using awk with if and z option

Question

when I want to check if a returned value is integer or not I use this in bash script:

  if [ -z "$value" ]
    then
    echo 0
    else
    echo $value
  fi

I was trying to use z option in awk with if. for example i have this line:

PRIMARY SECONDARY CONNECTED 350 800

I tried using this:

/bin/awk '{if( -z $1){print "0"}else{print $1}}' script

no matter i replace $1 with $2 or $3 or $4 or $5 it always return 0. am I using awk in a wrong way?

score 2 · Answer 1 · 2019-02-05T15:02:05.683

2

-z in shell will not tell you "if a value is integer or not", but whether its length is > 0.

You can do the same in awk with length -- if(length($1)) ...

If you want to check if a variable or field is a number or not in awk, you could do this:

 if($1+0 || $1~/^[+-]?0/) ...

Your if(-z $1) expression will simply negate the unset variable z, which will yield 0, and then append the 0 to the $1 field, creating a non-empty string which will be always considered TRUE, even if $1 was the empty string "" or the number 0.

edited Feb 05 '19 at 15:02

answered Feb 05 '19 at 08:04

would you please explain what is ($1+0 || $1~/^0/) doing? – BlackCrystal Feb 05 '19 at 09:34
$1+0 checks if the numeric value of $1 is non-zero (which will be if $1 is already a non-zero number or is a string which starts with a non-zero number), and $1~/^0/ checks if it's the number 0 or a string starting with 0 (not "foo", or "", which will also evaluate to 0 when converted to a number) – Feb 05 '19 at 09:44
If you're only interested in non-negative integers, you can get away with this $1~/^[0-9]/. For numbers in general, you'll have to implement the whole of strtod() in slow awk if you don't like my hack. – Feb 05 '19 at 09:50
One idiom in awk is to test $1+0 == $1, that is to force the value to a number and see if it compares equal to the original. (Strings get turned to 0 in the process, and that doesn't compare equal to the original string.) This accepts anything awk thinks is a number, like 123.34e5, or -0. – ilkkachu Feb 05 '19 at 10:10
@ilkkachu that won't do. not even for your example: echo | awk '{$1="123.34e5"; print $1, $1+0, $1+0 == $1}'. You shouldn't try with fields, because awk not only splits a line with IFS, it also turns into numbers the fields which look like numbers (pass the strtod-test). echo 0 | awk '{print $1 ? "yes" : "no"; $1="0"; print $1 ? "yes" : "no"}' – Feb 05 '19 at 14:43
@pizdelect, mm, it works for me in echo 123.34e5 | awk '{ print $1, $1+0, $1+0 == $1 }'. But perhaps you're right, perhaps it does some autodetecting. – ilkkachu Feb 05 '19 at 17:16
It seems you still don't believe me, despite the evidence. That's documented behavior (in mawk's manpage and in the POSIX standard -- "Each array element shall have a string value when created and, if appropriate, the array element shall be considered a numeric string" -- though not in gawk's, even if gawk is acting the same). Some other awk implementation have different bugs, they may work differently because == will do a numeric comparison, etc. -- anyways, my point stands: your trick is not reliable. – Feb 05 '19 at 17:41

Kusalananda · Accepted Answer · 2019-02-05T10:40:32.787

2

As others have said, there is no -z test in awk, and the -z test in the shell tests for empty strings, not integers.

In awk you would do the following to test whether a variable is empty (and output a zero if it is, or its value if it's not):

if (value == "")
    print 0
else
    print value

or,

print (value == "" ? 0 : value)

For example:

$ awk -F '\t' '{ print ($1 == "" ? 0 : $1) }' file

to print a zero for those lines whose first tab-delimited value in the input is empty, and the actual value otherwise.

Possibly related (if you want to find the type of a variable in awk):

Can I determine type of an awk variable?

edited Feb 05 '19 at 10:40

answered Feb 05 '19 at 08:34

Kusalananda

333,661

would you please explain how can i use these with awk? i get syntax error. – BlackCrystal Feb 05 '19 at 09:33
@BlackCrystal The code in my answer is awk code. I'll add an example that calls awk from the shell as soon as I'm at a computer (now on phone). – Kusalananda Feb 05 '19 at 09:57
@BlackCrystal Updated. – Kusalananda Feb 05 '19 at 10:40

score 1 · Answer 3 · answered Feb 05 '19 at 08:04

1

try

/bin/awk '{if( x$1 == x){print "0"}else{print $1}}' script

there is no -z option in awk.

answered Feb 05 '19 at 08:04

Archemar

31,554

that will only check if $1 is non empty (ie is not ""), which you could do just as well with if($1) ... – Feb 05 '19 at 08:35
1

@pizdelect if ($1) would not trigger if $1 is zero. – Kusalananda Feb 05 '19 at 08:35
@Kusalananda not if it's "0". – Feb 05 '19 at 08:36
1

@pizdelect The test if ($1) would not test whether $1 is empty. It would test whether $1 is non-zero. That's what I mean. – Kusalananda Feb 05 '19 at 08:40
@Kusalananda Yeah, it's a bit of confusion. I know that echo 0 | awk '$1' won't print anything. echo | awk '{$1="0";if($1)print$1} will print 0, though. And it will test whether $1 is empty: echo :foo | awk -F: '$1' won't print anything, etc. – Feb 05 '19 at 08:42

using awk with if and z option

3 Answers3