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man:system

SYSTEM(3) Linux Programmer's Manual SYSTEM(3)

NAME

     system - execute a shell command

SYNOPSIS

     #include <stdlib.h>
     int system(const char *command);

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 *) 0);
     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.

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.

  • 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 termina-

tion status of the child shell used to execute command. (The termi-

        nation  status of a shell is the termination status of the last com-
        mand 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.

ATTRIBUTES

     For  an  explanation  of  the  terms  used   in   this   section,   see
     attributes(7).
     +----------+---------------+---------+
     |Interface | Attribute     | Value   |
     +----------+---------------+---------+
     |system()  | Thread safety | MT-Safe |
     +----------+---------------+---------+

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008, C89, C99.

NOTES

     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 cre-
     ate 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) (WEXITSTA-
     TUS(), etc.) are made available when including <stdlib.h>.
     As mentioned, system() ignores SIGINT and SIGQUIT.  This may make  pro-
     grams  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  val-
     ues  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 func-
     tions 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  ver-
     sion  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.

SEE ALSO

     sh(1),  execve(2),  fork(2),  sigaction(2),  sigprocmask(2),   wait(2),
     exec(3), signal(7)

COLOPHON

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.
                                2017-09-15                         SYSTEM(3)
/data/webs/external/dokuwiki/data/pages/man/system.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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