CpE001L Final Lab Lecture 2 - Linux Files and File Permissions

7/29/2019 CpE001L Final Lab Lecture 2 - Linux Files and File Permissions

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CpE001L Final Lab Lecture 2

by: Engr. Ricrey E. Marquez, CpE, MSCS


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Lecture Objectives

After this lecture, students should be

able to:

identify the Linux file system,

execute basic file system commands, and

give permission to Linux file system.


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PART IWorking with Linux Files


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Linux Files

File

a collection of data

a stream of characters or a "byte

stream"

no structure is imposed on a file by the

operating system


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Linux File Types

Figure 1. Linux File Types


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Linux File Types

Special Characters for Different File Types


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Linux File Types

Regular files are the most common type of files store any kind of data(data can be stored as plain text, an application- specific format, or aspecial binary format that the system can execute)

Symbolic link is a special file that points to another file on the system. Device files are access points to the device within the file systems. two

main types of device files are:

Character special files - provide a mechanism for communicating with adevice one character at a time

Block special files - also provide a mechanism for communicating withdevice drivers via the file system

Socket files are created when communication to a process on anothermachine located on a network is required.


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Linux File Types

In Linux/UNIX there are three basic types of files:

Ordinary Files - a file on the system that contains data, text,or program instructions. e.g. myfile.txt, myscript.sh

Directories - store both special and ordinary files. For usersfamiliar with Windows or Mac OS, Linux/UNIX directoriesare equivalent to folders. e.g. /myfolder, /yocirem, /home

Special Files - provide access to hardware such as hard drives,CD/DVD-ROM drives, modems, and Ethernet adapters. Otherare similar to aliases or shortcuts and enables to access a singlefile using different names. e.g. .profile, /dev, /dev/sda


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Linux Filenames

Some important facts about Linux filenames:

Should be descriptive of the content (readable)

Should only use alphanumeric characters (uppercase, lowercase,number, @, _ )

Should not include embedded blanks

Should not contain shell metacharacters: * ? > < / ; & ! [ ] | \ ' " () { }

Should not begin with + or - sign

Case sensitive (uppercase is different from lowercase letters)

Filenames starting with a . are hidden files

Maximum number of characters for a filename is 255


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Linux Pathnames

Full Pathnames

A pathnames that starts from / or /root (theroot directory)

Relative Pathnames

A pathnames that starts from the presentworking directory


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Linux Pathnames

Figure 2. Example Linux Directory Structure Types


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Example Linux Pathnames

Assume that the working directory is at/home/tux1

/home/tux1/doc/mon_report (full)

doc/mon_report (relative)

../tux3/pgms/suba (relative)

./test (a file in the current dir)

~/test (same as above)


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Example Hidden Files

An invisible/hidden file is one whose first

character is the dot or period character (.).Some common examples of hidden files include

the files:

.profile (the Bourne shell ( sh) initialization script)

.kshrc (the Korn shell ( ksh) initialization script) .cshrc (the C shell ( csh) initialization script)

.rhosts (the remote shell configuration file)


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Listing Files

ls - list the files and directories stored in the currentdirectory. With the ls command: $ ls dir/file

$ ls /hometux1 tux2 tux3

Important options

-l - long listing (more information) -a - lists all files (including hidden)

-t - lists files sorted by change date

-R - lists contents recursively

-F - appends a character indicating the file type of each of the items itlists


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Knowing the Current Directory

pwd (print working directory) -

command used to find out what yourcurrent working directory is.

$ pwd/home/tux1


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Changing Current Directory

With the cd (change directory) command: $cddirname

$ cd doc (relative)$ cd /home/tux1/doc (full)$ cd ~tux1/doc (home)

$ cd (Go to your home directory)$ cd .. (Go one directory up)$ cd - (Go to previous directory)


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Creating Directories

With themkdir (make directory)command:$ mkdir dirname

$ mkdir /home/tux1/doc (full pathname)$ cd /home/tux1

$ mkdir doc (relative pathname)


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Removing Directories

With the rmdir (remove directory) command: $rmdir dirname

$ pwd/home/tux1$ rmdir doc test

rmdir: doc: Directory not empty

Note: directory must be empty


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Working with Multiple

Directories

Create and remove multiple directoriessimultaneously with the -p flag

$ mkdir -p dir1/dir2/dir3 or$ mkdir dir1 dir2 dir3

$ rmdir -p dir1/dir2/dir3 or$ rmdir -r dir1 dir2 dir3


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Modifying File Time

touch command creates an empty file, or updates the modification time of anexisting file

$ ls -l-rw-rw-r-- 1 tux1 penguins 512 Feb 24 11:10 docs

$ touch docs$ ls -l-rw-rw-r-- 1 tux1 penguins 512 Mar 5 15:37 docs

$ touch new$ ls -l-rw-rw-r-- 1 tux1 penguins 512 Mar 5 15:37 docs-rw-rw-r-- 1 tux1 penguins 0 Mar 5 15:37 new


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Copying Files

cp command copies files: cp source/starget

Copying one file to another:$ cp .bashrc bashrc.old

Copying multiple files into a target directory:$ cp doc/mon_report doc/walrus/tmp


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Copying Files

cp can recursively copy directories with the-R flag

$ cp -R /home/tux1/doc /tmp

To prevent cp from overwriting existingfiles, use:

$ cp -R -i /home/tux1/doc /tmp

cp: overwrite `/tmp/doc/walrus?


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Removing and Renaming Files

With themv command: $ mv source/s target

To move a file do another directory:$ mv doc/walrus ../../tmp

To rename a file:

$ mv doc documents

Use the -i option to prevent mv from overwriting existingfiles


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Removing and Renaming Files

Moving and renaming files can be combined bymv$ cd

$ pwd/home/tux1$ mv /tmp/walrus ./test/rob

To move a directory:$ mv ./test /tmp

Note:mv is recursive by default


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Listing the File Contents

With the cat (concatenate) command: $ cat file1file2 ...

$ cat walrus"The time has come", the walrus said,"To talk of many things:Of shoes - and ships - and sealing wax -

Of cabbage - and kings -And why the sea is boiling hot -And whether pigs have wings."


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Creating File Content with cat

With the cat (concatenate) command: $ cat > file1

$ cat > walrus2Welcome to LinuxBasic Linux ShellEnd of the test text

$cat walrus2Welcome to LinuxBasic Linux ShellEnd of the test text


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Displaying Page by Page

With themoreor less commands: $ less/morefilename

"The time has come", the walrus said,"To talk of many things:Of shoes - and ships - and sealing wax -Of cabbage - and kings -

And why the sea is boiling hot -And whether pigs have wings."/tmp/test/walrus 1-6/6 (END)


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Displaying Binary Files

With the od command

With the strings command:

$ strings /usr/bin/passwd/lib/ld.so.1

__gmon_start____deregister_frame_info__register_frame_info


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Removing Files

With the rmcommand: $ rm filename$ ls test/rob

ls: rob: No such file or directory

If unsure, use -i option$ rm -i test/robrm: remove `test/rob?

To remove files and directories recursively:$ rm -ir test


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Splitting Files

You can split a file into smaller files with the split command $ split -b file [prefix]

$ ls -l-rw-r--r-- 1 root root 4194304 Feb 21 13:31 large

$ split -b 1024k large large.

$ ls -l

-rw-r--r-- 1 root root 4194304 Feb 21 13:31 large-rw-r--r-- 1 root root 1048576 Feb 21 13:33 large.aa-rw-r--r-- 1 root root 1048576 Feb 21 13:33 large.ab-rw-r--r-- 1 root root 1048576 Feb 21 13:33 large.ac-rw-r--r-- 1 root root 1048576 Feb 21 13:33 large.ad


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PART IILinux File Permissions


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File Permission

Linux files are setup so access to them is controlled. There are threetypes of access:

read (r)

write (w)

execute (x)

Each file belongs to a specific user (u) and group (g).

Access to the files is controlled by user, group, and what is calledother(o).

The term, other, is used to refer to someone who is not the user(owner) of the file, nor is the person a member of the group the filebelongs to.


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File Permission

Setting permissions for "other" users to use, it is commonly referredto as setting the world execute (x), read (r), or write (w) bit sinceanyone in the world will be able to perform the operation if thepermission is set in the other category.


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File Names and Permission

Characters

File names can be up to 256 characters long with "-","_", and "." characters along with letters andnumbers.

When a long file listing is done, there are 10 charactersthat are shown on the left that indicate type andpermissions of the file. File permissions are shownaccording to the following

Syntax: d rwx rwx rwx


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Characters

Permissions

File permissions are assigned to:

owner of a file

members of the group the file is assigned to

all other users

Permissions can only be changed by the owner androot


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Characters

Viewing Permissions To show the permissions of a file, use the ls command with

the -l option


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Characters

Permissions Notation


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Characters

Required Permission


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Changing File Permissions

To change the permission of a file use the chmod command

Syntax: chmod[MODE] [FILE/S]

Mode can be symbolic

$ chmod go-rx /home/tux1$ ls -ld /home/tux1drwx------ 4 tux1 users 512 Jan 5 12:43 /home/tux1

Mode can be octal:

$ chmod 700 /home/tux1$ ls -ld /home/tux1drwx------ 4 tux1 users 512 Jan 5 12:43 /home/tux1


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Table 1.

whoTable 2.

Permission

Table 3.

Action


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Calculating numeric (octal) mode:


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umask Command

New files should not be created with 666, to avoid this problem apermission mask exists. Syntax: $ umask 022


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Set User Identification Attribute

File permissions bits include an execute permission bit for file

owner, group, and other. When the execute bit for the owner is set to "s" the set user ID

bit is set.

This causes any persons or processes that run the file to haveaccess to system resources as though they are the owner of the

file.

When the execute bit for the group is set to "s", the set groupID bit is set and the user running the program is given accessbased on access permission for the group the file belongs to.


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The following command:

$ chmod +s myfile- sets the user ID bit on the file "myfile".

$ chmod g+s myfile - sets the group ID bit on the file "myfile".

The listing below shows a listing of two files that have the group or user IDbit set.

-rws--x--x 1 root root 14024 Sep 9 1999 chfn

-rwxr-sr-x 1 root mail 12072 Aug 16 1999 lockfile

Note: The files chfn and lockfile are located in the directory "/usr/bin".

CpE001L Final Lab Lecture 2 - Linux Files and File Permissions

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