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Full Discussion: How to capture actual error message when a command fails to execute
Top Forums Shell Programming and Scripting How to capture actual error message when a command fails to execute Post 302310716 by durden_tyler on Sunday 26th of April 2009 05:03:01 PM
Old 04-26-2009
Okay, let's see:

Quote:
Originally Posted by prathima
...
ls -l abc.txt 2>>errorlog.txt

In this case I understand the error message is written to the errorlog.txt
...
That's correct.

Quote:
... and I assume its bacause the return code from the command ls -l abc might return 2 if "abc" doesnt exists.
...
Nope, your assumption is incorrect.
The number 2 is not some return code that you are capturing in errlog.txt. Instead, it is the file descriptor of the stderr file. The three files - stdin, stdout and stderr are always open and have the descriptors 0, 1 and 2 assigned to them respectively. By default, stdin is your keyboard, stdout is your screen and stderr is your screen as well. Which means, by default, all error messages are directed to your screen. If you want to redirect an error message to something other than the screen (a file, for instance), then you'd use the construct "2>err.txt" after the command, which redirects the error message to the file "err.txt".

Quote:
My question is: How about if the command return non zero return code and is not equal to 2?
If the command returns a return code other than 2, it will still be captured in "err.txt" if you put the construct "2>err.txt" after that command.
An example follows:

Code:
$
$ # The telnet command returns an error code 1 if the target host is unknown
$ # The error code is stored in the "$?" variable that is echo'd right after the command.
$
$ telnet xxx
telnet: xxx: Name or service not known
xxx: Unknown host
$
$ echo $?
1
$

So now if I put "2>err.txt", then this error message should be redirected to "err.txt". As you can see, the error code here is something other than 2.

Code:
$ # try the command again and redirect the stderr to a file
$ 
$ telnet xxx 2>err.txt
$
$ # Check the file "err.txt"
$
$ cat err.txt
telnet: xxx: Name or service not known
xxx: Unknown host
$
$ # The error message was redirected to the file, and the error code was *not equal to* 2
$
$

Hope that helps,
tyler_durden
 

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LEARN ABOUT DEBIAN

lwres_hstrerror

LWRES_HSTRERROR(3)						       BIND9							LWRES_HSTRERROR(3)

NAME

       lwres_herror, lwres_hstrerror - lightweight resolver error message generation

SYNOPSIS

       #include <lwres/netdb.h>

       void lwres_herror(const char *s);

       const char * lwres_hstrerror(int err);

DESCRIPTION

       lwres_herror() prints the string s on stderr followed by the string generated by lwres_hstrerror() for the error code stored in the global
       variable lwres_h_errno.

       lwres_hstrerror() returns an appropriate string for the error code gievn by err. The values of the error codes and messages are as follows:

       NETDB_SUCCESS
	   Resolver Error 0 (no error)

       HOST_NOT_FOUND
	   Unknown host

       TRY_AGAIN
	   Host name lookup failure

       NO_RECOVERY
	   Unknown server error

       NO_DATA
	   No address associated with name

RETURN VALUES

       The string Unknown resolver error is returned by lwres_hstrerror() when the value of lwres_h_errno is not a valid error code.

SEE ALSO

       herror(3), lwres_hstrerror(3).

COPYRIGHT

       Copyright (C) 2004, 2005, 2007, 2012 Internet Systems Consortium, Inc. ("ISC")
       Copyright (C) 2000, 2001 Internet Software Consortium.

BIND9								   Jun 30, 2000 						LWRES_HSTRERROR(3)
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