Double pipe || between two commands - if the first command fails, then second will execute.
<command-1> || <command-2>
Double && between two commands - the second command would run only when first succeeds.
<command-1> && <command-2>
cd joininternet || mkdir joininternet && cd joininternet
The above command changes directory to join-internet, and suppose if does directory does not exists, then the command would fail, but as we have provided another after ||, then the second would execute and create the directory join-internet. After that, as change directory command is also given after &&, it would also get completed. Finally, we reach to directory join-internet.
Command History - If we press up arrow keyboard key, the lastest command executed in the history gets displayed.
Last command argument with !$
cd !$ Here, i$ = join-internet
cd join-internet || mkdir !$ && cd !$
!v would look in the history for a command which starts with letter 'v'. Similarly, if we provide !?75, it would look for 75, anywhere as an argument or its part.
date --date "75 days ago"