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I try, with no success, to use an awk command inside a for loop. I'v got a variable which contains a series of strings that I want to cut with awk to get the data. I know how to do that but what I really want is to cut the data successively.

So I've got this variable:

var="data1,data2,data3"

And here where I am right now:

for ((i=1; i<=3; i++))

do

echo $(awk -F, '{print $1}' <<<$var)

done

I try to replace the $1 by the loop $i but without success.

You can accomplish what you're trying to do by using double quotes in the awk script to inject the shell variable into it. You still want to keep one literal $ in it, which you can do by escaping it with backslash:

echo $(awk -F, "{print $$i}" <<<$var)

This will expand the $i to 1, 2 and 3 in each of the iterations, therefore awk will see $1, $2 and $3 which will make it expand each of the fields.

Another possibility is to inject the shell variable as an awk variable using the -v flag:

echo $(awk -F, -v i="$i" '{print $i}' <<<$var)

That injects the variable into awk with the same name. Variables in awk don't use a $, which is used for fields, so $i is enough to refer to the i-th field if i is a variable in awk.

Injecting a variable with -v is generally a safer approach, particularly when it can contain arbitrary sequences of characters, in that case there's less risk that the contents will be executed as awk code against your intentions. But since in your case the variable holds a single integer, that's less of a concern.

Yet another option is to use a for loop in awk itself. See awk documentation (or search this site) for more details on how to do that.

Using awk seems excessive in this circumstance, how about a tr and a while-loop:

tr , 'n' <<<"$var" | while read; do

  echo $REPLY

done

Output:

data1

data2

data3

awk can accept both j (as variable) and $j (as field index):

for i in [1-3]; do echo $var | awk -v j=$i -F , '{print $j}'; done

$i in the example "confused" awk which one to use (shell or its own variable - taking precedence) as both are referred to with $ prefix.

note

sh shell which is standard for "portable" scripting do not support the:

(( i=1; i<=3; i++; )) and <<< $var constructs