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       system - execute a shell command


       #include <stdlib.h>

       int system(const char *command);


       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.


       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

       *  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.


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


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

       │InterfaceAttributeValue   │
       │system()                                                       │ Thread safety │ MT-Safe │


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


       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))

       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.

       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.


       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");


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


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                                            2021-03-22                                  SYSTEM(3)