Pipes and commands

Andy Libby — Fri, 08 Jun 2007 11:52:00 -0400

I received a question from a co-worker the other day about why “ls | grep [something]” returns single file names. When ls is run by itself in a terminal window, it shows output in columns (multiple files per line). The grep command works at the line level, not the field level. Why does ls | grep not return a line with multiple file names?

Commands like ls can detect if the input or output streams (standard input and standard output) are pipes. Armed with this information they alter their output to be more sensible for the context in which they are run.

The following example C program demonstrates:

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

int main() {
    struct stat *st = malloc(sizeof stat);
    if(fstat(1, st) != 0) {
        fprintf(stderr, "Could not stat standard output...\n");
        exit(1);
    }

    if(S_ISFIFO(st->st_mode)) {
        printf("Standard output is a pipe.\n");
    } else {
        printf("Standard outpt is not a pipe.\n");
    }
}

I then compiled and ran the program both with and without a pipe:

alibby@alibby-laptop:~$ gcc -o pipedetect pipedetect.c
alibby@alibby-laptop:~$ ./pipedetect 
Standard outpt is not a pipe.
alibby@alibby-laptop:~$ ./pipedetect | cat
Standard output is a pipe.
alibby@alibby-laptop:~$

So ls and other commands are nice enough to detect when a pipe is being used on standard out and present one line. In this case the ls command presents one file per line (which is what we want when grepping).

GNU ls is nice enough to provide the --format=single option to provide this behavior when a pipe is not used.

For more information about what’s going on in the C app above, you can consult manual pages for stat(2).

"Pipes and commands" by Colin

Sun, 10 Jun 2007 18:24:34 -0400

@Justin: Exactly. I wanted to grep through the output of an ls and I was surprised when this worked. I figured that, since ls would spit out multiple file names per line, the grep wouldn't be that useful. But alas, I am proven wrong once again by resourcefulness of open source.

"Pipes and commands" by Justin

Fri, 08 Jun 2007 14:24:20 -0400

The reason thats the default behavior is because its easier to parse through other command line scripting, correct?

has_many :thoughts: Pipes and commands

Pipes and commands

"Pipes and commands" by Colin

"Pipes and commands" by Justin