выполнить команду оболочки (execute a shell command)
Имя (Name)
system - execute a shell command
Синопсис (Synopsis)
#include <stdlib.h>
int system(const char *command);
Описание (Description)
The system() library function uses fork(2) to create a child
process that executes the shell command specified in command
using execl(3) as follows:
execl("/bin/sh", "sh", "-c", command, (char *) NULL);
system() returns after the command has been completed.
During execution of the command, SIGCHLD will be blocked, and
SIGINT and SIGQUIT will be ignored, in the process that calls
system(). (These signals will be handled according to their
defaults inside the child process that executes command.)
If command is NULL, then system() returns a status indicating
whether a shell is available on the system.
Возвращаемое значение (Return value)
The return value of system() is one of the following:
* If command is NULL, then a nonzero value if a shell is
available, or 0 if no shell is available.
* If a child process could not be created, or its status could
not be retrieved, the return value is -1 and errno is set to
indicate the error.
* If a shell could not be executed in the child process, then
the return value is as though the child shell terminated by
calling _exit(2) with the status 127.
* If all system calls succeed, then the return value is the
termination status of the child shell used to execute command.
(The termination status of a shell is the termination status
of the last command it executes.)
In the last two cases, the return value is a "wait status" that
can be examined using the macros described in waitpid(2). (i.e.,
WIFEXITED(), WEXITSTATUS(), and so on).
system() does not affect the wait status of any other children.
Ошибки (Error)
system() can fail with any of the same errors as fork(2).
Атрибуты (Attributes)
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│system() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
Стандарты (Conforming to)
POSIX.1-2001, POSIX.1-2008, C89, C99.
Примечание (Note)
system() provides simplicity and convenience: it handles all of
the details of calling fork(2), execl(3), and waitpid(2), as well
as the necessary manipulations of signals; in addition, the shell
performs the usual substitutions and I/O redirections for
command. The main cost of system() is inefficiency: additional
system calls are required to create the process that runs the
shell and to execute the shell.
If the _XOPEN_SOURCE feature test macro is defined (before
including any header files), then the macros described in
waitpid(2) (WEXITSTATUS(), etc.) are made available when
including <stdlib.h>.
As mentioned, system() ignores SIGINT and SIGQUIT. This may make
programs that call it from a loop uninterruptible, unless they
take care themselves to check the exit status of the child. For
example:
while (something) {
int ret = system("foo");
if (WIFSIGNALED(ret) &&
(WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))
break;
}
According to POSIX.1, it is unspecified whether handlers
registered using pthread_atfork(3) are called during the
execution of system(). In the glibc implementation, such
handlers are not called.
In versions of glibc before 2.1.3, the check for the availability
of /bin/sh was not actually performed if command was NULL;
instead it was always assumed to be available, and system()
always returned 1 in this case. Since glibc 2.1.3, this check is
performed because, even though POSIX.1-2001 requires a conforming
implementation to provide a shell, that shell may not be
available or executable if the calling program has previously
called chroot(2) (which is not specified by POSIX.1-2001).
It is possible for the shell command to terminate with a status
of 127, which yields a system() return value that is
indistinguishable from the case where a shell could not be
executed in the child process.
Caveats
Do not use system() from a privileged program (a set-user-ID or
set-group-ID program, or a program with capabilities) because
strange values for some environment variables might be used to
subvert system integrity. For example, PATH could be manipulated
so that an arbitrary program is executed with privilege. Use the
exec(3) family of functions instead, but not execlp(3) or
execvp(3) (which also use the PATH environment variable to search
for an executable).
system() will not, in fact, work properly from programs with set-
user-ID or set-group-ID privileges on systems on which /bin/sh is
bash version 2: as a security measure, bash 2 drops privileges on
startup. (Debian uses a different shell, dash(1), which does not
do this when invoked as sh.)
Any user input that is employed as part of command should be
carefully sanitized, to ensure that unexpected shell commands or
command options are not executed. Such risks are especially
grave when using system() from a privileged program.
Ошибки (баги) (Bugs)
If the command name starts with a hyphen, sh(1) interprets the
command name as an option, and the behavior is undefined. (See
the -c option to sh(1).) To work around this problem, prepend
the command with a space as in the following call:
system(" -unfortunate-command-name");
Смотри также (See also)
sh(1), execve(2), fork(2), sigaction(2), sigprocmask(2), wait(2),
exec(3), signal(7)
