为什么execlp不允许我使用管道？

Question

#include <stdio.h>
#include <unistd.h>
main()
{
    execlp("ls" , "ls", "-l", "|" , "sort", NULL);    
}

What's going wrong? Please help.

If I write this: execlp("ls" , "ls", "-l", NULL); then the result displays on screen and if I try this execlp("ls" , "ls", "-l", "|" , "sort", NULL); screen displays error message cannot access .

Why? Does anyone have any idea? Why doesn't it allow me to use pipe ( | ) when I execute the command execlp ?

Answer 1

Piping is a shell construct. execlp() isn't subject to shell parsing; it passes the arguments directly to the named command. To get shell features you need to explicitly start a shell process.

execl("/bin/sh", "/bin/sh", "-c", "ls -l | sort", NULL);

In that case it's probably best to specify the absolute path to the shell /bin/sh , which means you can use execl instead of execlp .

Answer 2

Oups, you a making a confusion. exec... family functions allow execution of one single program replacing the caller one. So your program just execute ls command with "-l" , "|" , "sort" as parameters.

Here is what it gives interactively :

$ ls "-l" "|" "sort"
ls: sort: No such file or directory
ls: |: No such file or directory

On the other end, then you execute normaly ls -l | sort ls -l | sort in a shell, the | is interpreted by the shell . And the shell :

• starts ls -l as a first command
• starts sort as a second command
• with output of first command connected to input of second.

You can do that explicitely with the help of fork , pipe and execlp , but you will have to prepare the pipe, and explicitely fork in order to have 2 processes.

Alternatively, you can ask the shell to do the job :

execlp("/bin/sh" , "sh", "-c", "ls -l | sort", NULL);

Answer 3

When you write the following command in a shell

ls -l | sort

two programs are executed: ls and sort .

The pipe character ( | ) means that the output of the first should be redirected to the standard input of the second. This pipe is interpreted by your shell. It is not a parameter of your command.

execlp takes the name of your program and its parameters as arguments.

So when you write

execlp("ls" , "ls", "-l", "|" , "sort", NULL);

it doesn't make any sense since | and sort are not parameters of ls .

What you want is do what the shell does for you:

1. declare a pipe

2. make a fork where you redirect stdout to your pipe (with dup ) and execute ls :

    execlp("ls" , "ls", "-l", NULL); 

3. make a fork where you redirect your pipe to stdin and execute sort :

    execlp("sort" , "sort", NULL); 

4. close your pipe, wait, etc.

Answer 4

There is a simple way of doing this. It involves using function popen :

#include <stdio.h>
#include <unistd.h>

int main() {

    FILE* out  = popen("ls -l | sort", "w");

    pclose(out);

    return 0;  
}

However it might be not what you were going for as it doesn't use exec.