Four more Linux commands we should all use more often

Looking up and spell checking words:
So if you’re like me, occasionally you may forget how to spell a word. It you’ve got the gist of it on the tip of your tongue then the ‘look’ command is what you’ve been needing. The look command works by searching for words that start with the given string, so for example giving it the word hat will return words like: hater, hatted, hats and so on.
look StringToSearchFor
Adding users:
This one is fairly self explanatory. The ‘useradd’ command simply adds a new user to the system. This works by inputting the name of the new user account and then following a menu driven process to complete the creation of the new user. Bear in mind that you will need sudo access to use this command.
useradd NewUserName
System up time:
The ‘uptime’ command does just that and will output the amount of time the current system has been running since the last boot. This is a great little command to check how long you’ve left a machine running or how long a machine has been sitting in one place.
uptime
What day is it?
The ‘cal’ command outputs a fairly simplistic and self explanatory calendar onto the terminal. By default the calendar is set to the current day’s date and is a nice little feature to check the day.
cal
Image Source: Ubuntu
More Reading: Linux Command Book