fork and exec. fork() copies your process but returns a different value to the copy so it can take a new direction. exec() replaces it with a different program. the original program can do other things and/or wait for the child to complete. see man fork, man execv, man wait.
Code:
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
int main(void)
{
pid_t pid=fork();
if(pid < 0)
{
perror("Couldn't fork");
exit(1);
}
else if(pid == 0) // child code
{
execv("/bin/echo", "/bin/echo", "asdf", NULL);
// this should never happen since execv REPLACES the process that calls it
exit(1);
}
else // parent code
{
int status;
// do other stuff. child will keep running.
sleep(5);
// wait for child to complete
wait(&status);
printf("child returned %d\n", WEXITSTATUS(status));
return(0);
}
}
Hi
I have a doubt in the way the variables inside a function are treated .
if a function is called from the main script directly, the variables inside them act as global variables.
however if the return value of the function is stored to some other variable in the main script as shown,... (3 Replies)
Hi all,
I'm doing automation task for my team and I just started to learn unix scripting so please shed some light on how to do this:
1) I have 2 sets of datafiles - datafile A and B. These datafiles must be loaded subsequently and cannot be loaded concurrently.
2) So I loaded datafile A... (10 Replies)
I have write a shell function to get the maximum of a vector.
However, the returned value from the function is not always
the correct one.
Here is the script:
maxval()
{
local max j i size arrval
size=$1 ; shift
max=-999999999
i=0
while
do
arrval="$1"
if
then
... (5 Replies)
What I need to learn is how to use a script that launches background processes, and then kills those processes as needed.
The script successfully launches the script. But how do I check to see if the job exists before I kill it?
I know my problem is mostly failure to understand parameter... (4 Replies)
Hi,
I have created a function f1 defined in script A.sh .I have called this function in background . But I want to use its return value for another function f2 in script A.sh.
I tried declaring it as a global variable, yet it always returns the status as 0. Is there any way with which I can get... (7 Replies)
Not sure where the problem is. I can run the script without any issue using the following command.
. /opt/app/scripts/cdc_migration.sh
But it fails with the below error when I try it this way
/opt/app/scripts/cdc_migration.sh
/opt/app/scripts/cdc_migration.sh: line 65: return: can only... (1 Reply)
Hi All,
Good Day, seeking for your assistance on how to not perform my 2nd, 3rd,4th etc.. function if my 1st function is in else condition.
#Body
function1()
{
if
then
echo "exist"
else
echo "not exist"
}
#if not exist in function1 my all other function will not proceed.... (4 Replies)
Discussion started by: meister29
4 Replies
LEARN ABOUT ULTRIX
fork
fork(2) System Calls Manual fork(2)Name
fork - create a new process
Syntax
#include <sys/types.h>
#include <unistd.h>
pid = fork()
pid_t pid;
Description
The system call causes creation of a new process. The new process (child process) is an exact copy of the calling process except for the
following:
o The child process has a unique process ID.
o The child process has a different parent process ID (that is, the process ID of the parent process).
o The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that, for
instance, file pointers in file objects are shared between the child and the parent, so that a on a descriptor in the child process
can affect a subsequent read or write by the parent. This descriptor copying is also used by the shell to establish standard input
and output for newly created processes as well as to set up pipes.
o The child processes resource utilizations are set to 0. For further information, see
Return Values
Upon successful completion, returns a value of 0 to the child process and returns the process ID of the child process to the parent
process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to
indicate the error.
Diagnostics
The system call fails and no child process are created under the following conditions:
[EAGAIN] The system-imposed limit {PROC_MAX} on the total number of processes under execution would be exceeded.
[EAGAIN] The system-imposed limit {CHILD_MAX} on the total number of processes under execution by a single user would be exceeded.
[ENOMEM] There is insufficient swap space for the new process.
See Alsoexecve(2), wait(2)fork(2)