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Full Discussion: Unix script (sh): state of ftp process
Top Forums Shell Programming and Scripting Unix script (sh): state of ftp process Post 302320156 by pludi on Wednesday 27th of May 2009 07:34:23 AM
Old 05-27-2009
No, your sleep does not block the FTP command. Let's sidetrack into kernel process management for a bit:

Since their invention CPU cores can only execute one process at a time. This meant that a single process waiting for user input could block the whole system. So someone came up with the idea of time slices. Each process is allowed execution for a certain time. After that time it returns control to the kernel. All was good until some processes didn't return control on purpose.
Most modern kernels use preempting instead. Again, each process is allocated execution time. After that time the OS kernel is woken via a timer interrupt (or sooner if the process gives it's rights back because it's waiting for something), the first process is sent into a sleep mode and another is given CPU time. The first process will continue when it's its turn.

Now back to your case: you're starting the FTP process and send it into the background. It gets its time-slices just like any other process, but it's not executing the whole time. Sometimes other processes are executed, so ftp is marked as 'sleeping' or, better said, waiting for its turn again. If you only have one CPU core and get the current list with ps, all processes except for ps will (most likely) be marked as 'sleeping', since ps is currently using its allocated time to gather the stats.

My suggestion for your problem would be:
  1. ping the host in question (maybe mark those pingable accordingly and skip the others)
  2. test if the FTP port is open. You can either use netcat (eg: netcat -z $host 21 and check $?) or parse the output of nmap)
  3. create a small checkfile on the FTP server, and fetch that first. For example, start the FTP command for that in the background. If it's still running after 5 seconds, consider the host dead.
  4. if all (necessary) checks are good, continue with the large transfer
 

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LEARN ABOUT PLAN9

ps

PS(1)							      General Commands Manual							     PS(1)

NAME

       ps, psu - process status

SYNOPSIS

       ps

       psu [ user ]

DESCRIPTION

       Ps prints information about processes.  Psu prints only information about processes started by user (default $user).

       For  each  process  reported, the user, process id, user time, system time, size, state, and command name are printed.  State is one of the
       following:

       Moribund      Process has exited and is about to have its resources reclaimed.

       Ready	     on the queue of processes ready to be run.

       Scheding      about to be run.

       Running	     running.

       Queueing      waiting on a queue for a resource.

       Wakeme	     waiting for I/O or some other kernel event to wake it up.

       Broken	     dead of unnatural causes; lingering so that it can be examined.

       Stopped	     stopped.

       Stopwait      waiting for another process to stop.

       Fault	     servicing a page fault.

       Idle	     waiting for something to do (kernel processes only).

       New	     being created.

       Pageout	     paging out some other process.

       Syscall	     performing the named system call.

       no resource   waiting for more of a critical resource.

FILES

       /proc/*/status

SOURCE

       /sys/src/cmd/ps.c
       /rc/bin/psu

SEE ALSO

       kill(1), db(1), ps(1), proc(3)

																	     PS(1)
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