Difference between WHILE and FOR loops Post: 302941955

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Top Forums Shell Programming and Scripting Difference between WHILE and FOR loops Post 302941955 by mccmjc on Thursday 23rd of April 2015 05:38:17 AM

04-23-2015

Registered User

Difference between WHILE and FOR loops

This might be a simple question, but I can't find the answer out there anywhere. I'm trying to write a loop to SSH to a server for me to then copy/paste some commands in. Once I'm done, I type 'exit' and the loop should then automatically SSH to the next server in line.

The 'dbs_group' file contains 5 hostnames, one per line. Nothing else.

Seems to work fine in a FOR loop, but not in a WHILE loop.

For example (server names replaced with 'xxx'):

Code:

[chris@x] for HOST in $(cat dbs_group); do ssh $HOST; done
Last login: Thu Apr 23 10:08:51 2015 from xxx.com
[chris@xxx1] exit
logout
Connection to xxx1 closed.
Last login: Sat Apr 18 13:56:28 2015 from xxx.com
[chris@xxx2] exit
logout
Connection to xxx2 closed.

etc., which is what I was expecting.

But using a WHILE loop:

Code:

[chris@x] cat dbs_group | while read HOST; do ssh $HOST; done
Pseudo-terminal will not be allocated because stdin is not a terminal.
-bash: line 1: xxx1: command not found
-bash: line 2: xxx2: command not found
-bash: line 3: xxx3: command not found
-bash: line 4: xxx4: command not found
-bash: line 5: xxx5: command not found
[chris@x]

I understand that the error above might be resolved by using 'ssh -t -t' but I'm just trying to figure out why the two different loops respond differently. I thought both of them would process the dbs_group file line per line and assign the variable to HOST.

Thanks,

Chris

mccmjc

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libbash

LIBBASH(7)							  libbash Manual							LIBBASH(7)

NAME

     libbash -- A bash shared libraries package.

DESCRIPTION

     libbash  is  a package that enables bash dynamic-like shared libraries. Actually its a tool for managing bash scripts whose functions you may
     want to load and use in scripts of your own.

     It contains a 'dynamic loader' for the shared libraries ( ldbash(1)), a configuration tool (ldbashconfig(8)), and some libraries.

     Using ldbash(1) you are able to load loadable bash libraries, such as getopts(1) and hashstash(1).  A bash shared library that can be  loaded
     using
     ldbash(1) must answer 4 requirments:

     1.     It must be installed in $LIBBASH_PREFIX/lib/bash (default is /usr/lib/bash).

     2.     It must contain a line that begins with '#EXPORT='.  That line will contain (after the '=') a list of functions that the library
	    exports.  I.e. all the function that will be usable after loading that library will be listed in that line.

     3.     It must contain a line that begins with '#REQUIRE='.  That line will contain (after the '=') a list of bash libraries that are
	    required for our library.  I.e. every bash library that is in use in our bash library must be listed there.

     4.     The library must be listed (For more information, see ldbashconfig(8)).

   Basic guidelines for writing library of your own:
     1.     Be aware, that your library will be actually sourced. So, basically, it should contain (i.e define) only functions.

     2.     Try to declare all variables intended for internal use as local.

     3.     Global variables and functions that are intended for internal use (i.e are not defined in '#EXPORT=') should begin with:
		  __<library_name>_
	    For example, internal function myfoosort of hashstash library should be named as
		  __hashstash_myfoosort
	    This helps to avoid conflicts in global name space when using libraries that come from different vendors.

     4.     See html manual for full version of this guide.

AUTHORS

     Hai Zaar <haizaar@haizaar.com>
     Gil Ran <ril@ran4.net>

SEE ALSO

     ldbash(1), ldbashconfig(8), getopts(1), hashstash(1) colors(1) messages(1) urlcoding(1) locks(1)

Linux								       Epoch								     Linux

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