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1. Add file extension to all files

   $ for f in *; do mv "$f" "$f.json"; done
   

2. Check last command exit status

   no error

   $ echo hello
   $ echo $?
   0
   

   error

   $ asdfg
   $ echo $?
   -bash: 127: command not found
   

3. Echo raw bytes

   $ echo -ne 'x41'