system - execute a shell command
#include <stdlib.h>
int system (const char * string);
system() executes a command specified in string by calling /bin/sh -c
string, and returns after the command has been completed. During execution
of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT
will be ignored.
The value returned is -1 on error (e.g. fork failed), and the return
status of the command otherwise. This latter return status is in the
format specified in wait(2). Thus, the exit code of the command will
be WEXITSTATUS(status). In case /bin/sh could not be executed, the
exit status will be that of a command that does exit(127).
If the value of string is NULL, system() returns nonzero if the shell
is available, and zero if not.
system() does not affect the wait status of any other children.
ANSI C, POSIX.2, BSD 4.3
It is extremely unfortunate that the libc version of system() ignores
interrupts. This makes programs that call it from a loop uninterruptable.
This means that for such purposes one should not use system()
but a private version like (warning: untested code!)
int my_system (const char *command) {
int pid, status;
if (command == 0)
return 1;
pid = fork();
if (pid == -1)
return -1;
if (pid == 0) {
char *argv[4];
argv[0] = "sh";
argv[1] = "-c";
argv[2] = command;
argv[3] = 0;
execve("/bin/sh", argv, environ);
exit(127);
}
do {
if (waitpid(pid, &status, 0) == -1) {
if (errno != EINTR)
return -1;
} else
return status;
} while(1);
}
Do not use system() from a program with suid or sgid privileges,
because strange values for some environment variables might be used to
subvert system integrity. Use the exec(3) family of functions instead,
but not execlp(3) or execvp(3). system() will not, in fact, work properly
from programs with suid or sgid privileges on systems on which
/bin/sh is bash version 2, since bash 2 drops privileges on startup.
(Debian uses a modified bash which does not do this when invoked as
sh.)
The check for the availability of /bin/sh is not actually performed; it
is always assumed to be available. ISO C specifies the check, but
POSIX.2 specifies that the return shall always be non-zero, since a
system without the shell is not conforming, and it is this that is
implemented.
It is possible for the shell command to return 127, so that code is not
a sure indication that the execve() call failed; check errno to make
sure.
sh(1), signal(2), exec(3)
GNU 1998-05-11 SYSTEM(3)
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