Redirecting stdin and stdout? - c

Question

So im trying to redirect the I/O to read command from file then when user runs the output command it will print the compiled command to output file.
For example on the terminal:
./run 2 < test.txt // This would take file using dup and take the input
Then when you want to output the compile:
./run 1 > output.txt // and it would put into an output file
So far i know how to output to a file but my problem is with the input. how do i get the command from the file using the dup2() function? I tried researching this but no luck.
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>
char inputForOutput[100];
void functionOutput(int argc, char **argv){
int ofd; //Init of file desc.
ofd = open(argv[1], O_CREAT|O_TRUNC|O_WRONLY);
dup2(ofd, 1);//Duplicates to stdout
system("ls");//Copies commnd given to output_file
}
//Function is called when argument number is == 1
void functionInput(int argc, char **argv){
FILE *ifd;
printf("\n %s \n ", argv[2]);
ifd = fopen(argv[2] , "r");
if (ifd == NULL){
perror("No file found");
exit(1);
}
fscanf(ifd,"%s",inputForOutput);
printf("\n**%s**\n",inputForOutput);
}
int main(int argc, char **argv)
{
int output;
int input;
output = strcmp("1", argv[1]);
input = strcmp("2" ,argv[1]);
if (output == 0 ) { //Fail safe for number of arguments
functionOutput(argc, argv);
}
else if ( input == 0){
functionInput(argc, argv);
}
else{
fprintf(stderr, "How to use: %s function output_file\n", argv[0]); // FAIL SAFE IF INPUT DOES NOT MATCH BOTH FUNCTIONS
}
return 0;
}

Answer 1

To redirect input and output, use this format
myprogram > out.txt < in.txt //read from in.txt, write to out.txt
myprogram < in.txt > out.txt //read from in.txt, write to out.txt
myprogram < in.txt //redirect stdin only
myprogram > out.txt //redirect stdout only
myprogram //no redirection
...
This should work with any program. Example:
int main(void)
{
char buf[1000];
if(fgets(buf, sizeof(buf), stdin))
printf("write: %s\n", buf);
return 0;
}
To redirect stdin/stdout in the program, use the standard method
freopen("output.txt", "w", stdout);
printf("Testing...");
fclose(stdout);
freopen("input.txt", "r", stdin);
char buf[100];
fgets(buf, sizeof(buf), stdin);
fclose(stdin);
Alternatively, set FILE *fin = stdin; FILE* fout = stdout; to redirect the opposite way.
Next, to write a program using argv elements, always test argc first. The code below shows an example.
#include <stdio.h>
#include <string.h>
int redirect(int argc, char **argv, int *index)
{
//no more redirection!
if(*index >= argc)
return 1;
//not enough parameters
if(*index + 1 >= argc)
{
printf("wrong usage\n");
return 0;
}
if(strcmp(argv[*index], "<") == 0)
{
*index++; //next parameter is to redirect input
if(!freopen(argv[*index], "r", stdin))
printf("error, redirect input failed");
}
else if(strcmp(argv[*index], ">") == 0)
{
*index++; //next parameter is to redirect output
if(!freopen(argv[*index], "w", stdout))
printf("error, redirect output failed");
}
else
{
printf("wrong usage\n");
return 0;
}
return 1;
}
int main(int argc, char **argv)
{
int index = 1;
if(!redirect(argc, argv, &index))
return 1;
if(!redirect(argc, argv, &index))
return 1;
//read
char buf[1000];
if(fgets(buf, sizeof(buf), stdin))
{
//write
printf("write: %s\n", buf);
}
fclose(stdin);
fclose(stdout);
return 0;
}

Answer 2

With functionOutput() you have a good first attempt at capturing the output of a system command to a file. Actually, that is the function called when the first argument is 1, so you might want to update your comment. Also, you're creating a file with the name stored in argv[1], which we already know is 1 so it's probably not doing what you expect, and you probably want:
ofd = open(argv[2], O_CREAT|O_TRUNC|O_WRONLY);
With functionInput() you're reading the first non-whitespace entry from the file. If you're telling it to read the file which you output using the functionOutput() function, that is likely to be (some of) the name of the first file which was listed by ls.
I'm finding it unclear what you're wanting to do which isn't that. If you want to find out what the command was which you ran to generate the output, that information is not available from the file itself, because you didn't write it there. If that's what you want, you may want to consider writing the command as the first line of the file, followed by the output. Then when you read it, you can assume that the first line is the command run, followed by the output of that command.

Answer 3

If I understand your question, and you want to run your program in essentially two different modes, (1) you want to take input if there is input to be taken on stdin; and (2) if there is no input waiting, you want to do an output, then select/pselect or poll are what you are looking for.
For example select allows you to check whether there is input ready to be read on a file descriptor (or set of descriptors) and it will return the number of descriptors with input waiting (or -1 and set errno on error). You could simply use the STDIN_FILENO (a/k/a fd 0) to check if there is input on stdin, e.g.
#include <stdio.h>
#include <unistd.h> /* for STDIN_FILENO */
#include <sys/select.h> /* for pselect */
int input (int filedes)
{
fd_set set;
/* declare/initialize zero timeout */
struct timespec timeout = { .tv_sec = 0 };
/* Initialize the file descriptor set. */
FD_ZERO (&set);
FD_SET (filedes, &set);
/* check whether input is ready on filedes */
return pselect (filedes + 1, &set, NULL, NULL, &timeout, NULL);
}
int main (void)
{
if (input (STDIN_FILENO))
puts ("doing input routine");
else
puts ("doing output routine");
return 0;
}
(note: from the man page "select() uses a timeout that is a struct timeval (with seconds and microseconds), while pselect() uses a struct timespec (with seconds and nanoseconds).")
Example Use/Output
$ ./bin/select_peekstdin < file
doing input routine
$ ./bin/select_peekstdin
doing output routine