Alt-Shift-3 in Bash inserts a ‘#’ at the beginning of the line, and then runs it. # is the comment character, so running it does nothing. So why would you want to do this?

The simplest use case is when you have a command typed out, but you realize there’s another command you need to run first. By pressing alt-#, you push the command into your history, so you can quickly recall it after running whatever other commands first.

Example:

# Set some dataset properties...but wait, what was the name of the dataset again?
# I have the whole line except for the dataset ready, so press Alt-#
$ #zfs set primarycache=none secondarycache=metadata tank3/
# It's in my bash history now, so I can run whatever other commands first
$ zfs list
...
# Okay, figured it out, so now I press up-arrow/^P (or search with ^R) to recall the command, then home/^A to go to the start of the line, then del/^D to remove the #
$ zfs set primarycache=none secondarycache=metadata tank3/path/to/my/data/set

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