How to edit (find-and-replace) in a file without opening it?

I needed to edit couple of files in a directory in order to find and replace one name/string with another one. Here is what I did using vi:

$ cat test_file
This is a test file.
Today is Monday.

$ vi -c "%s/Monday/Tuesday/g|wq" test_file
"test_file" 2 lines, 38 characters "test_file" 2 lines, 39 characters

$ cat test_file
This is a test file.
Today is Tuesday.

How to install Flash player plugin for Linux 64-bit version

Step by step: (Good for Linux 64-bit OS + Firefox 64 bit combination)

1) Download flash player plugin from Adobe site. To get a link to flash player, open http://www.youtube.com and try to play a video. If it does not play, then click on link provided to download flash player.

2) Go to ~/.mozilla/plugins directory and copy the plugin file in user's home directory/.mozilla/plugins. In my case, plugin directory was not created hence I created it manually for root user.

Shutdown/close firefox and restarted it. Voila!!! It worked like a charm!!! :)

Note (1) : The name of plugin file in this case was libflashplayer.so
Note (2) : In case of multiple users on the system, it has to be repeated for all users.