Shell Programming and Scripting

@Scruti

Ah ok ... i got it now (gush!, how did i miss it !) , thx !

By the way, in the code:

Code:

awk '{$1=$1; sub(FS,RS); sub(FS,RS)}1' infile

The $1=$1 is not necessary, is it ?

The $1=$1 is not strictly necessary with the sample provided, but it gives the script robustness since if the data were to include TABs or multiple spaces or if there were a space before the first field, then it might break otherwise...

Hi Scruti,

Ok, so you mean using $1=$1 trim blank stuff around the retained $1 ? or do i understand it in a wrong way ?

Do you have a short example just to illustrate what you mean ?

Thank in advance for your time & for your help

Yes the $1=$1 trims the blanks, so that we can be sure there is no leading whitespace and and only a single space separating the fields, so that the subs can be successful..


For example:

Code:

$ printf 'alpha 200 this is a comment for this record\n' | awk '{sub(FS,RS); sub(FS,RS)}1'
alpha
200
this is a comment for this record
$ printf '  alpha \t 200   this is a comment for this record\n' | awk '{sub(FS,RS); sub(FS,RS)}1'


alpha 	 200   this is a comment for this record
$ printf '  alpha \t 200   this is a comment for this record\n' | awk '{$1=$1; sub(FS,RS); sub(FS,RS)}1'
alpha
200
this is a comment for this record

could you please explain how does the below code works ?

sub(FS,RS); sub(FS,RS)

Code:

 
 
awk '{$1=$1; sub(FS,RS); sub(FS,RS)}1' infile

sub(FS,RS) substitue the Field Separator (whose default value is a space " ") with a Record Separator (whose default value is a newline "\n")

It does this substitution only once , so the first FS met is changed into a RS.
That is the reason why it is important to make sure that the first FS encountered is one between the fist field $1 and second field $2.

# cat t1
1 2 This is a comment

Code:

awk '{a=$1"\n"$2;sub(".*"$3,$3);print a"\n"$0}'  t1

sub(".*"$3,$3) replace in the current line ($0) everthing before and including $3 by the current value of $3
print a"\n"$0 print the variable a and the current line separated by a new line

by refering above file t1,current value of $3 is 'this' right ? , i am bit confused,could you please explain hw does this code works

Code:

sub(".*"$3,$3)

, will it returns the value 'this' or 'This is a comment'

please help me ............i am a newbie ..................

---------- Post updated at 06:34 PM ---------- Previous update was at 04:45 PM ----------

got it....
looks like above code does not give desired results, if the file contains string (this)which is repeated as below

# cat t1
1 2 This is a comment this is not a comment this this1

Code:

awk '{a=$1"\n"$2;sub(".*"$3,$3);print a"\n"$0}' t1