How to Grep files in Linux, but only certain file extensions?

Want to search your file system on Linux?  It's easy!

    grep -r --include=\*.txt 'searchterm' ./

...or case-insensitive version...

    grep -r -i --include=\*.txt 'searchterm' ./

• grep : command
  
• -r : recursively
  
• -i : ignore-case
  
• --include : all *.txt: text files (escape with \ just in case you have a directory with asterisks in the filenames)
  
• 'searchterm' : What to search
  
• ./ : Start at current directory.