SHELL

Hi, I need someone to implement part 2 of the homework requirement below in this code.

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#include

#include

#include

#include

#include

#include

#include

/* The array below will hold the arguments: args[0] is the command. */

static char* args[512];

pid_t pid;

int command_pipe[2];

#define READ 0

#define WRITE 1

/*

* Handle commands separatly

* input: return value from previous command (useful for pipe file descriptor)

* first: 1 if first command in pipe-sequence (no input from previous pipe)

* last: 1 if last command in pipe-sequence (no input from previous pipe)

*

* EXAMPLE: If you type "ls | grep shell | wc" in your shell:

* fd1 = command(0, 1, 0), with args[0] = "ls"

* fd2 = command(fd1, 0, 0), with args[0] = "grep" and args[1] = "shell"

* fd3 = command(fd2, 0, 1), with args[0] = "wc"

*

* So if 'command' returns a file descriptor, the next 'command' has this

* descriptor as its 'input'.

*/

static int command(int input, int first, int last)

{

int pipettes[2];

/* Invoke pipe */

pipe( pipettes );

pid = fork();

/*

SCHEME:

STDIN --> O --> O --> O --> STDOUT

*/

if (pid == 0) {

if (first == 1 && last == 0 && input == 0) {

// First command

dup2( pipettes[WRITE], STDOUT_FILENO );

} else if (first == 0 && last == 0 && input != 0) {

// Middle command

dup2(input, STDIN_FILENO);

dup2(pipettes[WRITE], STDOUT_FILENO);

} else {

// Last command

dup2( input, STDIN_FILENO );

}

if (execvp( args[0], args) == -1)

_exit(EXIT_FAILURE); // If child fails

}

if (input != 0)

close(input);

// Nothing more needs to be written

close(pipettes[WRITE]);

// If it's the last command, nothing more needs to be read

if (last == 1)

close(pipettes[READ]);

return pipettes[READ];

}

/* Final cleanup, 'wait' for processes to terminate.

* n : Number of times 'command' was invoked.

*/

static void cleanup(int n)

{

int i;

for (i = 0; i < n; ++i)

wait(NULL);

}

static int run(char* cmd, int input, int first, int last);

static char line[1024];

static int n = 0; /* number of calls to 'command' */

int main()

{

printf("SIMPLE SHELL: Type 'exit' or send EOF to exit. ");

while (1) {

/* Print the command prompt */

printf("$> ");

fflush(NULL);

/* Read a command line */

if (!fgets(line, 1024, stdin))

return 0;

int input = 0;

int first = 1;

char* cmd = line;

char* next = strchr(cmd, '|'); /* Find first '|' */

while (next != NULL) {

/* 'next' points to '|' */

*next = '\0';

input = run(cmd, input, first, 0);

cmd = next + 1;

next = strchr(cmd, '|'); /* Find next '|' */

first = 0;

}

input = run(cmd, input, first, 1);

cleanup(n);

n = 0;

}

return 0;

}

static void split(char* cmd);

static int run(char* cmd, int input, int first, int last)

{

split(cmd);

if (args[0] != NULL) {

if (strcmp(args[0], "exit") == 0)

exit(0);

n += 1;

return command(input, first, last);

}

return 0;

}

static char* skipwhite(char* s)

{

while (isspace(*s)) ++s;

return s;

}

static void split(char* cmd)

{

cmd = skipwhite(cmd);

char* next = strchr(cmd, ' ');

int i = 0;

while(next != NULL) {

next[0] = '\0';

args[i] = cmd;

++i;

cmd = skipwhite(next + 1);

next = strchr(cmd, ' ');

}

if (cmd[0] != '\0') {

args[i] = cmd;

next = strchr(cmd, ' ');

next[0] = '\0';

++i;

}

args[i] = NULL;

}

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Part 2: Implement Built-in Commands: exit, cd, history exit - Simply exits your shell after performing any necessary clean up. cd [dir] - Short for "change directory", and will be used to change the current working directory of your shell. Do not worry about implementing the command line options that the real cd command has in Bash. Just implement cd such that it takes a single command line parameter: The directory to change to. history [-c] [offset] - Similar to the Bash built-in history command, but much simpler. o history (without arguments) displays the last 100 commands the user ran, with an offset next to each command. The offset is the index of the command in the list, and valid values are 0 to 99, inclusive. 0 is the oldest command. Do not worry about persisting this list to a file; just store it in memory. Once more than 100 commands are executed, remove the oldest entry from the list to make room for the newer commands. Note that history is also a command itself and therefore should also appear in the list of commands. If the user ran invalid commands, those should also appear in the list. o history -c clears the entire history, removing all entries. For example, running history immediately after history -c should show history as the sole entry in the list. o history [offset] executes the command in history at the given offset. Print an error message of your choosing if the offset is not valid. o Example output for built-in history: o $cd /home/w4118 o $/bin/ls o my_file.txt o $history o 0 cd /home/w4118 o 1 /bin/ls o 2 history o $history 1 o my_file.txt o $history o 0 cd /home/w4118 o 1 /bin/ls o 2 history o 3 history 1 o 4 history o $history -c o $history o 0 history $