How do I execute a Shell built-in command with a C function?

Question

I would like to execute the Linux command "pwd" through a C language function like execv().

The issue is that there isn't an executable file called "pwd" and I'm unable to execute "echo $PWD", since echo is also a built-in command with no executable to be found.

Charlie Burns, the Virtual Machine I was provided lacks an entry for getcwd. Thank you, anyway. — user2851770, Oct 06 '13 at 14:37
http://theunixshell.blogspot.com/2013/01/executing-shell-command-in-cc-and.html — Vijay, Apr 16 '14 at 10:44

score 52 · Answer 1 · answered Oct 06 '13 at 14:05

52

If you just want to execute the shell command in your c program, you could use,

   #include <stdlib.h>

   int system(const char *command);

In your case,

system("pwd");

The issue is that there isn't an executable file called "pwd" and I'm unable to execute "echo $PWD", since echo is also a built-in command with no executable to be found.

What do you mean by this? You should be able to find the mentioned packages in /bin/

sudo find / -executable -name pwd
sudo find / -executable -name echo

answered Oct 06 '13 at 14:05

smRaj

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I'm still new at this sort of thing. I tried using "which pwd" on the shell and it only returned that "pwd" is a built-in command. Thank you, anyway! – user2851770 Oct 06 '13 at 14:32
1

Forgive my possible ignorance, but that will just return with the return code rather than stdout, correct? – Jpatrick Aug 30 '19 at 15:02
1

@Jpatrick You are correct. `system` helps with executing shell commands, and also be able to read the returncode with a couple other system calls. If you want to read the stdout of the program, need to use popen() instead: https://www.gnu.org/software/libc/manual/html_node/Pipe-to-a-Subprocess.html – smRaj Nov 22 '19 at 22:47

score 27 · Accepted Answer · answered Oct 06 '13 at 13:03

27

You should execute sh -c echo $PWD; generally sh -c will execute shell commands.

(In fact, system(foo) is defined as execl("sh", "sh", "-c", foo, NULL) and thus works for shell built-ins.)

If you just want the value of PWD, use getenv, though.

answered Oct 06 '13 at 13:03

Kerrek SB

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I used the getenv function. Thank you! – user2851770 Oct 06 '13 at 14:33

score 14 · Answer 3 · edited Feb 26 '22 at 09:25

You can use the execl() function:

int execl(const char *path, const char *arg, ...);

Like shown here:

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

int main (void) {

   return execl ("/bin/pwd", "pwd", NULL);

}

The second argument will be the name of the process as it will appear in the process table.

Alternatively, you can use the getcwd() function to get the current working directory:

#include <stdio.h>
#include <unistd.h>
#include <dirent.h>
#define MAX 255

int main (void) {
char wd[MAX];
wd[MAX-1] = '\0';

if(getcwd(wd, MAX-1) == NULL) {
  printf ("Can not get current working directory\n");
}
else {
  printf("%s\n", wd);
}
  return 0;
}

How do I execute a Shell built-in command with a C function?

3 Answers3

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