CMPT 300 - Assignment 1 - Shell Commands and Processes - Solved

1. Overview
In this assignment, you will develop a simple Linux shell. The shell accepts user commands and then executes each command in a separate process. The shell provides the user a prompt at which the next command is entered. One technique for implementing a shell interface is to have the parent process rst read what the user enters on the command line and then create a separate child process that performs the command. Unless otherwise speci ed, the parent process waits for the child to exit before continuing. However, UNIX shells typically also allow the child process to run in the background - or concurrently - as well by specifying the ampersand (&) at the end of the command. The separate child process is created using the fork() system call and the user's command is executed by using one of the system calls in the exec() family.

Note: As usual, all code must be written in C and run on a Linux machine. We will grade your code on a Linux machine. You should create a directory for this assignment, such as ~/cmpt300/a1/ and put all les related to this assignment in it.

2. A Simple Shell
 A C program that provides the basic operation of a command line shell is given below. The main() function rst calls read_command(), which reads a full command from the user and tokenizes it into separate words (arguments). These tokens can be passed directly to execvp() in the child process. If the user enters an "&" as the nal argument, read_command() will set the in_background parameter to true (and remove the "&" from the array of tokens). For example, if the user enters "ls -l" at the '$' prompt, tokens[0] will contain "ls", tokens[1] will contain (or point to) the string "-l", and tokens[2] will be a NULL pointer indicating the end of the arguments. (Each of these strings is a NULL terminated C-style string). Note that the character array buff will contain the text that the user entered; however, it will not be one single NULL terminated string but rather a bunch of NULL terminated strings, each of which is a token pointed to by the tokens array.

#include <stdio.h>

#include <stdbool.h> 

#include <stdlib.h>

#include <string.h>

#include <unistd.h>

#define COMMAND_LENGTH 1024 

#define NUM_TOKENS (COMMAND_LENGTH / 2 + 1) 

 

 
/**

*        Read a command from the keyboard into the buffer 'buff' and tokenize it

*        such that 'tokens[i]' points into 'buff' to the i'th token in the command.

*        buff: Buffer allocated by the calling code. Must be at least 

*        COMMAND_LENGTH bytes long. 

*        tokens[]: Array of character pointers which point into 'buff'. Must be at

*        least NUM_TOKENS long. Will strip out up to one final '&' token. 

*        'tokens' will be NULL terminated. 

*        in_background: pointer to a boolean variable. Set to true if user entered

*        an & as their last token; otherwise set to false. 

 */ void read_command(char *buff, char *tokens[], _Bool *in_background) {

         // ... Full code available in shell.c... 

}

/**

*        Main and Execute Commands 

 */ int main(int argc, char* argv[])

{    char input_buffer[COMMAND_LENGTH];       char *tokens[NUM_TOKENS];   while (true) {

                 // Get command 

                  // Use write because we need to use read()/write() to 
 
 
work with

      

      

      

      

      

      

      

      
// signals, and they are incompatible with printf(). write(STDOUT_FILENO, "$ ", strlen("$ ")); 

_Bool in_background = false; read_command(input_buffer, tokens, &in_background); 

/** 

*  Steps For Basic Shell: 

*  1. Fork a child process 

*  2. Child process invokes execvp() using results in 
 
 
token array. 

      

               }
*        3. If in_background is false, parent waits for 

*        child to finish. Otherwise, parent loops back to 

*        read_command() again immediately. 

 */ 
 
         return 0;

}

3. Problems
There are three mandatory problems (100 points) that you will need to solve:

Create the child process and executing the command in the child.

Implement some internal commands. Implement a history feature.

There is also one optional problem for a 20-point bonus, but you must attempt the mandatory problems rst, before attempting the bonus problem. Attempting the bonus problem without work done in the mandatory problems will not get you any points for the bonus problem.

Problem 1. Creating Child Process [30 points]
First, modify main() so that upon returning form read_command(), a child is forked and executes the command speci ed by the user. As noted above, read_command() loads the contents of the tokens array with the command speci ed by the user. This tokens array will be passed to the execvp() function, which has the following interface:

execvp(char *command, char * params[]); 

where command represents the command to be performed and params stores the parameters to this command. For this project, the execvp() function should be invoked as

execvp(tokens[0], tokens); 

 Be sure to check the value of in_background to determine if the parent process is to wait for the child exit or not. Hint: use waitpid() instead of wait() because you want to wait on the child you just launched. If you only use wait() and have previously launched any child processes in the background that have terminated, wait() will immediately return having "consumed" the previous zombie process, and your current process incorrectly acts as though it was run in the background. Note that we won't be testing with interactive command-line processes run in the background (think vim), or test using signals while running a command in the background. If execvp() returns an error (see man execvp) then display an error message. Note that using printf() may not work well for this assignment and that you should use write() instead (look up more with man write). The issue is that we need to use the read() function for getting the user's command and use write() when working with signals (later). And, it turns out that printf() and read()/write() don't always work well together. Therefore, when printing to the screen, use the write() command. For common things, such as displaying a simple string, or writing a number to the screen, you may want to make your own functions which make it easier. You can convert an integer to a string using sprintf().

 Waiting Aside: When a process in Linux nishes, it still exists in the kernel with some status information until the parent process waits on that child. These un-waited-on terminated child processes are known zombies. For this assignment, you won't lose any marks if you don't correctly wait() on zombie processes from background tasks; however, it's a good habit to correctly cleanup the zombies on your system! Above it is suggested that you use waitpid() to wait on the correct child. However, this will leave any background process as zombie processes (having exited, the parent process will never wait() on the child). You can correct this by occasionally trying a nonblocking wait to handle any zombie child processes. We can pass the WNOHANG option to waitpid() to be non-blocking, and setting the PID to -1 will wait on any child process. For example, have the following code run after every user command is processed:

// Cleanup any previously exited background child processes 

// (The zombies) while (waitpid(-1, NULL, WNOHANG) > 0)

         ; // do nothing.

Problem 2. Shell Prompt and Internal Commands [30 points]
First, make sure your shell prompt always shows the current working directory. For example, if in the /home/cmpt300 folder, the prompt should be:

/home/cmpt300$ 

Next, let's implement some internal commands. Internal commands are built-in features of the shell itself, as opposed to a separate program that is executed. Implement the commands listed below. Note that for these you need not fork a new process as they can be handled directly in the parent process. All the commands here are case-sensitive.

exit: Exit the shell program. If the user provided any argument, abort the operation (i.e., command not executed) and display an error message.

pwd: Display the current working directory. Use the getcwd() function. Run man getcwd for more. Again, abort the operation and display an error message if the user provided any argument. cd: Change the current working directory. Use the chdir() function. Pass chdir() the rst argument the user enters (it will accept absolute and relative paths). If the user passed in more than one argument, abort the operation and display an error message. If chdir() returns an error, display an error message.

help: Display help information on internal commands.

If the rst argument is one of our internal commands, print "<argument> is a builtin command" plus a brief description on what the command does. For example, if argument is 'cd', the output should be:

'cd' is a builtin command for changing the current working directory.

If the rst argument is not an internal command, this command prints "<argument> is an external command or application". For example, if argument is 'ls', the output must be:

'ls' is an external command or application

If there is more than one argument, display an error message

If there is no argument provided, list all the supported internal commands. For each command, include a short summary on what it does.

Problem 3. History Feature [40 points]
 The next task is to modify your shell to provide a history feature that allows the user access up to the 10 most recently entered commands. Start numbering the user's commands at 0 and increment for each command entered. These numbers will grow past 9. For example, if the user has entered 35 commands, then the most recent 10 will be numbered 15 through 34.

History Commands: First implement an internal command "history" which displays the 10 most recent commands executed in the shell. If there are not yet 10 commands entered, display all the commands entered so far.

Display the commands in descending order (sorted by its command number).

The output should include both external application commands and internal commands.

Display the command number on the left, and the command (with all its arguments) on the right.

Hint: Print a tab between the two outputs to have them line up easily.

If the command is run in the background using &, it must be added to the history with the &. A sample output of the history command is shown below:

/home/cmpt300$ history 

30       history

29       cd /home/cmpt300

28       cd /proc

27       cat uptime

26       cd /usr

25      ls 

24       man pthread_create 

23      ls 

22       echo "Hello World from my shell!" 

21       ls -la

/home/cmpt300$

Next, implement the ! commands which allows users to run commands directly from the history list:

Command "!n" runs command number n, such as "!22" will re-run the 23rd command entered this session. In the above example, this will re-run the echo command.

If n is not a number, or an invalid value (not one of the previous 10 command numbers) then display an error.

You may treat any command starting with ! as a history command. For example, if the user types

"!hi", just display an error. Note that atoi("hi") returns 0, which should not be treated as a valid command.

Command "!!" runs the previous command.

If there is no previous command, display an error message.

When running a command using "!n" or "!!", display the command from history to the screen so the user can see what command they are actually running.

Neither the "!!" nor the "!n" commands are to be added to the history list themselves, but rather the command being executed from the history must be added. Here is an example.

/home/cmpt300$ echo test test

 

 

 Note

bash).

 

 

Hints: 

3

4

 

5
if (length < 0) {    perror("Unable to read command. Terminating.\n");    exit(-1);  /* terminate with error */ 

}
 
to the following:

if ((length < 0) && (errno !=EINTR)) {    perror("Unable to read command. Terminating.\n");    exit(-1);  /* terminate with error */ 

}
Problem 4 (Optional). Better cd Command [15 bonus points]

: You must nish Problems 1, 2 and 3 before attempting Problem 4. Submitting Problem 4 alone without solutions to Problems 1, 2 or 3 will leave you with 0 mark for Problem 4.

Implement the following features that are often supported by the cd command in modern shells (e.g.,

Change to the home directory if no argument is provided. [4 points]

Support ~ for the home folder. [6 points]

For example, cd ~/cmpt300 should change to the "cmpt300" directory under the current user's home directory. Issuing cd ~ will switch to the home directory. Support - for changing back to the previous directory. [5 points]

For example, suppose that the current working directory is /home and you issued cd / to change to the root directory. Then, cd - will switch back to the /home directory.

You may nd the getuid and getpwuid functions useful. They allow you to gather useful

information about the current user.

. Notes

You do not need to support either > or | from the terminal. These are features of the normal Linux terminal that we are not implementing. Your code must not have any memory leaks or memory access errors (using Valgrind). Your shell must free all memory before it exits. However, your child processes may exit without freeing all their memory (if exec() fails then the child process will have a copy of all the memory that the parent holds; freeing this memory would be unnecessarily time consuming for the OS). Therefore, you may ignore any Valgrind messages when a child process terminates to the effect of memory being held when program terminated.

. Resources

Sample code: shell.c, Make le Guide to Signals