How do I use ag to look for text in files with certain extensions?

Per the manual, you could use ag with -G

   -G --file-search-regex PATTERN
          Only search files whose names match PATTERN.

e.g.

ag -G '\.java$' 'ftp' .

Per the same manual

   It is possible to restrict the types of files searched [...]
   For a list of supported types, run ag --list-file-types.

So you could also run

ag --java 'ftp' .

though that would restrict the search to file names ending in .java or .properties

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