Most of the time with a file is spent reading it, and reading it well is a skill. cat for small files, head and tail for big ones, less for scrolling, and grep for finding. Choosing the wrong one fills your terminal with ten thousand lines.
About 2 min of reading
Sample a large file without dumping it.
Make a big file
Before you run it, say out loud what it will print. Then open the answer.
Sample both ends
Before you run it, say out loud what it will print. Then open the answer.
wc -l file tells you the size before you cat it. A 400,000 line file printed to the terminal is unreadable and slow, and the count would have told you in one command.
cat on a binary prints garbage and can leave odd characters in your terminal. grep -l or file tells you the type first, and the habit of checking type is what keeps a session usable.
head -20 shows the start of a file and tail -20 shows the end, which is where logs put the newest line. Neither loads the whole file into your view, so they are the default for anything you have not already sized.
less opens the file in a pager you can page through and search. It is the right tool when you want to read rather than sample, and it leaves the terminal in a state you have to quit from, which is worth knowing.
grep pattern file gives the matching lines with their numbers when you ask for -n. That is a different question from 'what is in this file', and asking the wrong one gives a confident but useless answer.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.