File System

Objective

• Perform more operations on the file system.
• Get familiar with file permission assignment.
• Get familiar with text editors.

Summary

Aliases

An alias lets you create a shortcut name for a command, file name, or any shell text. By using aliases, you save a lot of time when performing frequent tasks. An alias is a command in various command-line interpreters (shells), which enables a replacement of a word by another string. It is mainly used for abbreviating a system command, or for adding default arguments to a regularly used command, for example:

> alias ll="ls -l"
> ll
total 56
drwxr-xr-x. 1 tux tux     0 Jan 25 00:16 Desktop
drwxr-xr-x. 1 tux tux    30 Jan 29 13:59 Documents
drwxr-xr-x. 1 tux tux    62 Jan 29 14:03 Downloads
...

An alias will last for the life of the shell session. Regularly used aliases can be set from the shell’s rc file, such as ~/.bashrc, so that they will be available upon the start of the corresponding shell session:

echo 'alias l="ls"' >> ~/.bashrc
echo 'alias ll="ls -la"' >> ~/.bashrc
source ~/.bashrc
l
ll

Note: alias, without arguments, prints the list of defined aliases.

Searching Files

grep

Looks for strings in a file. Its name comes from the ed, a text editor, command g/re/p (global / regular expression search / and print), which performs the same function.

Syntax

grep [-options] pattern file [file ...]

Example

> grep root /etc/passwd
root:x:0:0:Super User:/root:/bin/bash
operator:x:11:0:operator:/root:/usr/sbin/nologin
 
> grep "FTP User" /etc/passwd
ftp:x:14:50:FTP User:/var/ftp:/usr/sbin/nologin

Exercise 1

1. Display the contents of /etc/hosts to show all known systems on your network.
2. Locate all occurrences of the word nologin in the /etc/passwd file.
3. Make a directory called cse.
4. Create another directory under cse called cs.
5. Create a file called cmps405. Move cmps405 into cs. Verify the contents of cs.

Locating Files

find

Used to locate files within the directory structure that meet specified search criteria.

Syntax

find path predicate-list

path is the pathname of the directory to start the search from. All subdirectories are searched recursively by default.

Example

> find / -ctime +10 -name include

Exercise 2

1. Create the following file structure under the current running user:

2. Add sample C source code to the *.c files.
3. List all C file names whose content has main using one command.
4. Copy all C files only from the docs directory to the source directory using one command.
5. Move the backup directory and put it inside the images directory using one command.
6. Starting from your home directory, find all files whose name start with the letter D using one command.

Sorting Lines

sort

Sorts lines, lexicographically or numerically, in a file.

Syntax

sort [-nr] filename

Example

> sort /etc/passwd
abrt:x:173:173::/etc/abrt:/sbin/nologin
adm:x:3:4:adm:/var/adm:/usr/sbin/nologin
apache:x:48:48:Apache:/usr/share/httpd:/sbin/nologin
avahi:x:70:70:Avahi mDNS/DNS-SD Stack:/var/run/avahi-daemon:/sbin/nologin
bin:x:1:1:bin:/bin:/usr/sbin/nologin
chrony:x:990:989:chrony system user:/var/lib/chrony:/sbin/nologin
...

Counting Words

wc

Counts lines, words, and characters in a file.

Syntax

wc [-lwc] filename

Example

> wc -l /etc/passwd
46 /etc/passwd
> wc -w /etc/passwd
106 /etc/passwd
> wc /etc/passwd
  46  106 2646 /etc/passwd

Text Editing

nano

To start GNU nano, open a terminal and type nano followed by the name of an existing file or a new file that you want to create. The available shortcuts are displayed at the bottom of the terminal window.

vim

• To start the vim text editor, open a terminal and simply type vim followed by the name of an existing file or a new file that you want to create.
• vim works in two main modes, one for editing text and the other for giving commands. To switch between the two modes you use the i and <escape> keys for editing and commands respectively.
• Use the command :wq to save (write) changes and quit.
• Use the command :w to save (write) changes without quitting.
• Use the command :q! to ignore changes and quit.

emacs

• To start the Emacs text editor, open a terminal and simply type emacs followed by the name of an existing file or a new file that you want to create.

• To save a file, press C-x C-s (<ctrl> + x followed by <ctrl> + s).

• If you don’t want to save but quit, use the command C-x C-c (<ctrl> + x followed by <ctrl> + c). If there is unsaved data, press y for yes.

• emacs has the following editing keys: C stands for <ctrl> and A stands for <alt>:

  Command	Key Combination
  Moving to the next line	C-n (n for Next)
  Moving to the previous line	C-p (p for Previous)
  Moving one character forward	C-f (f for Forward)
  Moving one character backward	C-b (b for Backward)
  Moving one word forward	A-f (f for Forward)
  Moving one word backward	A-b (b for Backward)
  Moving to the start of a line	C-a
  Moving to the end of a line	C-e (e for End)
  Moving one page down	C-v (or <page-down>)
  Moving one page up	A-v (or <page-up>)

gnome-text-editor

To start the GNOME text editor, open a terminal and type gnome-text-editor followed by the name of an existing file or a new file that you want to create. Notice that the terminal is occupied until you exit from the editor.

Linking Files

Symbolic links are files that point to other files or directories. They are similar to shortcuts and hyperlinks.

ln

Creates a symbolic or a hard link.

Syntax

ln [-s] original-file symbolic-link

Example

> touch file
> ln -s file link
> ls -l
total 4
-rw-r--r--. 1 tux tux 0 Feb  4 20:49 file
lrwxrwxrwx. 1 tux tux 4 Feb  4 20:49 link -> file
> echo "Hello, world!" >> file
> cat link
Hello, world!
> echo "Hello, multiverse!" >> link
> cat file
Hello, world!
Hello, multiverse!
> ls -l
total 4
-rw-r--r--. 1 tux tux 0 Feb  4 20:50 file
lrwxrwxrwx. 1 tux tux 4 Feb  4 20:49 link -> file
> rm file
> ls -l
total 4
lrwxrwxrwx. 1 tux tux 4 Feb  4 20:49 link -> file

File Security

• File security limits access to the files. The system provides you with the commands to specify who can access a file and, once accessed, what type of operations can be done on the file.
• The three types of permissions are: r (read), w (write) and x (execute).
• To deny a certain permission - is used in place of rwx, e.g. r-- r-x
• Access may be granted to or revoked from the following types of users: the owner or user (u), members of the group (g), other users (o). The a option may be used instead of ugo.
• Operators used are + (grants a permission), - (denies or revokes a permission), and = (sets all permissions for a specified user).
• Permissions are listed and applied in the following order: u [rwx], g [rwx], o [rwx].
• The ls -l command is used to view or list permissions.
• The chmod command is used to set the file permission (access mode).

Listing Permissions

File information may be displayed using ls -l and has the following format for each entry:

1. Type, permissions, and attributes
   1. Entry type
   2. User permissions
   3. Group permissions
   4. Others permissions
   5. Extended attributes
2. Hard-link count
3. Size (bytes)
4. User (owner)
5. Group (owner)
6. Last modification date
7. Name

Entry Type

Users

To display your user id and group information, use the id command:

> id -a
uid=1000(tux) gid=1000(tux) groups=1000(tux),10(wheel) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
> id tux
uid=1000(tux) gid=1000(tux) groups=1000(tux),10(wheel)
> id root
uid=0(root) gid=0(root) groups=0(root)

Permissions

Changing Permissions

chmod

• Permissions may be changed for several reasons:

  • to hide private files from other users
  • to share files with other users
  • to make files modifiable
  • to make files executable
  • to prevent or allow modification of directory contents

• File and directory permissions are changed using the chmod command.

• Only the owner of the file or the superuser (root) can change the permissions.

• Permissions can be specified using either the symbolic method or the octal method.

• You can change the owner and group using the chown and chgrp commands, respectively, for example:

  sudo chown root file
  sudo chgrp wheel file
  ls -l file

Symbolic Method

chmod [who][action][access] file [file ...]

1. who: u, g, o, a.
2. action: +, -, =.
3. access: r, w, x.

Example

> mkdir directory
> cd directory
> touch tigers bears elephants
> ls -l
total 0
-rw-r--r--. 1 tux tux 0 Feb  4 20:39 bears
-rw-r--r--. 1 tux tux 0 Feb  4 20:39 tigers
-rw-r--r--. 1 tux tux 0 Feb  4 20:39 elephants
> chmod u-w tigers
> ls -l tigers
-r--r--r--. 1 tux tux 0 Feb  4 20:39 tigers
> chmod go-r bears
> ls -l bears
-rw-------. 1 tux tux 0 Feb  4 20:39 bears
> chmod u=rx,g=r elephants
> ls -l elephants
-r-xr--r--. 1 tux tux 0 Feb  4 20:39 elephants
> chmod o= elephants
> ls -l elephants
-r-xr-----. 1 tux tux 0 Feb  4 20:39 elephants

Exercise 3

1. Use touch to create a file called file_modes:

touch file_modes

2. List the file information by typing:

ls -l file_modes

3. Remove all permissions from this file using:

chmod ugo= file_modes

4. List the file information and confirm the change.
5. Verify that you can neither read from nor write to this file.

cat file_modes
echo "date" >> file_modes

6. Add write permission for yourself:

chmod u+w file_modes

7. Verify that you can write now to the file file_modes:

echo "date" >> file_modes

8. Now add write permission for everyone else. Also add read permission for yourself:

chmod go+w,u+r file_modes

9. Verify that you can now read from the file file_modes:

cat file_modes

10. Assign everyone read and write permission to your file:

chmod ugo=rw file_modes

11. Now deny everyone write permission, but grant everyone execute permission:

chmod a-w,a+x file_modes

12. Finally, verify that your file can now be executed:

./file_modes

As file_modes contains the command date, the output should be the current system date.

Octal Method

The permission categories rwx can be represented numerically: A granted permission is represented by 1 and a revoked permission by 0. The rwx combination for any user category may now be represented as a 3-digit binary number which is equivalent to one octal digit:

The effective access permission for the 3 user categories (ugo) is represented by just one 3-digit octal number, as shown in the following table:

Example

> ls -l
total 0
-r--------. 1 tux tux 0 Feb  4 20:39 bears
-r--r--r--. 1 tux tux 0 Sep 15 21:22 tigers
-r-xr-----. 1 tux tux 0 Sep 15 21:22 elephants
> chmod 664 *
> chmod 000 tigers
> ls -l
total 0
-rw-rw-r--. 1 tux tux 0 Feb  4 20:39 bears
----------. 1 tux tux 0 Feb  4 20:39 tigers
-rw-rw-r--. 1 tux tux 0 Feb  4 20:39 elephants

Exercise 4

1. Create a file called practice using touch.
2. List the files in your directory along with their permissions.
3. Change the permissions on the file practice so that only you can read and write it.
4. Change the permissions so that nobody can access the practice file, not even yourself.
5. Verify that you have no access to the file.
6. Try to create a file in another user’s directory.

Resources

• Linux file permissions explained (opens in a new tab)