Basic Unix Commands

Learning Outcomes

By the end of this lecture, you should be able to:

1. Identify and use common Unix/Linux commands for file and directory management.
2. Choose and operate basic text editors in a Unix environment.
3. Understand and apply file permissions and user/group management commands.
4. Use input/output redirection and pipes to manipulate command-line data.
5. Utilize regular expressions for pattern matching in file operations.
6. Locate and interpret system and user information using Unix commands.

Text Editors

Choosing a text editor is an important step in your Unix/Linux journey. You will use your editor frequently, so investing time to learn one well will pay off.

Types of Text Editors

• Terminal-based editors:

  • Vi, Vim, Neovim: Lightweight, available on almost every system, but have a steep learning curve. Once learned, they are very fast and efficient.
    • See the cheat sheets.
    • Try vimtutor in the terminal for a hands-on tutorial.
  • Emacs: Highly customizable and powerful, but also has a learning curve.
  • nano: Simple and beginner-friendly, available on most systems.

• Graphical editors:

  • VS Code: Popular, extensible, and user-friendly.
  • VSCodium: Free/Libre Open Source VS Code
  • gVim: Graphical version of Vim.
  • Xemacs: Graphical Emacs.
  • gedit: Simple, similar to Notepad.
  • Sublime Text: Powerful, modern editor (unlimited free trial).
  • Kate, Tea, Jed: Other graphical options.

TIP

People can be passionate about their editor of choice. Try a few and see what works best for you.

Vi, Vim, Neovim

Lightweight with a high learning curve, but fast once learned.

:q	to quite from vim
:wq	to write change and quit
:q!	to quit without saving
ESC	to exit from editing mode
i	to insert
x	to delete a char
dd	to delete a line
/	to text search

• The same on every machine.
• See the cheat sheets.
• Type vimtutor in the terminal for an iterative tutorial.

Meme emphasizing the speed and clarity users have with the tried-and-true Vi or it's directives.

Example of using Vim to edit C++ code

EMACS (Editor MACroS)

• Lots of features -- tons of capabilities
• LISP
• Your hand may be warped from all the hotkeys.
• Not always the same on every machine

EMACS is powerful and highly customizable.

nano

• Simple! The same on every machine.
• The commands are at the bottom of your screen.
  • ^ stands for Ctrl.
  • Ctrl + G displays the help screen.
• Good for quick edits, but not powerful enough for software development.

Example of using nano to edit C++ code.

Don't use nano as your primary editor for coding.

Directory Operations

ls -- List files and directories.

ls displays a list of files in the current working directory, like dir in DOS.

-a	Display hidden (start with a ‘.’) files.
-A	Do not display ‘.’ and ‘..’.
	‘.’ means current directory.
	‘..’ means upper layer directory.
-l	Display file lists with long format.
-s	Display file size.
-h	Display sizes in a more human readable format.

cd -- Change directories (change the working directory).

Usage: cd <DIRECTORY>

cd /usr/bin	Change the directory to /usr/bin.
cd ..	Change to the parent directory.
cd /	Change to the root directory.
cd ~	Change to the user’s home directory.

Making Directories

mkdir DIRECTORY -- Create the DIRECTORY(ies) that do not already exist.

mkdir temp	Make a directory named temp.
mkdir temp2	Make a directory named temp2
mkdir temp/insideTemp	Make insideTemp directory within temp.

Removing Directories

rmdir DIRECTORY -- Remove the DIRECTORY(ies) if they are empty.

rmdir temp	Remove a directory named temp.
rmdir temp2	Remove a directory named temp2
rmdir temp/insideTemp	Remove the insideTemp directory that is within temp.

rm -r DIRECTORY -- Remove directories and their contents recursively.

Directory Usage

• Any command that accepts a file or folder as a parameter assumes that the file is in the present working directory (PWD) unless told otherwise.

• Use relative or absolute paths to indicate where a file is.

• Absolute paths start with a / and the path begins from the highest level.

• Relative paths NEVER start with a / and are how to get to the file from the PWD.

• .. is your parent directory, useful in relative paths

File Operations

cp -- Copy SOURCE to DEST. Usage: cp [OPTIONS] SOURCE DEST

• cp data1.txt data2.txt

• cp data3.txt temp

• Options:

  -i	Ask before overwrite.
  -v	Display copying process.
  -R	Recursive copy, including sub-directories.

• cp -R * backup -- Copy all files in current directory into backup.

rm -- Remove the FILE(s).

Usage: rm [OPTIONS] [FILES]

• rm data1.txt

• rm *

• Options:

  -f	Force remove files.
  -r	Recursive remove files, including sub-directories.
  -v	Display removing process.

• rm -f *.txt -- Removes all files ending in .txt from the current directory.

• rm -r * -- Removes all files and folders from the current directory.

Other File Operations

• mv -- Move files or rename a file.

  • mv data1.txt ..

  • mv data1.txt data4.txt

• pwd -- Print (present) working directory.

• file -- Display file type.

  • file filename

  • file *

• head -- Displays the first 10 lines of each FILE.

  • head data1.txt shows first 10 lines of data1.txt

  • head -20 data1.txt shows first 20 lines

• tail -- Displays the last 10 lines of each FILE.

  • tail data1.txt shows last 10 lines of data1.txt

• wc -- Displays line count, unique word count, and total word count for each FILE (and a total line if more than one FILE is specified).

• find -- Find files and directories (and perform operations on them).

  • -name -- the filename you are looking for

  • -print -- output results

  • find /usr -name config -print
    Displays where all the files named compress are located.

• grep -- Search for PATTERNS in each FILE.

  • Usage: grep [OPTIONS] PATTERNS [FILE].

• ln -- Create a link to TARGET with the name LINK_NAME.

  • Usage: ln [OPTIONS] TARGET LINK\_NAME

  • -s Create a symbolic link.

  • ln -s target.txt newname.txt

  • More later on linking.

Redirects

Input/Output (I/O)

There are three standard input/output devices.

stdin	keyboard input
stdout	Monitor output
stderr	Error output device

Input/Output for a Process

Input/Output for a Process

The Pipe Operator (|)

• Redirect output of one program to another.

• For Example,
  ls -al /etc | more

  

  The output of ls is piped as the input to more

  

  The output of ls is piped as the input to more

• This can continue on

• What Data Structure does this look like?

I/O Redirect to Files

• Use < and > to redirect I/Os

• cat > output.dat

• Use Ctrl-d to quit

• cat >> output.dat

  • Append new output to the end of output.dat.

• cat !> output.dat

  • Force overwrite output.dat.

• more -- Pause viewing output.

  • ls -al | more

  • more data1.txt

  • Type :q to exit.

• less -- Hmmm, what does less do?

• cat -- Display or concatenate files.

  • cat data1.txt | more

  • cat data1.txt >> data2.txt

  • cat data1.txt data2.txt >> data3.txt

• tac -- Hmmm, what does tac do?

Looking up how a command works.

• The --help argument displays how to use (almost) any command.

  • ls --help

• man -- Display the manual for a command.

  • man ls

• info -- Displays the information about a command.

  • info ls

Regular Expressions (RegEx)

Sets of special characters that help you match a pattern.

*	All possible combination(s)
	des* – desk, desktop, description,
?	Any single character
	des? – desk, des9,
	ls -al *.ps
	cat ??go
	more *a*b?
[ ]	Symbols in the square brackets
	ls -al [ms]oon displays files named: moon and soon
	more [a-p]ount displays contents of files named:
	aount, bount, count, , pount

User Administration

User-Management Tools

• useradd -- Adds a new user account to the system. Its options permit the sysadmin to specify the user's home directory and initial group or to create the user with the default home directory and group assignments.

• useradd -D -- Displays default settings for new users

• useradd -G -- Sets the system defaults for creating the users' home directory, account expiration date, default group, and command shell. See the specific options in man useradd. Used without any arguments, it displays the defaults for the system. The default set of files for a user are found in /etc/skel.

  • ls -al /etc/skel -- will list the files with the defaults.

• userdel -- Will completely remove a user's account (thereby eliminating that user's home directory and all files it contains).

• passwd -- Change a user's password.

• usermod -- Changes several user attributes. The most commonly used arguments are -s to change the shell and -u to change the UID. No changes can be made while the user is logged in or running a process.

• chsh -- Changes the user's default shell. For Fedora, Debian, and others, the default shell is /bin/bash, known as the Bash (or Bourne Again Shell).

useradd winnie -p gu1tarplayeR -s /bin/bash -u 507

Adding a user with a GUI.

Group Hug

Groups establish relationships among users where they
share a common set of permissions.

• All the groups are listed in /etc/group file.

• Group-Management Tools:

  • Add a new group with the groupadd command.
    groupadd www-data

  • Change the group ownership of a file with the chgrp command.
    chgrp www-data /srv/www

  • Add the approved user to the group with usermod.
    usermod -G www-data shelley

  • Make shelley the group administrator with gpasswd, so that she can add new users to the group.
    gpasswd -A shelley

User Accounts

• Usually, /etc/passwd holds user account information. Each user, regardless of type, will have a one-line entry of account information stored in this text file.

• The superuser is known by the name root.

• User IDs and Group IDs

• File Permissions:

  chgrp	Changes the group ownership of a file.
  chown	Changes the owner of a file.
  chmod	Changes the access permissions of a file.

• The password file is /etc/passwd, and it is the database file for all users on the system. The format of each line is as follows:
  username:password:uid:gid:gecos:homedir:shell

• Example:

  root:x:0:0:root:/root:/bin/bash
  sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
  rpc:x:32:32:Portmapper RPC user:/:/sbin/nologin
  rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/sbin/nologin
  nfsnobody:x:65534:65534:Anonymous NFS User:/var/lib/nfs:/sbin/nologin
  mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin

File Permissions using chmod Try ls -l.

- rwxr-xr-x joe acctg archive.sh
- rw-rw-r-- joe acctg orgchart.gif
- rw-rw-r-- joe acctg personnel.txt
- r--rw-r-- joe acctg public.html
d rwxr-xr-x joe acctg sales
- rw-r----- joe acctg topsecret.inf
- rwxr-xr-x joe acctg wordmatic

The first set of three letters after the file type describes the permissions the owner of the file has.

An r in the first position means you are permitted to read the file.
A w in the second position means you may write to or delete the file.
An x in the third position means you may execute the file.

A hyphen represents a denied permission at its position.

File Permissions using chmod The most succinct way to use chmod is with numbers. Each permission is assigned a value:
r = 4, w = 2, x = 1, Therefore,

Value	Meaning	Binary
0	No permission	000
1	Execute only	001
4	Read only	100
5	Read & Execute	101
6	Read & Write	110
7	Full permission	111

Using chmod With chmod you have three numbers: first for the owner, second for group, third for everyone else.

Let's look at some examples.

Before:	-rwxr-xr-x archive.sh
Command:	chmod 754 archive.sh
After:	-rwxr-xr-- archive.sh
Before:	-rw-r--r-- topsecret.txt
Command:	chmod 600 topsecret.txt
After:	-rw------- topsecret.txt
Before:	-rw------- publicity.html
Command:	chmod 665 publicity
After:	-rw-rw-r-- publicity.html

Become Root!

• Temporarily changing the user identity with the su command.
  su - root

• In many Linux distributions, the root user is disabled by default. Instead, sudo can be added to the beginning of a command to temporally grant the command root permissions. This gives admins greater control of what has the power to do anything on the system.

System Information

• Get the name, version, and other details about the current machine and the operating system.
  uname -a

• The -a argument stands for all. Other arguments provide more specific information.

Lab 01

On Blackboard:

1. Follow the instructions for Lab 01.

2. This lab will be manually graded.

3. Future labs will be submitted via your fork of the class git repository.

Don't forget:

0. Submit your GitHub username on Blackboard so I can grant you access.

1. Fork repository from GitHub.

2. Clone your forked copy to your computer.