Linux Overview

Linux

• Linux is an operating system, like Windows or OSX.

• Linux and Kutztown University

  • The Computer Science department has a Linux server named csitrd that CS and IT students can access

  • This is where you will do your assignments for this class

  • The interface to the Linux server is a command line interface, not a graphical user interface

Connecting to csitrd

• Windows: Open the command prompt program

• OSX: Open the terminal program

• In the command prompt or terminal program

  • Run the command:

    ssh username@host

  • Your username is the first part of your KU email address

  • The host name is csitrd.kutztown.edu

  • Example:

    ssh abcde123@csitrd.kutztown.edu

Connecting to csitrd (Continued)

• The first time you connect to csitrd you may see something similar to:

  The authenticity of host 'csit.kutztown.edu (156.12.127.18)' can't be established.
  ECDSA key fingerprint is SHA256:2oJ7zjD4/XbLbyWwWbv15+GxAwdQdittOcm0Jsv4cDg.
  Are you sure you want to continue connecting (yes/no/[fingerprint])?

  • If you see this, type in “yes”, then press enter.

• Then you will be prompted for password. Your password is the same password that you use to access other Kutztown resources. Type in your password, then press enter.

• On a successful login, you should see something similar to:

  Last login: Sun Sep 26 12:16:10 2021 from kuvapcsitrd01.kutztown.edu
  [user@kuvapcsitrd01 ~]$

Command Line

• A command line is a text based interface to the operating system

• Commands are entered by typing the command on the keyboard then pressing enter

• The command line presents you with a prompt; as a command is typed it is displayed after the prompt.

• Example: in the previous slide, the prompt is:

  [user@kuvapcsitrd01 ~]$

Directories and Files

• Linux uses directories (similar to Windows folders) to organize files

• Directories can contain files and other directories (called subdirectories)

• A directory is a also a file; it is a special file that can contain other files

• A directory or file that has a name that starts with a . (dot) is called a hidden directory or file

• Directories and files should be named using the letters, digits, underscores, and dots; other names are possible, but are not as nice to deal with using the command line interface

Directory System Structure

• The Linux file system is hierarchical; all directories and files are organized in a tree-like structure with the directories corresponding to branches and the files corresponding to leaves

• The topmost directory is call the root directory and is denoted with / (slash)

• Subdirectories are located “under” the root directory

• The working directory is the current location in the file system

• When you first log in, your working directory is your home directory

Absolute Path Names

• Every directory or file has a full name called an absolute pathname

• An absolute pathname uniquely identifies a specific directory or file

• Absolute pathnames always start with a slash (/) followed by any subdirectories in its path separated by slashes and ending with the name of the directory or file

Absolute Path Name Example

• The following absolute path is similar to the absolute path of your home directory

  /home/students.kutztown.edu/username/f.txt

• This can be read as:

  • There is a directory under the root directory named home

  • that contains a subdirectory named students.kuztown.edu

  • that contains a subdirectory named username

  • that contains a file named f.txt

Relative Path Names

• Directories and files can also be referred to using relative pathnames

• A relative pathname is a pathname relative to the working directory

• Example: If the working directory is username and contains a file named f.txt, then the relative path for f.txt is simply f.txt

Linux Commands

• A Linux command consists of three parts:

  • Command: name of the command; this always comes first

  • Options (or flags): an flag has a dash (-) in it; flags are usually optional and alter the way the command executes relative to its default behavior

  • Arguments: some commands need specific information to run, for example a file name, which are specified as command line arguments

Conventions for Command Descriptions

• The following conventions will be used in the command descriptions that follow:

  • [] (square brackets) are used to indicate that something is optional

  • <> (angle brackets) are used to indicate a path (relative or absolute)

  • $ (dollar sign) indicates the prompt in examples

pwd

• Print the absolute pathname of the working directory

• Example:

  $ pwd
  /home/students.kutztown.edu/username

ls

• List the contents of a directory

  • ls: list the contents of the working directory

  • ls <directory>: list the contents of the specified directory

  • ls -l [<directory>]: list the contents in long format

  • ls -a [<directory>]: list the contents including hidden directories and files

cd

• Change directory

  • cd: change to home directory

  • cd <directory>: change to the directory indicated by the path <directory>

• Special directory names

  • . (dot): the working directory

  • .. (dot dot): the parent of the working directory

  • ~ (tilde): your home directory

file

• Determine the file type

  • file <path>: determine the file type of the file at <path>

• Example:

  $ file f.txt
  f.txt: ASCII text

mkdir

• Make directory

  • mkdir <dirname>: create the directory at the path <dirname> if it does not already exist in the parent directory

  • mkdir -p <dirname>: create the directory at the path <dirname> and also create all non-existing parent directories in the path

rmdir

• Remove directory

  • rmdir <path>: remove the directory at <path> as long as it is empty

• Note: the rmdir command will permanently remove the directory; you cannot undo this action

touch

• Create an empty file

  • touch <filename>: create an empty file named <filename>

• Example:

  $ touch a.txt
  $ ls
  a.txt

cp

• Copy files and directories

  • cp <source> <destination>: copy the file at <source> to <destination>

  • cp -r <source> <destination>: copy the directory at <source> to <destination>

mv

• Move (rename) a file or directory

  • mv <source> <destination>: move (rename) the file or directory at <source> to <destination>

• Note: unlike the cp command, the mv command does not need the -r flag when moving (renaming) a directory

rm

• remove files or directories

  • rm <file>: remove the file at the path <file>

  • rm -r <directory>: remove the directory at path <directory> and its contents

  • rm -i <file>: (RECOMMENDED) remove the file at the path <file> in interactive mode; this will prompt the user before removing the file

  • rm -rI <directory>: (RECOMMENDED) remove the directory at the path <directory> in interactive mode; this will prompt the user before removing the directory and all its contents

• Note: the rm command will permanently delete the file or directory; you cannot undo this action

nano

• nano is a command line text editor

  • nano <filename>: open the file <filename> in a nano interface; if the file <filename> does not exist, then it is created

• A text editor is a program that allows a user to edit the contents of a text file

• There are several text editor programs installed on the Linux server; nano is the simplest one for beginners

The nano interface

The nano interface (Continued)

• The top line of the interface has:

  • The name of the program and version number

  • The name of the file

  • An indicator that the file has been modified since it was last saved

• The third line from the bottom is a “system message”

• The last two lines are the shortcut lines; this is what makes nano more user-friendly compared to other Linux text editors

nano Shortcuts

• All nano shortcuts are prefixed with either ^ (caret) or M

• ^ refers to the control key; when typing a control sequence, you must hold down the control key and press the accompanying key at the same time

• M refers to the alt key; when typing a control sequence, you must hold down the alt key and press the accompanying key at the same time

• Example: ^G means hold down the control key and press g (not shift+g)

Common nano Shortcuts

• ^S: save

• ^O: save as

• ^X: exit (if the file is modified a prompt will appear)

• ^G: display the help text

g++

• The GNU compiler; compiles C++ source files

  • g++ <filename>: compile the source code in <filename>; produces an executable file named a.out on success and error messages on failure

  • g++ <filename> -o <out filename>: name the output of the successful compliation <out filename> instead of the default a.out name

  • g++ -g <filename>: produce an executable that is easier to debug in the debugging program

• The executable a.out can be run with ./a.out if the working directory contains the a.out file. Recall that . (dot) is a special name for the working directory