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Full Discussion: If statement for null
Top Forums Shell Programming and Scripting If statement for null Post 302557104 by gary_w on Tuesday 20th of September 2011 11:46:49 AM
Old 09-20-2011
For the sake of argument, the ksh built=in test "[[" handles a null variable. At least on our Sun box. Consider this:
Code:
#!/bin/ksh

unset efs
#efs=
#efs=x

if [[ $efs == "x" ]]; then
  print "efs equals 'x'"
else
  print "efs is unset, efs != 'x' or efs is null"
fi

Output:
Code:
$ ./testprog
efs is unset, efs != 'x' or efs is null
$

Of course if you explicitly need to test for null by itself, by all means use '-z' (or '-n' for not null) as testing like this is really testing for 3 conditions at once and may not be the desired action.
 

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LEARN ABOUT MINIX

shift

shift(1)                                                           User Commands                                                          shift(1)

NAME

       shift - shell built-in function to traverse either a shell's argument list or a list of field-separated words

SYNOPSIS

   sh
       shift [n]

   csh
       shift [variable]

   ksh
       * shift [n]

DESCRIPTION

   sh
       The positional parameters from $n+1 ... are renamed $1 ... . If n is not given, it is assumed to be 1.

   csh
       The  components of argv, or variable, if supplied, are shifted to the left, discarding the first component. It is an error for the variable
       not to be set or to have a null value.

   ksh
       The positional parameters from $n+1 $n+1 ... are renamed $1 ..., default n is 1. The parameter n can  be  any  arithmetic  expression  that
       evaluates to a non-negative number less than or equal to $#.

       On this man 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,  following a command preceded by ** that are in the format of a variable assignment, are expanded with the same rules as a vari-
           able assignment. This means that tilde substitution is performed after the = sign and word 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), ksh(1), sh(1), attributes(5)

SunOS 5.10                                                          15 Apr 1994                                                           shift(1)
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