How to check if the variable value in AWK script is null or empty? - unix

Question

I am using AWK script to process some logs.
At one place I need to check if the variable value is null or empty to make some decision.
Any Idea how to achieve the same?
awk '
{
{
split($i, keyVal, "#")
key=keyVal[1];
val=keyVal[2];
if(val ~ /^ *$/)
val="Y";
}
}
' File
I have tried with
1) if(val == "")
2) if(val ~ /^ *$/)
not working in both cases.

Answer 1

The comparison with "" should have worked, so that's a bit odd
As one more alternative, you could use the length() function, if zero, your variable is null/empty. E.g.,
if (length(val) == 0)
Also, perhaps the built-in variable NF (number of fields) could come in handy? Since we don't have access to your input data it's hard to say though, but another possibility.

Answer 2

You can directly use the variable without comparison, an empty/null/zero value is considered false, everything else is true.
See here :
# setting default tag if not provided
if (! tag) {
tag="default-tag"
}
So this script will have the variable tag with the value default-tag except if the user call it like this :
$ awk -v tag=custom-tag -f script.awk targetFile
This is true as of :
GNU Awk 4.1.3, API: 1.1 (GNU MPFR 3.1.4, GNU MP 6.1.0)

Answer 3

It works just fine for me
$ awk 'BEGIN{if(val==""){print "null or empty"}}'
null or empty
You can't differentiate between variable being empty and null, when you access "unset" variable, awk just initializes it with default value(here it is "" - empty string). You can use some sort of workaround, for example, setting val_accessed variable to 0 and then to 1 when you access it. Or more simple approach(somewhat "hackish") setting val to "unitialized"(or to some other value which can't appear when running your program).
PS: your script looks strange for me, what are the nested brackets for?

Answer 4

I accidentally discovered this less-used function specific in gawk that could help differentiate :
****** gawk-only ******
BEGIN {
$0 = "abc"
print NF, $0
test_function()
test_function($(NF + 1))
test_function("")
test_function($0)
}
function test_function(_) { print typeof(_) }
1 abc
untyped
unassigned
string
string
So it seems, for non-numeric-like data :
absolutely no input to function at all : untyped
non-existent or empty field, including $0 : unassigned
any non-numeric-appearing string, including "" : string
Here's the chaotic part - numeric data :
strangely enough, for absolutely identical input, only differing between using $0 vs. $1 in function call, you frequently get a different value for typeof()
even a combination of both leading and trailing spaces doesn't prevent gawk from identifying it as strnum
[123]:NF:1
$0 = number:123 $1 = strnum:123 +$1 = number:123
[ 456.33]:NF:1
$0 = string: 456.33 $1 = strnum:456.33 +$1 = number:456.33000
[ 19683 ]:NF:1
$0 = string: 19683 $1 = strnum:19683 +$1 = number:19683
[-20.08554]:NF:1
$0 = number:-20.08554 $1 = strnum:-20.08554 +$1 = number:-20.08554
+/- inf/nan (same for all 4):
[-nan]:NF:1
$0 = string:-nan $1 = strnum:-nan +$1 = number:-nan
this one is a string because it was made from sprintf() :
[0x10FFFF]:NF:1
$0 = string:0x10FFFF $1 = string:0x10FFFF +$1 = number:0
using -n / --non-decimal-data flag, all stays same except
[0x10FFFF]:NF:1
$0 = string:0x10FFFF $1 = strnum:0x10FFFF +$1 = number:1114111
Long story short, if you want your gawk function to be able to differentiate between
empty-string input (""), versus
actually no input at all
e.g. when original intention is to directly apply changes to $0
then typeof(x) == "untyped" seems to be the most reliable indicator.
It gets worse when null-string padding versus a non-empty string of all zeros ::
function __(_) { return (!_) ":" (!+_) }
function ___(_) { return (_ == "") }
function ____(_) { return (!_) ":" (!""_) }
$0--->[ "000" ] | __(""$0)-->{ !(""$0) : !+(""$0) }-->[ 0:1 ]
___($0)-->{ $0=="" }-->[ 0 ] | ____($0)-->{ ! $0 : (!""$0) }-->[ 1:1000 ]
$0--->[ "" ] | __(""$0)-->{ !(""$0) : !+(""$0) }-->[ 1:1 ]
___($0)-->{ $0=="" }-->[ 1 ] | ____($0)-->{ ! $0 : (!""$0) }-->[ 1:1 ]
$0--->[ " -0.0 -0" ] | __(""$0)-->{ !(""$0) : !+(""$0) }-->[ 0:1 ]
___($0)-->{ $0=="" }-->[ 0 ] | ____($0)-->{ ! $0 : (!""$0) }-->[ 0:1 -0.0 -0 ]
$0--->[ " 0x5" ] | __(""$0)-->{ !(""$0) : !+(""$0) }-->[ 0:1 ]
___($0)-->{ $0=="" }-->[ 0 ] | ____($0)-->{ ! $0 : (!""$0) }-->[ 0:1 0x5 ]