if a command has a structure like
command_1 | command_2 | command_3 | . . . . . . | command_n
then the tokenizer will split into a list of commands with | as delimiter in the following manner.
index list
[0] ----> command_1
[0] ----> command_2
[0] ----> command_3
. .
. .
. .
[n-1] ----> command_3
Following code describes how roosh_tokenizer works
// accepts a string line as input and
// tokenizes it according to delimeter
roosh_tokenizer(line, delimiter)
{
vector<string> tokens; // to store tokens
string token; // a single token
istringstream iss(line); // used to split string around delimiter
// Tokenizing w.r.t. specified delimiter
while (getline(iss, token, delimiter))
{
token = trim(token); // cleanup whitespaces from token
if (!token.empty())
{
tokens.push_back(token);
}
}
return tokens;
}Roosh launcher is used when a user types a line and clicks enter key. The launcher has to do all the preparations before sending the input for execution.
Following code describes how roosh_launch works
roosh_launch(string line)
1. Call roosh_tokenizer to generate a list of commands
2. Set the input source for the first command. (stdin or a file)
3. Send the list_cmds to the pipe_exec for execution
Roosh pipes are used for actually executing all the commands received from roosh_launch. The process flow of Roosh launch and pipe is explained in a detail in the activity diagram.
Following code describes how pipe_exec works
bool pipe_exec(const vector<string> &cmds, size_t pos, int in_fd)
{
string cmd = cmds[pos];
if (pos == cmds.size() - 1) /* last command */
{
return pipe_exec_last(cmd, in_fd);
}
else
{
int fd[2]; /* output pipe */
if (pipe(fd) == -1)
{
report_error_and_exit("pipe");
}
// parent checks for builtin
int idx = check_builtin(cmd);
// if it is a builtin command, parent runs it
// and moves on to the next command without forking
if (idx != -1)
{
pipe_redirect(in_fd, STDIN_FILENO); // read from read_end of old pipe/stdin
pipe_redirect(fd[1], STDOUT_FILENO); // write to the write_end of new pipe
run_builtin(cmd, idx); /* run builtin-command */
return pipe_exec(cmds, pos + 1, fd[0]); /* execute the rest */
}
/* fork if the command is not a builtin */
pid_t pid = fork();
/* fork failed */
if (pid < 0)
{
report_error_and_exit("fork");
}
/* child process */
else if (pid == 0)
{
child = 1;
Close(fd[0]); /* unused */
pipe_redirect(in_fd, STDIN_FILENO); /* read from in_fd */
pipe_redirect(fd[1], STDOUT_FILENO); /* write to fd[1] */
auto [args, num_args] = roosh_parse(cmd);
execvp(args[0], args);
report_error_and_exit("execvp");
}
/* parent process */
Close(fd[1]); /* unused */
/* closes the old pipe */
if (in_fd != STDIN_FILENO)
{
Close(in_fd);
}
/* waits for the child process to complete */
wait(NULL);
/* execute the rest of the commands*/
return pipe_exec(cmds, pos + 1, fd[0]);
}
}In the code above, first, we check whether the command to be executed is the last command. Because the last command works slightly differently, it does not create a new pipe and does not make a recursive call. It just ends the whole cycle of running new commands.
Roosh Google command is used to get meanings of a word via a request sent to Free Dictionary API. The command can return meaning to a word in more than six languages that can be specified using language code. User can access the help page of the google command via google --help command that lists the language codes too, along with the format of command, i.e. google <word> <language> or to use default language google <word> is enough.
Following is the code for sending a request and parsing the JSON response received from the API. CPR is used to make HTTPS requests and nlohmann to parse JSON from the string.
int google_a_description(string str, string lang)
{
try
{
using json = nlohmann::json;
str = "https://api.dictionaryapi.dev/api/v2/entries/" + lang + "/" + str;
cpr::Response r = cpr::Get(
cpr::Url{str}, cpr::Timeout{5000},
cpr::Header{{"accept", "application/json"},
{"user-agent",
"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.93 Safari/537.36"},
{"upgrade-insecure-requests", "1"}},
cpr::VerifySsl{false});
auto j = json::parse(r.text);
for (auto ar : j[0].at("meanings"))
{
cout << ar.at("partOfSpeech") << " " << ar.at("definitions")[0].at("definition") << endl;
}
}
catch (...)
{
cout << "Sorry this word cannot be found at this moment\n";
}
return 0;
}To Capture the parsed command format and handle the help page following code block is used:
int roosh_google(char **args, int num_args)
{
if (num_args == 1)
{
cout << "Please use google --help for any help.\n";
}
else if (num_args == 2)
{
if (!strcmp(args[1], "--help"))
{
Print Google Help Page
}
else
{
google_a_description(args[1], "en_US");
}
}
else if (num_args == 3)
{
if (!strcmp(args[2], "US"))
google_a_description(args[1], "en_US");
else if (!strcmp(args[2], "UK"))
google_a_description(args[1], "en_UK");
else
google_a_description(args[1], args[2]);
}
else
{
cout << "google: error invalid number of arguments\n";
cout << "Please use google --help for any help.\n";
}
return 1;
}The code checks the number of arguments passed by the Roosh parser and, based on it, analyze the condition and calls the function to fetch data accordingly.
Roosh Calc command is used to evaluate simple arithmetic expressions and displays the output. For this command we will use tinyexpr library written in C.
int roosh_calc(char **args, int num_args)
{
// 2 arguments required
// first is the calc command
// second is the expresssion given
if (num_args > 2)
{
invalid_arg_count_error(num_args, 1);
return 1;
}
if (num_args == 1 || !strcmp(args[1], "--help"))
{
Print Calc Help Page
return 1;
}
// tinyexpr evaluates the expresssion
double answer = te_interp(args[1], 0);
cout << answer << endl;
return 1;
}Our Tutorial only provides a basic usage overview of most commonly used commands in linux and certain test levels to test your understanding of these concepts.
The tutorial is implemented in two parts:
a) tutorial command displays the tutorial content.
b) tutorial level [level-number] to access the levels and tutorial level [level-number] password to submit the password for grading.
To implement these from the parsed command format, following function block is used:
int roosh_exec_tutorial(char **args, int num_args)
{
// number of arguments can be =>
// 1 i.e. tutorial
// 3 i.e. tutorial level <1-6>
// 4 i.e. tutorial level <1-6> password
if (num_args == 1)
{
return roosh_tutorial();
}
// case of 3 or 4 arguments
else if ((num_args == 3 || num_args == 4) && (strcmp(args[1], "level") == 0))
{
if (num_args == 4)
{
//error if fourth arg is not password
if ((strcmp(args[3], "password") != 0))
{
cerr << "no such command exists\n";
return 1;
}
}
//parse the level value to int
int level;
stringstream ss;
ss << args[2];
ss >> level;
//level must be b/w 1-6
if (level > 0 && level <= 6 && (strlen(args[2]) == level / 10 + 1))
{
if (num_args == 3)
{
if (level == 1 || unlocked[level - 1])
{
//display corresponding instructins
string path = (home_dir + "/tutorial/lvl_instrns/level" + args[2] + ".txt");
print_instructions(path);
//navigate to corresponding directory
path = (home_dir + "/tutorial/tutorial_levels/level" + args[2]);
chdir(path.c_str());
if (level == 4)
{
roosh_launch("chmod 000 " + home_dir + "/tutorial/tutorial_levels/level4/open_me");
}
}
else
{
print_no_access_to_level();
}
}
else
{ // for level 1 just check the password entered
if (level == 1)
{
check_password(level);
return 1;
}
//else first check whether the level is unlocked or not
else if (unlocked[level - 1])
{
check_password(level);
return 1;
}
else
{
print_no_access_to_level();
}
}
return 1;
}
else
{
//display error if level is not an integer bw 1 nd 6
cerr << "Please enter a level between 1 to 6.\n";
return 1;
}
}
else
{
cerr << "no such command exists\n";
return 1;
}
}