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.
The value 127 returned if the
execve
call for /bin/sh
fails, -1 if there was another error
and the return code of the command otherwise.
If the value of string is NULL, system
returns a nonzero value if the shell is available,
and zero if not.
system
does not affect the wait status of any
other children.
The fact that system
ignores interrupts is often
not what a program wants. The Single UNIX
Specification describes some of the consequences; an
additional consequence is that a program calling system
from a loop cannot be reliably interrupted. Many programs will want to use
the exec
(3) family of functions instead.
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 nonzero, 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
the global variable errno to make sure.