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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 X11R4

netrc

NETRC(5)						      BSD File Formats Manual							  NETRC(5)

NAME

     netrc -- user configuration for ftp

SYNOPSIS

     ~/.netrc

DESCRIPTION

     This file contains configuration and autologin information for the File Transfer Protocol client ftp(1).

     The .netrc file contains login and initialization information used by the auto-login process.  It resides in the user's home directory.  The
     following tokens are recognized; they may be separated by spaces, tabs, or new-lines:

     machine name
	       Identify a remote machine name.	The auto-login process searches the .netrc file for a machine token that matches the remote
	       machine specified on the ftp command line or as an open command argument.  Once a match is made, the subsequent .netrc tokens are
	       processed, stopping when the end of file is reached or another machine or a default token is encountered.

     default   This is the same as machine name except that default matches any name.  There can be only one default token, and it must be after
	       all machine tokens.  This is normally used as:

		     default login anonymous password user@site

	       thereby giving the user automatic anonymous ftp login to machines not specified in .netrc.  This can be overridden by using the -n
	       flag to disable auto-login.

     login name
	       Identify a user on the remote machine.  If this token is present, the auto-login process will initiate a login using the specified
	       name.

     password string
	       Supply a password.  If this token is present, the auto-login process will supply the specified string if the remote server requires
	       a password as part of the login process.  Note that if this token is present in the .netrc file for any user other than anonymous,
	       ftp will abort the auto-login process if the .netrc is readable by anyone besides the user.

     account string
	       Supply an additional account password.  If this token is present, the auto-login process will supply the specified string if the
	       remote server requires an additional account password, or the auto-login process will initiate an ACCT command if it does not.

     macdef name
	       Define a macro.	This token functions like the ftp macdef command functions.  A macro is defined with the specified name; its con-
	       tents begin with the next .netrc line and continue until a null line (consecutive new-line characters) is encountered.  If a macro
	       named init is defined, it is automatically executed as the last step in the auto-login process.

SEE ALSO

     ftp(1), ftpd(8)

Linux NetKit (0.17)						September 23, 1997					       Linux NetKit (0.17)
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