How to redirect out put of one C program to other C program using bash script?

Question

I have two programs ( Prog1.c and Prog2.c ) written in C and each of them take one command line argument.

Prog1.c takes a file name as argument , reads content from file and outputs it on the STDOUT (standard output) screen. Prog2.c takes data as argument and does some operation. I want to redirect output of Prog1.c to Prog2.c as input.

I have tried following bash script which gives me error

#!/bin/bash
prog2 "`prog1 file.txt`"  

I have also tried without quotes and in both cases, it gives me following error.

Prog2:: argument list too long.

Answer 1

To get the output of a command as a parameter for another command you can use backticks:

prog2 "`prog1 file.txt`"

or use $() (I believe this is a more preferred way for bash):

prog2 "$(prog1 file.txt)"

If you want to use the STDOUT of prog1 as STDIN for prog2 use the | (pipe) operator:

prog1 | prog2

Note: When you want to use pipes, you need to modify the code of prog2, as it needs to read from STDIN instead of the command arguments ( argv of the main() function). See How to read a line from the console in C? for an example on how to do this.

Answer 2

PIPES!!!!!

On the command line (or in your script) do prog1 file.txt | prog2 prog1 file.txt | prog2

Answer 3

You must use backticks in both places before and after prog1

prog2 "`prog1 file.txt`"

When you get "argument list too long", you have reached the system limit for the command line. You must then find another way to send the data to prog2 . You can read from stdin instead, and pipe the output of prog1 to prog2

prog1 file.txt | prog2

When you pipe the data from prog1 to prog2, you must change prog2 and read from stdin instead of using argv.

Example reading line by line:

char buf[1024];
while (fgets(buf, 1024, stdin)) {
    /* process line in buf */
}

Example reading white space delimited words

char buf[1024];
while (scanf("%s", buf) != EOF) {
    /* process word in buf */
}