using awk in tcl script

Question

I want to print a particular column number fields in a file while in TCL script.

I tried with exec awk '{print $4}' foo where foo is filename, but it is not working as it gives error

can't read "4": no such variable

How can I do above awk in tcl scripting?

Thanks,

Answer 1

The problem is that single quotes have no special meaning in Tcl, they're just ordinary characters in a string. Thus the $4 is not hidden from Tcl and it tries to expand the variable.

The Tcl equivalent to shell single quotes are braces. This is what you need:

exec awk {{print $4}} foo

The double braces look funny, but the outer pair are for Tcl and the inner pair are for awk.

Btw, the Tcl translation of that awk program is:

set fid [open foo r]
while {[gets $fid line] != -1} {
    set fields [regexp -all -inline {\S+} $line]
    puts [lindex $fields 3]
}
close $fid