Thread: standard input / output check

I'm trying to check the command line arguments for a -i input_filename flag. If it's present I want to read the input stream from the file named there. If it's not I want to read the input from standard input.

Also, I want to check the command line arguments for a -o output_filename, if it's present it'll write the output stream named there, if not it'll write the input to standard output.

My question is whats the best way to go about doing these checks for my program? Thanks, let me know.

So far this is what I have:

FILE *fp;

if (!strcmp(argv[1], "-i") && !strcmp(argv[3], "-o")) { // Ex: -i input.txt -o output.txt

/* Input File */ fp = fopen(argv[2],"r");
// Output File output = fopen(argv4], "w");
}

Something like

Code:

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
  FILE *fp;
  if ( argc == 2 ) {
    printf("Input from file\n");
    fp = fopen(argv[1],"r");
  } else {
    printf("Input from stdin\n");
    fp = stdin;
  }
  int ch;
  while ( (ch=fgetc(fp)) != EOF ) fputc(ch,stdout);
  fclose(fp);
  return 0;
}


$ ./a.out 
Input from stdin
hello
hello
$ ./a.out < input.txt 
Input from stdin
123
$ ./a.out input.txt 
Input from file
123

< redirections are handled by the shell, not your program.

For a.out < filename.txt or cat filename.txt | a.out
no filename is needed in your program, use stdin like:

Code:

char line[BUFSIZ];
while( fgets(line,BUFSIZ,stdin) )
{
  ...
}

Originally Posted by benjammin5

how do I use fopen on the file name if they use standard input < filename.txt? my program uses f open if they put in the -i filename.txt

You don't. It's their choice to use redirection instead of `-i` option, and your program shouldn't care.