Shell Programming and Scripting

That is because these are but three lines of a 70 line AWK script. Isn't it enough to say code is AWK code or BASH code or Korn code? indeed there is. I am passing filenames from the Bash wrapper to the AWK script this way. I disagree. Those 3 lines are what I am doing (they work just fine). I am just looking for a more elegant solution You showed me several things including the solution I used in my very first post in this thread.

Code:

argument=$1; sub ( "^$", "default if empty", argument) (AWK)

That's becaue the sample script you showed me works just fine as do the scripts and portions of scripts I posted. My question was "Does awk have parameter substitution (or something with the same functionality)?" I'm trying my best to explain what I mean by that. I gave a BASH example of parameter substitution and I gave example of AWK code that performs the same function but using more commands.

Let me try again: In BASH you can equate one BASH variable to another BASH variable with a default in a single command.

Code:

a=${b:-"default"}

Is there a similar command in AWK (equate one AWK variable to another AWK variable with a default value in a single command)?

Mike

But - hasn't that been shown to you in posts #2 and #3?

Code:

awk -vb=XXX 'BEGIN {a=b?b:"Default"; print a}' 
XXX
awk  'BEGIN {a=b?b:"Default"; print a}' 
Default

Post 2, I recognised that it worked (the reason I thanked the post).
Post 3, the second example you reposted is applicable to my script but I failed to understand it.

It appears that I did not appreciate the difference between how AWK ? operates from the BASH conditionals based on std error I am used to.

Code:

 
$ echo | awk 'BEGIN { a=b?b:"Default"; print a }' # I expected this result
Default
 
$ echo | awk 'BEGIN { b=""; a=b?b:"Default"; print a }' # I did not expect this result
Default
 
$ echo | awk 'BEGIN { c="something"; a=b?b:"Default"; print a }' # expected
something
 
$ echo | awk 'BEGIN { b="0"; a=b?b:"Default"; print a }' # expected
0
mestora@MESTORA-MOBL1 ~
$ echo | awk 'BEGIN { b=0; a=b?b:"Default"; print a }' # was not sure what to think but this is useful.
Default

Mike

PS. Going back to thank post 3.
PPS. That example works for one part of my sciprt that does include truely null values. In another file I am working with, the column is never truely empty but it consists of double quoted emptyness, in which case I need to stick with the gsub.
PPS. I also was not aware that you could simultaneously do the assignement and test the assignment. I would have thought the required syntax was:

Code:

a= a=b? c : "Default"; #which also works

If you would just tell us the constraints under which you want to assign one of a group of values to a variable, we can probably help you construct a single command using one or more ternary expressions to get what you want. But just saying "I want to set a default value like bash does." doesn't really make sense. The awk utility doesn't have the concept of an "unset" scalar variable. (It does for elements of an array variable; but not for scalar variables.) And, in awk the empty string and the string "0" and the number 0 will be treated as identical in many awk expressions. But, if we know explicitly what you are trying to do we can differentiate between unset array elements, an empty string, and a 0 (although you can't differentiate a string 0 from a number 0 in awk). And, if you reference an array element (other than by using the in operator), you create that array element. For instance, if you split a line using:

Code:

n = split($0, a, ",")

and you want to set the variable v to "unset" if array[5] has not been set (i.e., split() returned a value less than 5), "empty" if a[5] is an empty string, "zero" if a[5] compares equal to 0 (such as with "0", "000", or "0.0"), and to the contents of a[5] for anything else you could use:

Code:

v = (5 in a) ? a[5] == "" ? "empty" : a[5] == 0 ? "zero" : a[5] : "unset"

The command:

Code:

b=""; a=b?b:"Default"

sets a to Default because the ternary operator evaluates the 1st expression (b in this case) and if that expression evaluates to TRUE returns the 2nd expression; otherwise it returns the 3rd expression. In awk a variable evaluates to TRUE if it is a non-zero string, a non-zero number, or a non-empty string; otherwise, it evaluates to FALSE.

Scalar variables in awk are never "unset" as in a shell variable that has never been set. In awk, every unset scalar variable has the value 0 if it is referenced as a numeric value and has the value empty string if referenced as a string.

That surprises me. c is "something", but neither assigned to a nor printed anywhere. So, the result should be "Default".

As to what pertains to the "double quoted emptyness", awk can't differentiate between empty and unset. But, neither can bash (man bash):

Also from man bash:

So to test for an unset variable, use:

Code:

${parameter-word}

Which is also standard for any POSIX-conformant shell..

--
Regarding awk, it can detect set or unset array elements through the relational expression "element in array", where the element does not get created...

With regular variables, it is not so much that awk cannot detect unset variables, but rather - as a design choice - awk works such, that referencing a variable creates it first with an empty value.

Should have read on...