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SYSTEM
!!!SYSTEM
NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
CONFORMING TO
BUGS
SEE ALSO
----
!!NAME


system - execute a shell command
!!SYNOPSIS


__#include
__ ''string''__);
__
!!DESCRIPTION


__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.
!!RETURN VALUE


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.
!!CONFORMING TO


ANSI C, POSIX.2, BSD 4.3
!!BUGS


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] =
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.
!!SEE ALSO


sh(1), signal(2), exec(3)
----

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Last edited on Tuesday, June 4, 2002 12:24:37 am by "perry"

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