perl function wait

The wait function in Perl waits for a child process to terminate and returns the process ID of the terminated child. If called in a scalar context, it waits for any child process. If called in a list context, it waits for any child process and returns additional information about the child's exit status.

Here is an example:

#!/usr/bin/perl

use strict;
use warnings;

my $pid = fork();

if (not defined $pid) {
    die "Failed to fork: $!";
}
elsif ($pid == 0) {
    # Child process
    print "Child process $$ started\n";
    sleep 5;
    print "Child process $$ finished\n";
    exit 0;
}
else {
    # Parent process
    print "Parent process $$ waiting for child process $pid\n";
    my $child_pid = wait();
    if ($child_pid == $pid) {
        print "Child process $child_pid terminated\n";
    }
}

In this example, the parent process forks a child process, which sleeps for 5 seconds and then terminates. The parent process waits for the child process to terminate and then prints a message indicating that the child process has finished. When the child process terminates, its exit status is returned to the parent process. If the child process exits normally (i.e., with an exit status of 0), the parent process will receive a child process ID of the terminated child, which is equal to the process ID of the child process. If the child process exits abnormally (i.e., with a nonzero exit status), the parent process can obtain additional information about the child's exit status by calling wait in a list context, like so:

my ($child_pid, $child_status) = wait();
if ($child_pid == $pid) {
    if ($child_status == 0) {
        print "Child process $child_pid terminated normally\n";
    }
    else {
        print "Child process $child_pid terminated with error code $child_status\n";
    }
}

In this case, the wait function returns two values: the process ID of the terminated child, and its exit status. The exit status is a 16-bit value that encodes both the exit code of the child process and any additional information about how the child process terminated. If the child process exits normally, the exit code will be 0 and the additional information will be 0. If the child process exits with an error code, the exit code will be nonzero and the additional information will indicate the cause of the error.