Command line arguments and read file/print text in C - c

Question

I am trying to work with command line arguments and parsing a text file in C. Basically I want to be able to put in two numbers, like, 1 and 4 and have it read a column of a text file then print it to stdout. I want to be able to do something like take this:
PID TTY TIME CMD
449 ttys000 0:00.35 -bash
1129 ttys001 0:00.35 -bash
25605 ttys001 0:00.15 vi prog.c
6132 ttys002 0:00.11 -bash
6208 ttys002 0:00.03 vi test
And do:
./your_prog 1 4 < data.txt
PID CMD
449 bash
1129 -bash
25605 vi
6132 -bash
6208 vi
So I need to enter the the columns i want to print out, redirect the file in "data.txt" and have it process the file and print like so.
So far I have this for my code:
#include <stdio.h>
int main(int argc, char **argv){
//int row = argc[0];
//int col = argc[1];
//if number entered is less than one, re-enter
int i;
for (i = 0; i < argc; i++) {
printf("argv[%d] = %s\n", i, argv[i]);
}
if(argc < 1){
fprintf(stderr, "Enter a valid input");
//quit
return 1;
}
else{
char ch[256];
//ch[255] = '\0';
while(fgets(ch, 256, stdin) != NULL){
printf("%s", ch);
}
}
return 1;
}
but am not sure if I am on the correct track and am confused as to what to do next. I am new to C, so I apologize if this is an easy question.

Answer 1

To get you started, this improves the way you gather the command line information. Note the first argument argv[0] is the name of the executable itself. Bear in mind comments above, I haven't improved any further. Syntax is progname filename col1 col2
#include <stdio.h>
#include <stdlib.h>
void fatal(char *msg) {
printf("%s\n", msg);
exit (1);
}
int main(int argc, char*argv[])
{
int col1, col2;
FILE *fp;
char ch[256];
if(argc < 4)
fatal("Need three arguments");
if (sscanf(argv[2], "%d", &col1) != 1)
fatal("Argument 1 error");
if (sscanf(argv[3], "%d", &col2) != 1)
fatal("Argument 2 error");
printf("Read columns %d and %d from file %s\n", col1, col2, argv[1]);
if ((fp = fopen(argv[1], "r")) == NULL)
fatal("Unable to open file");
// process the file
while (fgets(ch, 256, fp) != NULL) {
printf("%s", ch);
}
fclose(fp);
return 0;
}

Answer 2

Here is what I have so far. I can get it to parse apart then I can put it back how it was, but for the life of me I cant figure out how to tell it to print only the first column. If I could have some direction, then I could figure out how to do the rest, but I can find anything. Here is what code I have so far:
#include <stdio.h>
#include <string.h>
int main(int argc, char **argv){
int col1, col2;
int size = 512;
char ch[size];
char *temp[size];
char *token;
if(argc == 1){
fprintf(stderr, "I need more!\n");
return 1;
}
else{
//test to see what is stored
int i;
for (i = 0; i < argc; i++) {
printf("argv[%d] = %s\n", i, argv[i]);
}
if(sscanf(argv[1], "%d", &col1) != 1) return 1;
if(sscanf(argv[2], "%d", &col2) != 1) return 1;
while(fgets(ch, size, stdin) != NULL){
//get 1st token
token = strtok(ch, " ");
while(token != NULL){
printf(" %s", token);
temp[i++] = token;
token = strtok(NULL, " ");
}
}
return 0;
}
}