如何从C程序为Unix system（）调用构造字符串

Question

I'm trying to make my C program run a command in Unix using system() .

Can I make something like system("stat myFile") , where myFile is a variable in my program? Is there any other way possible?

Answer 1

system() takes a pointer-to-char or a char array. You can construct it as follows:

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

char command[256]; /* Big enough for "stat " + longest file name. */
char *myFile = "/my/file";
/* ... */
sprintf (command, "stat %s", myFile);
system(commmand);

This should get you going, but note that a hard limit like 256 is not bullet-proof coding due to a possible buffer overrun if myFile is a looooong name. Supercalifragilisticexpialidocious file names are not unheard of :-) If you want to do it like a pro, the next step would be to read the snprintf manual and ask your shell for getconf _POSIX_PATH_MAX .

Answer 2

The short answer: don't . system has lots of problems you don't want to deal with:

1. It runs the external program via the shell, so you have to worry about properly escaping any filenames or other options you pass when building your string. Failure to do so could create dangerous bugs.

2. system suspends your program until the program you run finishes.

3. There's no way to read back the output from the program you run via system .

Using popen solves all of the problems except shell escaping and is very convenient. Using fork and execvp , or posix_spawnp , is more complex but solves all of the above problems and more.

Answer 3

For that particular case of running stat(1) command, you could use instead the stat(2) syscall instead. See syscalls(2) . Hence use

  struct stat mys;
  memset (&mys, 0, sizeof(mys));
  if (stat(filename, &mys)) { perror(filename); exit(EXIT_FAILURE); };
  /// use mys

In general you might consider using snprintf(3) or asprintf(3) to build a command string, and using popen(3) or system(3) to run it. This is not advisable if you can avoid that (possibility of code injection , so you need to consider shell escapes, etc...).

You should also read Advanced Linux Programming , perhaps to know about fork(2) , execve(2) , pipe(2) etc...