I don't know ,why I can't print from to file. My command:

awk 'BEGIN{print "af">file}'


fatal: expression for `>>' redirection has null string value

file needs to be in quotes else awk will attempt to parse file as a variable rather than a string:

 awk 'BEGIN{print "af">"file"}'
  • ... or else file has to be assigned a value: awk 'BEGIN {file="mytest"; print "af">file}'. – Scott Sep 2 '15 at 5:16

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