Class Objectives

• Linux filesystem
  • File permissions
• Using Linux commands
• Compiling C programs

Special Symbols

• . (single period) represents current directory
• .. (two periods) represents directory above current directory
• Files and directories starting with . (single period) are hidden

Path Examples 1

• Absolute path (starts with /)

  /home/w123123/images/cats

  • Is “images” a directory or a file?
  • Is “cats” a directory or a file?
• Relative paths (no starting `/`, path gets added to current path)

  w123123/images/cats

  ../images/cats/my_cat.jpg

  • Remember that .. indicates the directory above the current directory

Path Examples 2

• Assume you are in /home/user (current directory)
• Which of the following are equivalent?
  1. /home/user/foo
  2. foo
  3. ./foo
  4. ../user/foo
  5. foo/../foo
• You can get the current path with `pwd`

Path Summary

• Directory structure is a tree where / is the root
• Paths can be relative to the current directory or absolute
• It is not possible to distinguish files from directories from the path
• . and .. are special characters for current path and up one directory
• When you log in, your directory is set to your home
• You can get the current path with pwd

Representation of Permissions

• Represented as a combination of one character and three triads of characters
  • Each triad represents rwx (read-write-execute)
  • User, group and other have their own triads
  • First character indicates file type (- is regular file) ```sh

-rwxr–r– 1 user group 0 Sep 19 17:10 test_file*

* Can see permissions with `ls -l`
* Can change permissions with `chmod` and ownership with `chmod`

Changing Permissions

• You can change permissions with chmod

  $ chmod {ugo}{+-}{rwx}

• Remove read access from group

  $ chmod g-r FILE

• Add execute access to other

  $ chmod o+x FILE

• Add read and write to group

  $ chmod g+rw FILE

Important Points

• You need read permission to be able to read the contents of the file
• You need write permission to be able to write to the contents of the file
• You need execute permission to be able to execute the file as a command
• To see the contents of a directory, you need both read and execute permission

Shell Basics

• General usage of the shell is to type commands and hit to execute them
• Commands have optional or required parameters
• General syntax is

  $ command [options] arguments

• The command prompt shows useful information like current directory (it can be customized)
  • The $ at the end of the prompt indicates you are are logged in as a regular user
• If the command fails, an error warning will be printed

The PATH

• How does the shell know how to interpret and run programs?
  • It searches programs that match your command in the $PATH
  • You can see your $PATH with

    $ echo $PATH

• To run local programs you must use an explicitly relative path starting with ./
  • Example:

    $ ./my_local_program

Basic Commands 1

• pwd

  $ pwd

  • Tells you the current directory
  • Useful in scripts
• List

  $ ls [dir]

  • Lists what is in the directory [dir]
  • If [dir] is not specified, lists current directory
• Change directory

  $ cd [path]

  • Changes the working directory to [path]
  • If [path] is not specified, changes to the home directory

Basic Commands 2

• Copy

  $ cp SOURCE DEST

  • Copies SOURCE to DEST
  • If DEST is a directory, will put SOURCE inside it
  • If DEST does not exist, it will rename the copy of SOURCE to DEST
• Move

  $ mv SOURCE DEST

  • Sames as copy, but removes SOURCE on success
• Remove

  $ rm FILE

  • Removes FILE permanently
  • You can list more than one file with separated by spaces

Basic Commands 3

• Make directory

  $ mkdir PATH

  • Creates a new directory at PATH
• Remove directory

  $ rmdir PATH

  • Removes a directory at PATH
  • Directory to remove must be empty
• File viewer

  $ less FILE

  • Viewer for text files
  • Can not edit the file
  • Use arrows keys and spacebar to move
  • q exits

Basic Commands 4

• Online reference manual

  $ man PAGE

  • Lets you look up documentation
  • Very useful when you forget commands
  • Opens up in the less file viewer
  • Exit with q
• For example try the following command:

  $ man man

Command Options

• It is possible to change command options with flags that start with -
• A description of the options and what they do are shown in the manual pages
• Some useful examples
  • Detailed list of files

    $ ls -l

  • Display all directory files (including hidden files)

    $ ls -a

• It is possible to combine flags with a single hyphen

  $ ls -al

Keybindings

• Moving the cursor
  • Ctrl+p, Ctrl+n (↑, ↓)
  • Ctrl+b, ctrl+f (←, →)
  • Ctrl+a (beginning of line)
  • Ctrl+e (end of line)
• Select
  • Ctrl-Space
• Cut
  • Ctrl-Space, Ctrl-w
• Copy
  • Ctrl-Space, M-w
  • For M-w, hit escape then w, or hold alt and press w
• Paste
  • Ctrl-y
• Save
  • Ctrl-x Ctrl-s

Practice

• Let’s try writing the sample C program I showed at the beginning
• Write the following in vim and save it to a file hello_world.c

  #include <stdio.h>
  int main( int argc, char *argv[] )
  {
   printf("Hello World!\n");
   return 0;
  }

• Once you have written it, try saving and exiting emacs, then loading the file again

C Code Compilation

• We shall now compile the previous file we created with emacs and run it
• Compile it using the GNU C Compiler (gcc)

  $ gcc hello_world.c -o hello_world

• Check to see if the file has been created and if it has executable permission

  $ ls -l

  • You should see a hello_world file
• Try to run the file

  $ ./hello_world

Next Class

• How to write a C program
• How to use variables in C programs
• Assignments and arithmetics with variables
• Using the standard input/output