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Full Discussion: Difference between WHILE and FOR loops
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
Old 04-23-2015
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
 

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LEARN ABOUT OPENSOLARIS

break

break(1)							   User Commands							  break(1)

NAME

       break, continue - shell built-in functions to escape from or advance within a controlling while, for, foreach, or until loop

SYNOPSIS

   sh
       break [n]

       continue [n]

   csh
       break

       continue

   ksh
       *break [n]

       *continue [n]

   ksh93
       +break [n]

       +continue [n]

DESCRIPTION

   sh
       The break utility exits from the enclosing for or while loop, if any. If n is specified, break n levels.

       The continue utility resumes the next iteration of the enclosing for or while loop. If n is specified, resume at the n-th enclosing loop.

   csh
       The  break  utility  resumes  execution after the end of the nearest enclosing foreach or while loop. The remaining commands on the current
       line are executed. This allows multilevel breaks to be written as a list of break commands, all on one line.

       The continue utility continues execution of the next iteration of the nearest enclosing while or foreach loop.

   ksh
       The break utility exits from the enclosed for, while, until, or select loop, if any. If n is specified,	then  break  n	levels.  If  n	is
       greater than the number of enclosing loops, the outermost enclosing loop shall be exited.

       The  continue utility resumes the next iteration of the enclosed for, while, until, or select loop. If n is specified then resume at the n-
       th enclosed loop. If n is greater than the number of enclosing loops, the outermost enclosing loop shall be used.

       On this manual page, ksh(1) commands that are preceded by one or two * (asterisks) are treated specially in the following ways:

	   1.	  Variable assignment lists preceding the command remain in effect when the command completes.

	   2.	  I/O redirections are processed after variable assignments.

	   3.	  Errors cause a script that contains them to abort.

	   4.	  Words that follow a command preceded by ** that are in the format of a variable assignment are expanded with the same rules as a
		  variable  assignment.  This  means  that tilde substitution is performed after the = sign, and also that word splitting and file
		  name generation are not performed.

   ksh93
       break is a shell special built-in that exits the smallest enclosing for, select, while, or until loop. It also exits the nth enclosing loop
       if n is specified. Execution continues at the command following the loop or loops.

       If n is specified, it must be a positive integer >=1. If n is larger than the number of enclosing loops, the last enclosing loop is exited.

       continue  is  a	shell special built-in that continues execution at the top of the smallest enclosing for, select, while, or until loop, if
       any; or of the top of the nth enclosing loop if n is specified.

       If n is specified, it must be a positive integer >=1. If n is larger than the number of enclosing loops, the last enclosing loop is used.

       On this manual page, ksh93(1) commands that are preceded by one or two + symbols are special built-in commands and are treated the  follow-
       ing ways:

	   1.	  Variable assignment lists preceding the command remain in effect when the command completes.

	   2.	  I/O redirections are processed after variable assignments.

	   3.	  Errors cause a script that contains them to abort.

	   4.	  Built-in commands are not valid function names.

	   5.	  Words  following  a command preceded by ++ that are in the format of a variable assignment are expanded with rules as a variable
		  assignment. This means that tilde substitution is performed after the = sign and field splitting and file  name  generation  are
		  not performed.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWcsu			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       csh(1), exit(1), ksh(1), ksh93(1), sh(1), attributes(5)

SunOS 5.11							    8 Apr 2008								  break(1)
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