Linux command line cheat sheet

The chapter on one page: what to remember, the commands and keys to have at your fingertips, and flashcards for quick revision.

The chapter in eight sentences

Linux is the kernel; a distribution such as RHEL packages it with thousands of tools into a complete, supported operating system. Rocky Linux rebuilds RHEL from the same source.
The shell (Bash) reads your commands, runs programs and shows their output in a terminal.
The prompt shows the user, the host and the current directory, and ends in $ for an ordinary user or # for root.
A command line is a command, then options (-n 3, --all), then arguments (what to work on).
ssh user@host logs you in to another machine; the first connection asks you to confirm its host key, and a changed key is a warning. exit or Ctrl+D logs out.
file tells you what a file really is; cat, less, head and tail show it in whole or in part, and wc counts it.
Tab completes commands, names and options; the history reruns commands with the arrows, !!, !N and !string, and Alt+. reuses the last argument.
Ctrl+A/E/U/K/R move, delete and search on the command line, so you edit instead of retyping.

Cheat sheet

Commands

Command	Does
`whoami` / `hostname`	Who and where you are
`date`, `date +%R`	The date and time; `+FORMAT` picks the parts
`passwd`	Change your password
`file FILE`	What kind of file it really is
`cat FILE`	Print the whole file
`less FILE`	Page through a file: Space, b, /search, q
`head -n N FILE` / `tail -n N FILE`	First / last N lines (default 10)
`wc FILE`, `wc -l FILE`	Lines, words, bytes; `-l` lines only
`ssh user@host`	Log in to another machine
`ssh host command`	Run one command there and see its output here
`sudo -i`	Root shell (asks for your password)
`history`	Numbered list of earlier commands
`COMMAND --help`	Short usage summary

Keys

Keys	Does
Tab, Tab Tab	Complete; list the choices
↑ / ↓	Step through the history
`!!`, `!N`, `!-N`, `!string`	Rerun the last / number N / N ago / last starting with string
Alt+.	Insert the previous command's last word
Ctrl+A / E	Start / end of the line
Ctrl+← / →	One word left / right
Ctrl+U / K	Delete to the start / end
Ctrl+R	Search the history backwards
Ctrl+C	Abandon the line, or stop a command
Ctrl+D	End of input: logs out of a shell
Ctrl+L	Clear the screen

Syntax

command [options] [arguments]
cmd1 ; cmd2          two commands on one line
long command \       one command on several lines (the next line continues it)
  continued
date +%Y-%m-%d       an argument beginning with + is a format for date

Flashcards

What is the difference between the kernel and a distribution?: The **kernel** manages the hardware for every program. A **distribution** packages the kernel with tools, an installer and updates into a complete system.
What does ` Linux command line Cheat Sheet (RHCSA) or `#` at the end of the prompt mean?: ` Linux command line Cheat Sheet (RHCSA) an ordinary user; `#` **root**, who can change anything.
Name the three kinds of part on a command line.: The **command**, its **options** (start with a dash) and its **arguments** (what it works on).
What does SSH do with a server's host key on the first connection?: Shows the fingerprint, and if you answer yes, **saves it** in `~/.ssh/known_hosts` to check silently on every later connection.
Two ways to log out of a shell?: `exit` or **Ctrl+D**.
Which command tells you what kind of file something is?: `file`. Linux ignores file name extensions.
Last 20 lines of a file?: `tail -n 20 FILE`
Count only the lines of a file?: `wc -l FILE`
Rerun the previous command?: `!!`, or the up arrow then Enter.
Rerun the latest command that started with `ssh`?: `!ssh`, or Ctrl+R then type ssh to see it first.
Insert the previous command's last argument?: **Alt+.** (or Esc then .); repeat to go further back.
Delete from the cursor to the end of the line?: **Ctrl+K**. Ctrl+U deletes back to the start.