How to fix executing awk in Tcl? - awk

Question

I am unable to read fields from awk command in Tcl while it runs in a terminal but not in Tcl script.
Tried making syntax changes, it works in terminal not in script
set a { A B C D E F G H I J K L M N O P Q R S T U V W X Y Z }
#store only cell var in file
exec grep -in "cell (?*" ./slow.lib | cut -d "(" -f2 | cut -d ")" -f1 > cells.txt
#take alphabets to loop
foreach b $a {
puts "$b\n"
if { [ exec cat cells.txt | awk ' $1 ~ /^$b/ ' ] } {
foreach cell [exec cat ./cells.txt] {
puts "$b \t $cell"
}
}
The condition should check for first char in the file and give boolean.
The error is:
can't read "1": no such variable
while executing "exec cat cells.txt | awk ' $1 ~ /^$b/ ' "

Answer 1

Your problem is that Tcl attaches no special meaning at all to the ' character. It uses {…} (which nest better) for the same purpose. Your command:
exec cat cells.txt | awk ' $1 ~ /^$b/ '
should become:
exec cat cells.txt | awk { $1 ~ /^$b/ }
Except… you also want $b (but not $1) to be substituted in there. The easiest way to do that is with format:
exec cat cells.txt | awk [format { $1 ~ /^%s/ } $b]
It would be more optimal to omit the use of cat here:
exec awk [format { $1 ~ /^%s/ } $b] <cells.txt
You are aware that your whole script can be written in pure Tcl without any use of exec?

Answer 2

can't read "1": no such variable
The (Tcl) error message is very informative. Tcl feels responsible for substituting a value of a Tcl variable 1 for $1 (meant for awk as part of the awk script). This is due to improper quoting of your awk scriplet. At the same time, you want $b to be substituted for from within Tcl.
Turn awk 'print $1 ~ /^$b/' into awk [string map [list #b# $b] {{$1 ~ /^#b#/}}]. Curly braces will preclude Tcl substitutions for $1, #b# will have already been substituted for before awk sees it thanks to [string map].
exec cat cells.txt | awk [string map [list #b# $b] {{$1 ~ /^#b#/}}]
That written, I fail to see why you are going back and forth between grep, awk etc. and Tcl. All of this could be done in Tcl alone.