Variable operator for numerical comparison in awk - awk

Question

Can awk use variable operators for numerical comparison? The following code works with a hard coded operator, but not with a variable operator:
awk -v o="$operator" -v c="$comparison" '$1 o c'

Answer 1

No, that cannot work. Awk's -v option defines actual Awk variables, and not token-level macro substitutions.
It doesn't work for the same reason that this doesn't work:
awk 'BEGIN { o = "+"; print 2 o 2 }' # hoping for 2 + 2
Awk is different from the POSIX shell and similar languages; it doesn't evaluate variables by means of textual substitution.
Since you're calling Awk from a shell command line, you can use the shell's substitution to generate the Awk syntax, thereby obtaining that effect:
awk -v c="$comparison" "\$1 $operator c"
We now need a backslash on the $1 because we switched to double quotes, inside of which $1 is now recognized by the shell itself.

Answer 2

Another way to the one proposed by Kaz would be to define your own mapping function which takes the two variables as argument and the corresponding operator string o:
awk -v o="$operator" -v c="$comparison" '
function operator(arg1, arg2, op) {
if (op == "==") return arg1 == arg2
if (op == "!=") return arg1 != arg2
if (op == "<") return arg1 < arg2
if (op == ">") return arg1 > arg2
if (op == "<=") return arg1 <= arg2
if (op == ">=") return arg1 >= arg2
}
{ print operator($1,c,o) }'
This way you can also define your own operators.

Answer 3

No but you have a couple of options, the simplest being to let the shell expand one of the variables to become part of the awk script before awk runs on it:
$ operator='>'; comparison='3'
$ echo 5 | awk -v c="$comparison" '$1 '"$operator"' c'
5
Otherwise you can write your own eval-style function, e.g.:
$ cat tst.awk
cmp($1,o,c)
function cmp(x,y,z, cmd,line,ret) {
cmd = "awk \047BEGIN{print (" x " " y " " z ")}\047"
if ( (cmd | getline line) > 0 ) {
ret = line
}
close(cmd)
return ret
}
$ echo 5 | awk -v c="$comparison" -v o="$operator" -f tst.awk
5
See https://stackoverflow.com/a/54161251/1745001. The latter would work even if your awk program was saved in a file while the former would not. If you want to mix a library of functions with command line scripts then here's one way with GNU awk for -i:
$ cat tst.awk
function cmp(x,y,z, cmd,line,ret) {
cmd = "awk \047BEGIN{print (" x " " y " " z ")}\047"
if ( (cmd | getline line) > 0 ) {
ret = line
}
close(cmd)
return ret
}
$ awk -v c="$comparison" -v o="$operator" -i tst.awk 'cmp($1,o,c)'
5