awk: using escape characters in for statement - for-loop

Question

I have the following awk command which works perfectly:
awk 'BEGIN { ORS = " " } {print 22, $1, NR; for(i=2;i<=NF;++i) print $i}{print "\n"}' file
What it does is insert two columns: the value 22 as the first, and the nr of the row as the third (see this question).
I tried running this command in a for loop as follows:
for p in 20 21 22
do
awk 'BEGIN { ORS = " " } {print $p, \$1, NR; for(i=2;i<=NF;++i) print \$i}{print "\n"}' file > file$p
done
So instead of the 22 in the first column, 3 files are made where each file has 20, 21 or 22 as the first column. Unfortunately, this doesn't work and I get the message: ^ backslash not last character on line.
It probably has something to do with how I escaped the $ characters, but I don't know how else to do it... any ideas?

Answer 1

You can assign a shell-parameter to awk using
-v parameter=value
i.e. use
awk -v p=$p ...
Like this:
for p in 20 21 22
do
awk -v p=$p 'BEGIN { ORS = " " } [TOO LONG LINE]' file > file$p
done
Complete command:
for p in 20 21 22; do awk -v p=$p 'BEGIN { ORS = " " } {print p, $1, NR; for(i=2;i<=NF;++i) print $i}{print "\n"}' file > file$p; done

Answer 2

You are in the right direction. Because your awk script is within ' (single quotes), bash doesn't replace $p with its value in the script (See Advanced Bash-Scripting Guide - Quoting).
You need to do one of the following:
1 (Recommened):
for p in 20 21 22
do
awk -v p=$p 'BEGIN { ORS = " " } {print p, \$1, NR; for(i=2;i<=NF;++i) print \$i}{print "\n"}' file > file$p
done
2: Use " instead of ' and within the awk escape the " by \"