File Management, File Attributes & Permissions

 

File Management

In Linux file management, we will learn about

  • Creating files
  • Copying files
  • Moving files
  • Removing files
  • Display the contents of files

File Management: Creating Files

Creating Files
Always remember that in Unix/Linux everything is treated as a file.

There are 7 types of file, that says the directory is also treated as a file.

Command to create a regular file in Linux

touch  filename

  Command to create a directory in Linux

mkdir  filename

Create a file sample.h under usr directory

touch /usr/sample.h

Create a directory cse under usr directory

mkdir  /usr/cse

 
Create a file with the cat command
Type the command

cat  > name_of_file

Now type in your text. Press the <Return> key to start a new line.

When you have finished typing in your text, enter Ctrl-d (Press and hold down the Ctrl key and type a “d”).

This stops the cat command and returns you to the system prompt.

 

Creating Files
Take care while creating a file using cat
Do not use the name of a file that already exists. If you do, it will be overwritten by the new copy.To avoid this:Set the shell variable “noclobber” so that existing files cannot be overwritten (or clobbered).set noclobber

File Management: Copying Files

To copy a file use cp command : 

cp source_path_filename  destination_path_filename

cp /usr/include/stdio.h  ~/myheader.h

 

If you are already in the directory where source file resides 

cp   stdio.h       ~/myheader.h

cp  ./stdio.h     ~/myheader.h

 

Note:   “~” refers to your home directory
To copy the contents of a directory, use the recursive option (-r). Because a directory can itself contain directories with subdirectories.

cp –r source_directory  destination_directory

cp –r /usr/include/  ~

 

If you are already in the directory where source file resides 

cp  -r  ./      ~

 

 

 

File Management: Moving Files

 

Moving Files
Move the file

  • To move the contents of a file to another.
  • The source file does not exist after the move operation.
  • It can also be used for renaming a file.

mv source_filename destination_filename

mv  /usr/sample.txt  /usr/sam.txt

  •  It is also used to move the directories.
  •  And also you can rename the directories.
  •  In the above two cases we need permission that we will study later.

 

Moving Files
  • To remove a file

rm –i filename

rm –f filename

–f option deletes the file without asking (non-interactive)

 -i option ask confirmation to proceed (interactive)

  • To remove a directory

rm dir_name

rmdir dir_name

     removes empty directory (can only delete empty directory)

rm –rf dir_name

      This will delete the directory including all files and  sub-directories (non-interactive)

 

File Management: Display the contents of a file

Display the contents of a file
To display the content of a file on the terminal itself

cat  path_of_filename

To display the contents of a file with the line number in front of each line, use option  -n.

cat  -n path_of_filename

Displaying file contents in reverse.

tac  path_of_filename
Other use of cat
Copy file contents

cat source_program_file  > backup_program_file

Concatenate Contents of Multiple Files

cat program1  program2  >  all_program

 

Commands covered in File management

      • touch
      • mkdir
      •  cp
      • mv
      • rm
      • rmdir
      • cat

 

File attributes and Permissions

In Linux File attributes and Permissions, we will learn about

  • ls  –l:  Listing File Attributes
  • The –d option: Listing directory attributes
  • File ownership
  • File permissions
  • chmod: Changing File Permissions
  • Directory Permissions
  • Changing File ownership: chown & chgrp

 

File attributes and Permissions:  Listing File attributes ( ls –l )

Sample output of  ls  –l 

list -l command
list -l command
1st COLUMN File Type and Permissions
2nd COLUMN No. of Links
3rd COLUMN Name of Owner
4th COLUMN Group Ownership (Name)
5th COLUMN File Size
6th COLUMN Last Modification time
7th COLUMN Filename

File attributes and Permissions: Listing Directory attributes ls –ld

Sample output of ls  –ld dir_name

Listing Directory attributes ls –ld
Listing Directory attributes ls –ld

 

File attributes and Permissions:  File ownership

File ownership

File attributes and Permissions:  File ownership
File attributes and Permissions:  File ownership
Referring to the above output of  ls –l command we can see

      • 3rd column refers to name of the owner.
      • 4th  column refers to the group name.

When the system administrator creates a user account, he has to assign these parameters to the user.

      • The user-id (UID) both its name and numeric representation.
      • The group-id (GID) both its name and numeric representation.

“/etc/passwd” file   maintains

      • UID (both number & name)
      • GID (only number)

“/etc/group” file maintains

      • Both the number & name of the group

File attributes and Permissions:  File Permissions

File Permissions User Group Others
File Permissions: User Group Others

User Permission refers to the owner of the file.

Group Permission refers to common users of file.

Other Permission refers to the rest of the world.

 

File attributes and Permissions:  File permissions
File

      • Read permission on a file allows you to read, display and copy the contents of a file.
      • Write permission on a file that allows you to write to a file.
      • Execute permission allows you to execute a file, such as executable program, shell program, etc..

Directory

      • If you have write permission for a directory, you can create new entries (files/folders/etc).
      • If you have read permission for a directory, you may list ls the directories contents.
      • If you have execute permission for a directory, you may switch to that directory (cd).

File attributes and Permissions:  File permissions
The difference in access permissions for files and folders
Access type File Folder
Read If the file contents can be read If the directory listing can be obtained.
Write If user or process can write to the file (change its contents) If the user or process can change directory contents somehow: create new or delete existing files in the directory or rename files.
Execute If the file can be executed If the user or process can access the directory, that is, go to it (make it be the current working directory).

 

File attributes and Permissions:  File permissions
Examples
–rwxr-xr-x File,
owner has read, write, execute permissions,
group: only read and execute permissions,
others: only read and execute permissions.
dr-x-—– Directory,
owner has read and execute access,
group and others have no access

 

File attributes and Permissions:  chmod (changing File permission)
  • The chmod (change mode) command is used to set the permission of one or more files, for all three categories of users (user, group and others).
  • It can run only by the user(owner) or the superuser

The command can be used in two ways

      • In a relative manner by specifying changes to the current permissions.
      • In an absolute manner by specifying the final permissions.
Relative Permission (Symbolic code)

Relative Permission (Symbolic code) user group others all ugoa read write execute rwx
Relative Permission (Symbolic code)
user group others all ugoa read write execute rwx

 

 

File attributes and Permissions: chmod Example ( Relative Permission)
chmod category operation permission filename

chmod category operation permission filename
chmod category operation permission filename

 

 

File attributes and Permissions:  chmod Octal Permission codes
Absolute Permission (Octal code)

      • Sometimes you don’t need to know what file permissions are.
      • Want to set all 9-bit permissions explicitly.
      • If we represent the permission of each category by one octal digit, this is how permission can be represented.
Octal Permission Basic codes ( read , write and execute)
Read Permission Write Permission Execute Permission
4 (Binary 100) 2 (Binary 010) 1 (Binary 001)

 

File attributes and Permissions:  chmod Octal Permission  codes
Octal digit Text equivalent Binary value Meaning
0 — 000 All types of access are denied
1 –x 001 Execute access is allowed only
2 -w- 010 Write access is allowed only
3 –wx 011 Write and execute access are allowed
4 r– 100 Read access is allowed only
5 r-x 101 Read and execute access are allowed
6 rw– 110 Read and write access are allowed
7 rwx 111 Everything is allowed

 

File attributes and Permissions:  chmod Examples
Example:

               chmod  644 any_File_name

 owner: read and write permissions,
group: only read permissions,
others: only read permissions. 
Example:

                chmod  755  any_Dir_name

 owner: read, write and execute permissions,
group: read and execute permissions,
others: read and execute permissions. 
Using chmod Recursively : It’s possible to make chmod descends a directory hierarchy and applies the expression to every file and subdirectory it finds. This is done with –R recursive option.
 chmod –R 755 . “.” is shortcut for current directory. Works on hidden file too.
 chmod –R a+x * “*” denotes a wildcard to include everything. This expression leaves out hidden file due to relative permission expression.
There are cases when you may come across four non-zero digits, in this case the first meaningful (non-zero) digit combines the following bits (in this order, high to low): SUID, SGID, sticky bit. We will study them later.     Ex: chmod 4775 filename

 

File attributes and Permissions: chmod Example ( absolute Permission)
chmod octal-permission-code filename

chmod octal-permission-code filename
chmod octal-permission-code filename

 

 

File attributes and Permissions:  chown and chgrp
chown command is used to change the owner and group of a file.

chgrp command is used to change the owner and group of a file.

Changing ownership requires super user permission, so first, switch to su or root account.

Usage :

chown option owner [:group] file(s)

It requires the owner name or (UID) & group name or (GID) for group.

chgrp option group file(s)

 

 

File attributes and Permissions:  chown and chgrp examples
chown user1 myfile user1 the new owner of myfile
chown command can also be used to change the group.
chown akash:CSE myfile akash is the new owner & CSE is the new group
chown  :CSE  myfile CSE is the new group
chgrp ECE myfile ECE is the new group