Problem Statement:

Come up with a mini shell (Command Line Interpreter) program, It should display a prompt (myshell $ ) and receive commands from

user, parse the arguments and execute it. After execution return back to the prompt to receive further commands from user.

Shell program should exit on receiving “exit”.

The shell should be able to interpret and execute multiple commands using a pipe symbol ( | )

Try implementing some internal commands in your shell i.e., commands like echo, pwd, etc.

Shell should also be able run commands in background when given a ‘&’ symbol at the end of arguments.

In case of a command running in background the shell should get back to the prompt immediately without waiting for command to exit.

Code Solution:-

#include<stdio.h>
#include<string.h>
#include<sys/types.h>
#include<unistd.h>

main() {
pid_t pid = 0;
int status = 0;
int lindex = 0;
int nopipes = 0;
int cmdcount = 0;
int argcount = 0;
int buffsize = 64;

char * pstr = NULL;
char * p_token = NULL;
char * cmds[16];
char * arg[16][16];

char inputbuffer[64];
int pipefd[16][2];

while (1)
{
lindex = 0;
cmdcount = 0;
buffsize = 63;
pstr = inputbuffer;
write(1, “MYSHELL>”, 8);
buffsize = read(0, inputbuffer, buffsize);
inputbuffer[buffsize] = ”;

if (buffsize > 0)
{
p_token = strsep(&pstr, “|\n”);
while(p_token)
{
cmds[cmdcount++] = p_token;
p_token = strsep(&pstr, “|\n”);
}
cmdcount–;
nopipes = cmdcount – 1;
while (lindex < cmdcount)
{
argcount = 0;
p_token = strtok(cmds[lindex], ” \n|”);
while (1)
{
arg[lindex][argcount++] = p_token;
if (NULL == p_token)
break;
p_token = strtok(NULL, ” \n|”);
}
lindex++;
}
if (1 == cmdcount) /*simple command*/
{
pid = fork();
if (0 == pid)
{
execvp(arg[0][0], arg[0]);
}
else if (0 < pid)
{
wait(&status);
}
}
else if (1 < cmdcount) /*pipes*/
{
//create pipes
for (lindex = 0;lindex < nopipes; lindex++)
{
pipe(pipefd[lindex]);
}
//create processes
for (lindex = 0;lindex < cmdcount; lindex++)
{
int liter = 0;
pid = fork();
if (0 == pid)
{
if (0 == lindex)
{
close(1);
dup(pipefd[lindex][1]);
for (liter = 0;liter < nopipes; liter++)
{
close(pipefd[liter][0]);
close(pipefd[liter][1]);
}
execvp(arg[lindex][0], arg[lindex]);
}
else if ((0 < lindex) && (lindex < cmdcount-1))
{
close(0);
close(1);
dup(pipefd[lindex-1][0]);
dup(pipefd[lindex][1]);
for (liter = 0;liter < nopipes; liter++)
{
close(pipefd[liter][0]);
close(pipefd[liter][1]);
}
execvp(arg[lindex][0], arg[lindex]);
}
else
{
close(0);
dup(pipefd[lindex-1][0]);
for (liter = 0;liter < nopipes; liter++)
{
close(pipefd[liter][0]);
close(pipefd[liter][1]);
}
execvp(arg[lindex][0], arg[lindex]);
}
}
}
for (lindex = 0;lindex < nopipes; lindex++)
{
close(pipefd[lindex][0]);
close(pipefd[lindex][1]);
}
for (lindex = 0;lindex < cmdcount; lindex++)
{
wait(&status);
}
}
}
}
return 1;
}