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NAME

       system - execute a shell command

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdlib.h>

       int system(const char *command);

DESCRIPTION

       The  system()  library function behaves as if it used fork(2) to create a child process that executed 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.

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.

ERRORS

       system() can fail with any of the same errors as fork(2).

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │system()                                                                      │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       C11, POSIX.1-2008.

HISTORY

       POSIX.1-2001, C89.

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 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.

       Before  glibc  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.

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");

SEE ALSO

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