hi,

assuming that any value not equal to 0 is $true ( like it is in C and afaik in mirc-script too ) you may try this:
it returns 0 ( -> $false ) if nothing was found, or the line number ( not 0 -> $true ) of the keyword's first occurance in the file.

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; $1 is the file to search in, $2 the keyword
alias isinFile {
var %temp = $read($1,w,* $+ $2 $+ *)
return $readn
}

if ($isinFile(the_file.txt, keyword)) { echo -a $ifmatch }
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