The Command-line

Getting started

Using ssh

Logging in to eustis

ssh NID@eustis.eecs.ucf.edu

Then enter your password

You can create an ssh key and add it to eustis’s .ssh/authorized_keys to login without a password.

# if you haven't already generated a key, run ssh-keygen
ssh-keygen
# accept the default location (as long as you haven't already generated


# now let's tell eustis about your public key (be sure to use .pub!)

# linux/mac
cat ~/.ssh/id_rsa.pub | ssh NID@eustis.cecs.ucf.edu 'cat >> .ssh/authorized_keys'

# windows (https://superuser.com/questions/1747549/alternative-to-ssh-copy-id-on-windows)
type %USERPROFILE%\.ssh\id_rsa.pub | ssh NID@eustis.cecs.ucf.edu "cat >> .ssh/authorized_keys"

# windows alternative (rsa_id.pub instad of id_rsa.pub)
type %USERPROFILE%\.ssh\rsa_id.pub | ssh NID@eustis.cecs.ucf.edu "cat >> .ssh/authorized_keys"

The command prompt

You should see a command prompt, something like this:

NID@net1547:~$

With a blinking cursor after the prompt (depending on your terminal program)

This is the bash shell program running and waiting for you to type commands.

Typing commands

Use the keyboard to type out the names of programs and other commands, e.g.,

NID@net1547:~$ ls

Notice that the prompt now has ls at the end.

Then hit the Enter key. The shell will run the named program (if it exists).

bash will only run programs found in directories listed in the $PATH environment variable or when given a path to the program.

This is why you need to type ./ before C programs after you compile them, e.g., ./a.out, because you are giving a path to a program to bash.

Getting help for commands

Use the man program to get help for shell commands.

NID@net1547:~$ man ls

Ctrl-L to clear your screen

To quickly clear your screen, and put the prompt at the top of the screen, use Ctrl-L (or type the clear command). You can also use Ctrl-Alt-L to remove previous screen output as well.

Comments in bash

ls # Runs ls, everything after # is ignored.

You can use Alt-Shift-3 (i.e., Alt-#) to insert a comment at the start of the current line, and then enter it.

File system hierarchy

Conventions

The root directory is called / (forward slash)
We also separate nested directories with /
All directories contain a . (dot) directory that points to itself
All directories contain a .. (double dot) directory that points to its parent
The home directory is aliased to ~ (tilde)

Example file system hierarchy

(Diagram)

/home/net/bro095187 (alias to tilde ~)
~/cop3402fal2026

Let’s look at an example hierarchy and see how bash commands move us around the file system.

bash (and every process) are associated with a /current working directory/, which affects how we reference the locations of files; any /relative/ paths will start from the current working directory. Relative paths are any paths not starting with the root of the file system /.

Paths

A string that contains the sequence of directory names along the file tree to the directory that contains the file and the file name itself. Allows you to uniquely identify files, even those with the same name.

Absolute paths

Absolute paths start with a /, i.e., the root of the file system.

Example: /usr/include/ is an absolute path that refers to the directory /usr/include/.

Relative paths

Relative paths do not start with a /. Instead, they are relative to the current working directory.

Example: If the current working directory is /usr, then the directory bin/ (notice the lack of a leading slash) refers to the directory with the absolute path /usr/bin.

Consider these commands

./hello
cd ..

Where are the “.” and “..” in our file tree?

In order to answer this, we need to introduce a new concept, the working directory.

The current working directory

Example: If the current working directory is /home/bpappas/, then the relative path ./Documents/ refers to the directory with absolute path /home/bpappas/Documents/.
Example: If the current working directory is /home/bpappas/, then the absolute path /usr/include/ refers to the directory with absolute path /usr/include (the current working directory is ignored for absolute paths).

Parent directory

Example: If the current working directory is /home/bpappas/Documents/, then the relative path ../Desktop/ refers to the directory with absolute path /home/bpappas/Desktop/.
Example: If the current working directory is /home/bpappas/Documents/, then the absolute path /etc/ refers to the directory with absolute path /etc/ (the current working directory is ignored for absolute paths).

Navigating the File System

Running commands

Type its name and hit Enter

Where am I?

Print Working Directory: pwd

What’s here?

LiSt the directory: ls

Running commands with arguments

Type its name, a space, then the argument.

Arguments are separate by spaces.

What if the argument has a space in it? - escape the space with a backslash \ - Use double-quotes (evaluates the contents, e.g., for bash variables and escape sequences) - Use single-quotes (does not evaluate contents or escape sequences)

Go to a new directory

Change (working) directory: cd path

cd /usr/include

What kind of path is this, relative or absolute?

Use eustis’s /usr/include as an example file hierarchy

These are where system-wide C header files are installed

Go the parent directory

cd ..

What is the .. directory? Show this in the diagram.

Where are we now? How do we find out?

Get used to running pwd and ls to orient yourself

Go to a subdirectory

cd include

What kind of path is this, relative or absolute?

What if we had done cd ./include instead? What’s the difference? Why do we have the . folder?

Programs versus shell builtins

We’ve used cd a few times now.

But what happens if we try to use man to learn more about cd?

man cd
No manual entry for cd

What’s going on?

Programs versus shell builtins

cd is actually what’s known as a a shell builtin.

It is implemented as a part of Bash (the shell we have been using).

Getting help for shell builtins

To get help for shell builtins, instead of man, use help

help cd

Checking a command’s type

To check if a command is a program, shell builtin, or some other kind of command, use type.

type cd

Use the -a flag to print all types.

type -a echo

The first one shown is what Bash defaults to using.

Can override default to use the program version (if one is present) by prefixing the command with the command builtin.

Can override default to use the builtin version (if one is present) by prefixing the command with the builtin builtin.

Making the command-line fast

At this point, you are typing a lot, and retyping some of the same paths over and over again.

Tab-completion

Autocomplete for the command-line

One of these most useful features for speed on the command-line

Works for files and directories, command-line arguments, and commands themselves.

You can even create completions for your own software.

Impatience

Don’t spend time typing when you can use tab completion

Laziness

Don’t try to remember yourself file names, let the shell do it

Example

Be sure we are in /usr/include

Type the following, but don’t hit Enter:

ls gnum

Hit Tab

The shell completes the file name for you!:

ls gnumake.h

Multiple completions

Type the following, but don’t hit Enter:

ls gnu

Hit Tab. Nothing seems to happen!

Hit Tab again to see all completions.

/usr/include$ ls gnu
gnu/            gnumake.h       gnu-versions.h
/usr/include$ ls gnu

Add the hyphen and hit Tab to complete.

ls gnu-

Completion happens only when there is a unique option. But hitting Tab again will show all possibilities with the prefix written so far. Continue typing until you have a unique prefix, then hit Tab again to complete the unique possibility.

Make it a habit to hit tab

Confirming existence of a file

Avoiding long typing

Narrowing down results

Press Tab twice

Editing commands

Keyboard	Description	Mnemonic
`Home` or `Ctrl-a`	Beginning of command (home)	`a` is beginning of the alphabet
`End` or `Ctrl-e`	End of command	`e` for end
`Backspace`	Delete a character behind you	It’s the `Backspace` key
`Alt-Backspace`	Delete a whole word behind you	`Alt` for alternative version
`Alt-b`	Go back a word	`b` for back, `Alt` for alternate
`Alt-f`	Go forward a word	`f` forward, `Alt` for alternate

https://www.gnu.html/software/bash/manual/bash.html#Commands-For-Moving

Bash has Emacs-like commands.

Additional commands

Keyboard	Description	Mnemonic
`Ctrl-d`	Delete character in front	d for delete
`Ctrl-b`	Move back	b for back
`Ctrl-f`	Go forward	f for forward
`Alt-d`	Delete a word in front	d for delete, alt for alternative version

Kill (cut) and yank (paste)

Keyboard	Description	Mnemonic
`Ctrl-u`	Kill (cut) until the start	`u` for kill
`Ctrl-k`	Kill (cut) until the end	`k` for kill
`Alt-Backspace`	Kill (cut) a word to back	`Alt` for alternative version
`Alt-d`	Kill (cut) a word in front	`d` for delete, `Alt` for alternative
`Ctrl-y`	Yank (paste)	`y` for yank

https://www.gnu.html/software/bash/manual/bash.html#Commands-For-Killing

Rerunning commands

History saves all commands your run
history will print them all
Last command added to bottom of the list

https://www.gnu.html/software/bash/manual/bash.html#Commands-For-History

Navigating the command history

Keyboard	Mnemonic	Description
`Ctrl-p` or `Up`	`p` for previous	Previous command
`Ctrl-n` or `Down`	`n` for next	Next command

Searching for a previous command

Ctrl-r (r for reverse lookup)
CAREFUL: hitting Enter will immediately run
- Use ctrl-a or ctrl-e to start editing the command
Ctrl-g to cancel search

“Save” commands by commenting them out

`Ctrl-a`, `#`, `Enter`

Adds “saved” command to last one in history

Viewing files

Command	Mnemonic	Description
`cat`	Concatenate	Write file(s) to terminal (stdout)
`more`	Ask user to show more	Write file out, pausing after each screenful
`less`	“Less is more”	Interactive terminal file viewer, `q` to quit

`cat` command

cat /usr/include/stdio.h

`less` command

less /usr/include/stdio.h

q to quit

Navigating `less`

q to quit (try Esc then q if not working, you might have hit other keys)
Pg-up, Pg-down, Up, Down, Left, Right
Ctrl-v, Alt-v, j, k, h, l
/ type pattern, then enter
- n for next
- N for previous
Type a number then G to go to line
Ctrl-g to show current command-line

The less command has vim-like commands.

Editing files with `vim` or `emacs`

Everyone should have completed the Emacs or vim tutorial as part of the reading for lecture 1.

Quick review

Keeping your editor running

Use Ctrl-z to “pause” your editor, returning to the command prompt.
Then use fg to resume it.

Warning: avoid opening multiple instances of the editor on the same file, or you may accidentally overwrite unsaved changes.

Note: eustis is configured to prohibit background processes from running after quitting and will kill them.

Downloading and untarring files

Command	Description
`wget 'URL'`	Download a file
`tar -xvf file.tar`	Unpack a tarball (e`x`pand; `v`erbose, `f`ilename)

cd ~/
wget 'https://www.cs.ucf.edu/~pappas/teaching/cop3402/fal2026/files/hw2.tar'
tar -xvf hw2.tar
cd hw2

You’ll need this for hw2 and later for projects.

Editing the file tree

Command	Description
`touch`	Create a file
`rm`	Remove a file
`mv`	Move a file
`cp`	Copy a file
`mkdir`	Make a directory
`rmdir`	Remove (an empty) Directory

Removing non-empty directories?

Remove contents of the directory first, then use rmdir
What’s an algorithm to remove a whole directory tree?

You can ask rm to remove non-empty directories and their contents recursively, but it’s a bit dangerous.

Review

Orienting yourself

pwd # Print the working directory
ls  # List current directory contents

Moving around

cd /usr/include # Change working directory
cd ../          # Move to /usr
cd ~            # Go to home directory
cd -            # Go to last directory
cd              # Also to go to home directory

Tab completion

Hit tab to auto-complete file and directory names
Hit tab again to show all files with a matching prefix
Use tab to quickly find paths

History

Hit Up (or Ctrl-p) to repeat the prior command
Go Up and Down (or Ctrl-n) to step through the command history
Hit Enter on a command to rerun it
Use !<prefix> to re-run the most-recent command beginning with <prefix>
Use fc to open the last entered command in your default text editor
- You can set your default editor by assigning its name to the variable EDITOR, e.g., EDITOR=vim.

Modifying the file tree

touch filename                # to create an empty regular file
rm filename                   # to remove a (non-directory) file
mv filename newname           # to rename a file
mkdir subdir                  # to make a new directory
mv filename subdir/           # to move a file
cp subdir/filename copiedname # to copy a file
rm subdir/filename            # to remove a file
rmdir subdir                  # to remove an empty directory

Getting started

Using ssh

Logging in to eustis

The command prompt

Typing commands

Getting help for commands

Ctrl-L to clear your screen

Comments in bash

File system hierarchy

Conventions

Example file system hierarchy

Paths

The current working directory

Parent directory

Navigating the File System

Running commands

Where am I?

What’s here?

Running commands with arguments

Go to a new directory

Go the parent directory

Go to a subdirectory

Programs versus shell builtins

Programs versus shell builtins

Getting help for shell builtins

Checking a command’s type

Making the command-line fast

Tab-completion

Impatience

Laziness

Example

Multiple completions

Make it a habit to hit tab

Confirming existence of a file

Avoiding long typing

Narrowing down results

Editing commands

Additional commands

Kill (cut) and yank (paste)

Rerunning commands

Navigating the command history

Searching for a previous command

“Save” commands by commenting them out

Ctrl-a, #, Enter

Adds “saved” command to last one in history

Viewing files

cat command

less command

Navigating less

Editing files with vim or emacs

Quick review

Keeping your editor running

Downloading and untarring files

Editing the file tree

Removing non-empty directories?

Review

Orienting yourself

Moving around

Tab completion

History

Modifying the file tree

`Ctrl-a`, `#`, `Enter`

`cat` command

`less` command

Navigating `less`

Editing files with `vim` or `emacs`