system.3s (2010 09)

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system(3S) system(3S)

NAME

system( ) - issue a shell command

SYNOPSIS

#include <stdlib.h>

int system(const char *command);

DESCRIPTION

system() executes the command speciﬁed by the string pointed to by command. The environment of

the executed command is as if a child process were created using

fork() (see fork (2)), and the child

process invoked the sh-posix (1) utility via a call to

execl() (see exec (2)) as follows:

execl("/usr/bin/sh", "sh", "-c", command, 0);

system()

ignores the SIGINT and SIGQUIT signals, and blocks the

SIGCHLD signal, while waiting

for the command to terminate. If this might cause the application to miss a signal that would have killed

it, the application should examine the return value from

system() and take whatever action is

appropriate to the application if the command terminated due to receipt of a signal.

system() does not affect the termination status of any child of the calling processes other than the pro-

cess or processes it itself creates.

system() does not return until the child process has terminated.

APPLICATION USAGE

If the return value of

system() is not − 1, its value can be decoded through the use of the macros

described in <sys/wait.h>. For convenience, these macros are also provided in <stdlib.h

Note that, while

system() must ignore SIGINT and SIGQUIT and block SIGCHLD while waiting for

the child to terminate, the handling of signals in the executed command is as speciﬁed by fork (2) and

exec(2). For example, if

SIGINT is being caught or is set to SIG_DFL

when system() is called, the

child is started with

SIGINT handling set to SIG_DFL.

Ignoring

SIGINT and SIGQUIT in the parent process prevents coordination problems (such as two

processes reading from the same terminal) when the executed command ignores or catches one of the sig-

nals.

RETURN VALUE

If command is null,

system() returns non-zero.

If command is not null,

system() returns the termination status of the command language interpreter

in the format speciﬁed by wait (2). The termination status of the command language interpreter is as

speciﬁed for sh-posix (1), except that if some error prevents the command language interpreter from exe-

cuting after the child process is created, the return value from system() is as if the command language

interpreter had terminated using _exit(127). If a child process cannot be created, or if the termina-

tion status for the command language interpreter cannot be obtained, system() returns − 1 and sets

errno to indicate the error.

DIAGNOSTICS

system() forks to create a child process which, in turn, exec()s /usr/bin/sh in order to execute

string . If the fork fails, system() returns −1 and sets errno. If the exec fails, system() returns

the status value returned by waitpid() (see wait (2)) for a process that terminates with a call of

exit(127).

ERRORS

If errors are encountered,

system() sets errno values as described by fork(2).

FILES

/usr/bin/sh