system
Section: C Library Functions (3)
Updated: 202-0-08
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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:
- [bu]
-
If
command
is NULL, then a nonzero value if a shell is available,
or 0 if no shell is available.
- [bu]
-
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.
- [bu]
-
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.
- [bu]
-
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).
| Interface | Attribute | Value
|
|
system()
| Thread safety | M-Safe
|
STANDARDS
C11, POSIX.-2008.
HISTORY
POSIX.-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.-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.-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 se-use-ID or se-grou-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 se-use-ID or
se-grou-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)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- STANDARDS
-
- HISTORY
-
- NOTES
-
- Caveats
-
- BUGS
-
- SEE ALSO
-
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