That depends on how you want to match it.
If you say "xtext is in file" do you mean an exact instance of xtext without wildcards?
Like:
word1
word2
xtext
word3
or more like:
word1
this is blablaxtextblabla
word2
Specific matching goes with: if !$read(myfile.txt,nw,xtext) { do things }
Wildcard matching like the second thing is: if !$read(myfile.txt,nw,*xtext*) { do things }
