`ls` - list directory contents

ls [options] [file]...

Options you will use

-l: Long listing: type+permissions, link count, owner, group, size, mtime, name.
-a, --all: Do not ignore entries starting with a dot (includes . and ..).
-A: Like -a but omit . and ..
-h, --human-readable: With -l, print sizes as 1K, 234M, 2G.
-R, --recursive: List subdirectories recursively.
-i, --inode: Print the inode number of each file.
-d, --directory: List the directory itself, not its contents. Essential with -l on a dir.
-t: Sort by modification time, newest first.
-S: Sort by file size, largest first.
-r, --reverse: Reverse the sort order.
-1: One entry per line.
-F, --classify: Append an indicator: / dir, * executable, @ symlink.
--color=auto: Colourise the output when stdout is a terminal (aliased by default on Ubuntu).
--time-style STYLE: How -l shows times: full-iso, long-iso, iso, or +FORMAT like +%F.
-n, --numeric-uid-gid: Like -l, but show numeric user and group IDs instead of names.
-Z, --context: Show the SELinux / AppArmor security context of each file.

$ ls -la

the everyday listing, hidden files included

$ ls -ld /srv/vault

permissions OF the directory instead of what is in it

$ ls -li

inode numbers - two names with the same inode are hard links

$ ls -lhS /var/log

biggest files in /var/log first

The first column of ls -l: d=directory l=symlink -=regular file; then user/group/other rwx. An s in place of x is setuid/setgid, a t on other is the sticky bit.

Try ls in a real terminal Free, in your browser - a real Ubuntu terminal to try it in, with missions that check your work.