Why is the following shell command working when executed directly in command line, but not working when executed through C program using popen/system?

Question

The command is : ps -c -p | tr -s " " | cut -d " " -f 2,6-10,13 | grep 'R'

I am running it through adb shell. Basically, I want a list of processes (and certain parameters) which are currently in the run queue. This is working fine if I run it directly through the shell.

However, if I put it in a C program and cross-compile it to run on Android, it's not working. Only ps -c -p is working (I have checked that). But on running this ps -c -p | tr -s " " | cut -d " " -f 2,6-10,13 | grep 'R', I get the output :

usage: tr [-cds] SET1 [SET2]

Translate, squeeze, or delete characters from stdin, writing to stdout

-c/-C  Take complement of SET1
-d     Delete input characters coded SET1
-s     Squeeze multiple output characters of SET2 into one character

tr: Needs 1 argument
usage: cut OPTION... [FILE]...

Print selected parts of lines from each FILE to standard output.

-b LIST select only these bytes from LIST.
-c LIST select only these characters from LIST.
-f LIST select only these fields.
-d DELIM    use DELIM instead of TAB for field delimiter.
-s  do not print lines not containing delimiters.
-n  don't split multibyte characters (Ignored).

cut: Needs -fcb

I think the output of ps -c -p is not being conveyed to tr, which doesn't convey it to cut. Can you please suggest what's the issue?

Here's the code I am using:

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

    #define BUFSIZE 128

    int main(int argc,char **argv)
    {
        char *cmd4 = "ps -c -p | tr -s " " | cut -d " " -f 2,6-10,13 | grep 'R'";    
        system(cmd4);

        FILE *fp;


        char buf[BUFSIZE];
     // Another method
    if ((fp = popen(cmd4, "r")) == NULL) {
        printf("Error opening pipe4!\n");
        return -1;
    }

    while (fgets(buf, BUFSIZE, fp) != NULL) {
        // Do whatever you want here...
        printf("cmd 4 running!");
        printf("OUTPUT: %s", buf);
    }

    if(pclose(fp))  {
        printf("Command not found or exited with error status4\n");
        return -1;
    }

    return 0;
}

If you're on Linux, run it under `strace`, follow all child processes using the `-f` option, and **learn what happens to command line arguments when passed through something like `popen()`**. — Andrew Henle, Sep 27 '19 at 18:45
For people who doubt that the pipeline is executing, take note that the error messages come from the second and third stages of the pipeline, not the first. — Ben Voigt, Sep 27 '19 at 18:47

score 2 · Accepted Answer · answered Sep 27 '19 at 18:45

In the shell, you're using the following command:

ps -c -p | tr -s " " | cut -d " " -f 2,6-10,13 | grep 'R'

In C, you're passing the following to system (and later to popen):

ps -c -p | tr -s  | cut -d  -f 2,6-10,13 | grep 'R'

See the difference? Quotes need to be escaped in C source code. Also, when you have trouble like this, be sure to output the relevant data so you can see what is actually happening instead of what you planned. A simple puts(cmd4) would have revealed this instantly.

In case you're wondering why this compiles without error, see https://stackoverflow.com/q/2837469/103167 — Ben Voigt, Sep 27 '19 at 18:49

Why is the following shell command working when executed directly in command line, but not working when executed through C program using popen/system?

1 Answers1