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Use of the ternary operator, an awk example.

Post first published in nixtip

Use of the ternary operator, an awk example.

Ok, suppose this source file:

$ cat infile
21/tcp   closed ftp
22/tcp   open   ssh
23/tcp   closed telnet
80/tcp   closed http
90/tcp   closed dnsix
95/tcp   closed supdup
100/tcp  closed newacct
162/tcp  closed snmptrap
205/tcp  closed at-5
335/tcp  closed unknown
435/tcp  closed mobilip-mn
555/tcp  closed dsf
8080/tcp closed http-proxy
8081/tcp closed blackice-icecap

Our mission will be to get a formatted output port:state:service like this


… and so on …

All closed ports should be marked as 1 the rest will be 0.

Like always, in *nix system we have plenty of tools (and approaches) to get the expected result, lets try the awk way…

At first sight we can identify three fields in our input file and tree tasks be solved.

  1. Get rid of the slash + tcp string of the first field.
  2. Change the value of the second field for 1 or 0.
  3. Field separator should be :

A simply text replacing, is a straightforward way to get the expected result:

$ awk '{sub(/\/.*closed +/,":1:");sub(/\/.*open +/,":0:")}1' infile

Here’s the internals:

  • We look for a string started by as slash (note de escape char \/) followed by any number of any character (dot + star .*) ,followed by the string closed and ended by any number of space chars * and replace it with :1: .For the first line: 21/tcp closed ftp will be replace for :1:

  • Same thing for “open” in this case “:0:” will be the substitution string , example: 22/tcp open ssh will be replace for :0:

Our initial tasks get solved ,but we can refine our efforts.

Let’s use the conditional operator.

expr ? action1 : action2

Its pretty straight forward : if expr then acction1 is performed/evaluated , if not action2.

For our example , field two must change to 1 if it’s value is closed, if not it should be 1.

The needed conditional operator:

$2=="closed" ? "1" : "0"

Depending of second field value, our program will perform a different action, in this case its returning a string : 1 or 0.

At this point, a variable is needed to store it:

n= $2=="closed" ? "1" : "0"

Finally we perform the text substitution:

awk '{n= $2=="closed" ? "1" : "0";sub(/\/.*(open|closed) +/,":"n":")}1' infile

Note that we reduce the calls to the sub function to just one.

A final (and total different) approach , field substitution instead of text replacing.

Remember our tasks:

a) Get rid of the slash+tcp string of the first field. b) Change the value of the second field for 1 or 0 c) Field separator should be :

Our input file has naturally three fields (by the default awk FS ):

21/tcp   closed ftp
22/tcp   open   ssh
23/tcp   closed telnet

It’s clear that we can think in a four fields based line, if we add the slash / to our field separators by using a regex as FS='( *)|(/)' where ( *) represents any number of spaces as separator and (/) represents the slash:


awk '{print $1,$2,$3,$4}' OFS='>' FS='( *)|(/)'  infile|head -3

Note that the Output Field Separator OFS is changed to > for clarify.

Now, we want to get rid of the second field, technically is not possible, but we can assign the null value (empty string) to it:

awk '{$2=""}1' OFS='>' FS='( *)|(/)'  infile|head -4

Attention, the use of the print statement is not needed, awk will print the input line if the result of applying the inner statements to the current input line is true.

The assignment $2="" is not an action statement but we force a true return by placing 1 at the end of the program.

If we set the OFS to null value:

awk '{$2=""}1' OFS= FS='( *)|(/)'  infile|head -4

We’re close to or goal, the last step is to process the third field:

$3=="closed" ? ":1:" : ":0:"

Like we saw before we need to assign it to a variable,… look the trick:

$3= $3=="closed" ? ":1:" : ":0:"

We say , hey! change `$3 depending of its previous value. So :

awk '{$2="";$3=$3=="closed" ? ":1:" : ":0:"}1' OFS= FS='( *)|(/)'  infile|head -4

A final optimization, the conditional operator performs always an action that imply the print statement, so:

awk '{$2="";$3=$3=="closed" ? ":1:" : ":0:"}1' OFS= FS='( *)|(/)' infile

Is equivalent to:

awk '$2="";$3=$3=="closed" ? ":1:" : ":0:"' OFS= FS='( *)|(/)'  infile

We’re done.

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© Juan Diego Godoy Robles