Using pipes to implement simple shell in c? [duplicate] - c

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Pipe implementation
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C Minishell Adding Pipelines
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I am building a simple shell in c using fork and execlp. I will be given a set of commands separated by pipes. eg: ls -l | wc -l .
I want to use pipes for intra process communication. So the output from ls -l is input for wc -l. There can any number of commands separated by pipes. I am not understanding whether to create a pipe between a child process and parent and then when I get a output from a child process somehow transfer that output to another child process... I have parsed the inputs. How can I go about this?
void excueteCommands() {
int i, j;
int fd[2];
int cid1;
commandNode* ptr = head;
while (ptr != NULL) {
for (i = 0; i <= pipeCount; i++) {
cid1 = fork();
if (!cid1) {
if (i != 0) {
dup2(fd[i - 1][0], 0);
}
if (i != pipeCount) {
dup2(fd[i][1], 1);
}
for (j = 0; j < pipeCount; j++) {
close(fd[j][0]);
close(fd[j][1]);
}
execlp(ptr->command, ptr->args, NULL);
exit(0);
}
ptr = ptr->next;
}
}
for (i = 0; i < pipeCount; i++) {
close(fd[i][0]);
close(fd[i][1]);
}
}

Answer 1

I also had same assignment last year. Your need not to handle input and output from one process to another separately. Just initialize pipes between the series of processes and the flow of input and output would take place itself. You only need to give input to first command and take output from the last command.
Following is code snippet I used.
#include <fcntl.h>
#include <errno.h>
#include <dirent.h>
#include <unistd.h>
#include <stddef.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
#include <stdlib.h>
#include <pwd.h>
#include <grp.h>
#include <time.h>
#include <sys/wait.h>
int exit_flag = 0;
#define ARG_SIZE 100
int main ()
{
int size = 1000,i,j;
int pipe_count = 0;
int fd[100][2],cid1,cid2,length,status;
char string[][100] = {"ls","wc"};
pipe_count = 1;
if(pipe_count)
{
for(i = 0;i < pipe_count;i++)
{
pipe(fd[i]);
}
for(i = 0;i <= pipe_count;i++)
{
cid1 = fork();
if(!cid1)
{
if(i!=0)
{
dup2(fd[i-1][0],0);
}
if(i!=pipe_count)
{
dup2(fd[i][1],1);
}
for(j = 0;j < pipe_count;j++)
{
close(fd[j][0]);
close(fd[j][1]);
}
execlp(string[i], string[i], NULL);
exit(0);
}
}
for(i = 0;i < pipe_count;i++)
{
close(fd[i][0]);
close(fd[i][1]);
}
waitpid(cid1,&status,0);
}
else
{
execlp(string[0], string[0], NULL);
}
return 0;
}
In above code string[] array contains all commands separated by '|' in sequence. fd[][2] is array of file descriptors. fd[i][0] is input of i+1 command and fd[i][1] is output of ith command. So output of i goes to input of i+1 using fd[i] pipe.
I don't remember now how the dup2 and close commands were handled. May you may get them since you have done them recently. I there is any doubt then I would try my best to clarify.