Read and use System Documentation

$ ls --help
Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).
...
  -a, --all             do not ignore entries starting with .
  -l, --long            use a long listing format
  -h, --human-readable  with -l, print sizes in human readable format

$ ls -l
bin/    lib/    libexec/    local/    sbin/    share/

$ journalctl --help
journalctl [OPTIONS...] [MATCHES...]
Query the journal.

Options:
  --system               Show the system journal
  --user                 Show the user journal
  -S, --since=DATE       Show entries not older than the specified date
  -U, --until=DATE       Show entries not newer than the specified cursor
  -b, --boot[=ID]        Show current boot or the specified boot
  --list-boots           Show terse information about recorded boots

$ man journalctl

$ man 1 printf   # The printf command
$ man 3 printf   # The printf C function

$ apropos directory
ls (1)       - list directory contents
mkdir (1)    - make directories
du (1)       - estimate file space usage

$ apropos -s 1,8 directory
ls (1)       - list directory contents
mkdir (1)    - make directories

$ systemctl <Tab><Tab>
add-requires    emergency       isolate       reboot
enable          exit            poweroff      show
...

$ systemctl list-dep<Tab>
systemctl list-dependencies

$ ls /u<Tab>      # expands to /usr/
$ ls /usr/<Tab>   # lists contents