Linux

Absolute Pathname

• An absolute path is defined as the specifying the location of a file or directory from the root directory (/).

To write an abosulte path-name:

• Start at the root directory (/) and work down.
• Write a slash (/) after every directory name (last one is optional)
• For example:

                                        
  $cat abc.sql
                                      
                                  

  Note: we don't have to write '$' its already written in terminal.
  'cat' command is used for displaying content of a file, in this case abc.sql.
  The above command only work when there is a file 'abc.sql' in the current directory.

                                        
  $cat /home/database/abc.sql
                                      
                                  

  In the above example, if the first character of a pathname is /, the file’s location must be determined with respect to root.
• An absolute path is defined as specifying the location of a file or directory from the root directory(/). In other words,we can say that an absolute path is a complete path from start of actual file system from / directory.

Relative path

• Relative path is defined as the path related to the present working directly(pwd).
• It starts at your current directory and never starts with a /.
• To be more specific let’s take a look on the below figure in which if we are looking for photos then absolute path for it will be provided as /home/jono/photos but assuming that we are already present in jono directory then the relative path for the same can be written as simple photos.
  

Using . and .. in Relative Path-names

• Linux offers a shortcut in the relative pathname that uses either the current or parent directory as reference and specifies the path relative to it.
• A relative path name uses one of these cryptic symbols:
  • . (a single dot) - this represents the current directory.
  • .. (two dots) - this represents the parent directory.
• If we are currently in directory /home/kt/abc and now you use .. as an argument to cd to move to the parent directory /home/kt as :

                                  
  $pwd 
  /home/kt/abc 
  $cd ..            *** moves one level up ***
  $pwd 
  /home/kt 
                                  
                              

  Note: Now when / is used with .. has a different meaning, instead of moving down a level, it moves one level up.

                          
  $pwd 
  /home/kt/abc 
  $cd ../..          *** moves two level up ***
  $pwd 
  /home
                          
                      

1. Changing directory with relative path concept:

                        
$pwd 
/home/kit 
$cd abc 
$pwd 
/home/kit/abc
                        
                    

2. Changing directory with absolute path concept:

                        
$pwd 
/home/kit 
$cd /home/kt/abc 
$pwd 
/home/kit/abc
                        
                    

pwd

• This command displays the present working directory, letting you know where you are now.

Example ↓

                        
$pwd
/home/database/sql/dataone/
                        
                    

tree

• The tree command displays filesystem information in a similar manner to a graphical interface. This can be handy for new Linux users who are more used to the hierarchical filesystem display in other operating systems.

cd

• This command moves you to the specified directory, changing your present working directory location.
• For example, to use an absolute path to move to the /etc/ssh directory, type the following command:

                                  
  $cd /etc/ssh
                                  
                              

  By using the absolute path, it doesn't matter where in the filesystem you currently are.

. .. ~

Take a shortcut. These make navigation easier. Three of them are:

1. Single dot, or .
2. Double dot, or ..
3. Tilde, or ~

• The single dot represents the present working directory, or where you are right now. Say you're in your home directory and you want to copy the sshd_config file from /etc/ssh. You can specify it with just a dot because you're copying the file to your current directory. The command looks like this:

                                  
  $sudo cp /etc/ssh/sshd_config .
                                  
                              

• Double dots represent the parent directory, or the directory immediately above the current one in the filesystem. If there's a subdirectory namedRockin theMusicdirectory, thenMusicis the parent directory ofRock. As another example, consider where log files are stored: /var/log. In that case, var is the parent directory of log (and the filesystem root / is the parent of var).
  So, to move from the current Rock directory to the Music directory above it, type:

                                  
  $cd ..
                                  
                              

• Similarly, the tilde character ~ represents the current, logged-on user's home directory. To copy the sshd_config file from /etc/ssh to your home directory (no matter where in the filesystem I'm presently located), type:

                              
  $cp /etc/ssh/sshd_config ~
                              
                          

Home directory

• The directory in which you find yourself when you first login is called your home directory.
• You can go in your home directory anytime using the following command −

                                  
  $cd ~
                                  
                              

• Here ~ indicates the home directory. Suppose you have to go in any other user's home directory, use the following command −

                              
  $cd ~username
                              
                          

• To go in your last directory, you can use the following command −

                                  
  $cd -
                                  
                              

ls

• Listing Directories: To list the files in a directory, you can use the following syntax -

                                  
  $ls dirname
                                  
                              

mkdir

• Creating Directories, directories are created by the following command -

                                  
  $mkdir mydir
                                  
                              

  Creates the directory mydir in the currect directory.
• Here is another example -

                              
  $mkdir /tmp/test-dir
                              
                          

  This command creates the directory test-dir in the /tmp directory. The mkdir command produces no output if it successfully creates the requested directory.
• If you give more than one directory on the command line, mkdir creates each of the directories. For example, −

                                  
  $mkdir docs pub
                                  
                              

  Creates the directories docs and pub under the current directory.

Creating Parent Directories

• Sometimes when you want to create a directory, its parent directory or directories might not exitst. In this case, mkdir issues an error message as follows-

                              
  $mkdir /tmp/amrood/test
  mkdir: Failed to make directory "/tmp/amrood/test";
  No such file or directory
                              
                          

• In such cases, you can specify the -p option to the mkdir command. It creates all the necessay directories for you. For example -

                              
  $mkdir -p /tmp/amrood/test
                              
                          

rmdir

• Removing Directories: Directories can be deleted using the rmdir command as follows -

                              
  $rmdir dirname
                              
                          

  Note − To remove a directory, make sure it is empty which means there should not be any file or sub-directory inside this directory.
• You can remove multiple directories at a time as follows -

                                  
  $rmdir dirname1 dirname2 dirname3
                                  
                              

  The above command removes the directories dirname1, dirname2, and dirname3, if they are empty. The rmdir command produces no output if it is successful.

mv

• Renaming Directores: The mv (move) command can also be used to rename a directory. The syntax is as follows -

                                  
  $mv olddir newdir
  $mv mydir yourdir
                                  
                              

ls -la

• If we enter the command to show a listing of the current working directories/files and use the -a option to list all the files and the -l option to provide the long listing, we will receive the following result

                                  
  pravin@Pravins-MacBook-Air BCA % ls -la
  total 64
  drwxr-xr-x@ 10 pravin  staff   320 Apr 18 19:01 .
  drwxrwxrwx@  8 pravin  staff   256 Feb 15 09:46 ..
  -rw-r--r--@  1 pravin  staff  8196 Jun  4 10:58 .DS_Store
  drwxr-xr-x  15 pravin  staff   480 Jun  5 12:29 .git
  -rw-r--r--   1 pravin  staff     8 Feb 18 12:13 .gitignore
  drwxr-xr-x   3 pravin  staff    96 Apr 18 19:01 .vscode
  -rw-r--r--@  1 pravin  staff  8661 May 28 09:15 index.html
  drwxr-xr-x   9 pravin  staff   288 May 21 10:34 public
  -rw-r--r--   1 pravin  staff   674 Apr 17 06:09 readme.md
  drwxr-xr-x   6 pravin  staff   192 Apr 27 16:18 resources
                                  
                              

Creating files

• You can use vi editor to create ordinary files on any Unix system. You simply need to give the following command:

                        
$vi filename
                        
                    

• The above command will open a file with the given filename. Now, press the key i to come into the edit mode. Once you are in the edit mode, you can start writing your content in the file as in the following program :

                        
This is unix file... and we are writing on this file.
I'm going to save this content in the file.
                        
                    

• Press esc to come out of the edit mode
• Press two keys Shift + Z together to come out of the file completely.
• You will now have a file with filename in the current directory.

Editing files

• You can edit an existing file using the vi editor.

                        
$vi filename
                        
                    

• Once the file in opened, you can come in the edit mode by pressing the key i and then you can proceed by editingg the file.

Display content of a file

• Use cat command to see the content of a file. Following is the simple content of the above created file:

                        
$cat filename
This is unix file... and we are writing on this file.
I'm going to save this content in the file.
$
                        
                    

Copying files

• To make a copy of a file use the cp command. The basic syntax of the command is:

                        
$cp source_file destination_file
                        
                    

• Following is the example to create a copy of the existing file filename.

                        
$cp filename copyfile
                        
                    

• You wil find one more file copyfile in your current directory. This file will exactly be the same as the original file filename.

Renaming files

• We same command for files as we have used for folders.
• To change the name of a file, use the mv command. Following is the basic syntax:

                        
$mv myold_file newfile
                        
                    

Deleting files

• To delete an existing file, use the rm command. Following is the basic syntax:

                        
$rm filename
                        
                    

• You can remove multiple files at a time with the command given below −

                        
$rm filename1 filename2 filename3
                        
                    

Using chmod in Symbolic Mode

The easiest way for a beginner to modify file or directory permissions is to use the symbolic mode. With symbolic permissions you can add, delete, or specify the permission set you want by using the operators in the following table.

                    
Sr. No          Chmod operator & Description
  1             +   Adds the designated permission(s) to file or directory
  2             -   Removes the designated permission(s) to file or directory
  3             =   Sets the designated permission(s).
                    
                

Here's an example using testfile. Running ls -l on the testfile shows that the file's permissions are as follows

                        
$ls -l testfile
-rwxrwxr-- 1 amrood users 1024 Nov 2 00:10 testfile
                        
                    

Then each example chmod command from the preceding table is run on the testfile, followed by ls –l, so you can see the permission changes

                        
$chmod o+wx testfile
$ls -l testfile
-rwxrwxrwx 1 amrood users 1024 Nov 2 00:10 testfile 
$chmod u-x testfile
$ls -l testfile
-rw-rwxrwx 1 amrood users 1024 Nov 2 00:10 testfile 
$chmod g = rx testfile
$ls -l testfile
-rw-r-xrwx 1 amrood users 1024 Nov 2 00:10 testfile
                        
                    

Here's how you can combine these commands on a single line

                        
$chmod o+wx,u-x,g = rx testfile
$ls -l testfile
-rw-r-xrwx 1 amrood users 1024 Nov 2 00:10 testfile
                        
                    

Using chmod with Absolute Permissions

The second way to modify permissions with the chmod command is to use a number to specify each set of permissions for the file.

Each permission is assigned a value, as the following table shows, and the total of each set of permissions provides a number for that set.

                            
Number   Octal Permission Representation                                     Ref 
  0          No permission                                                   ---
  1          Execute permission                                              --x
  2          Write permission                                                -w-
  3          Execute and write permission: 1 (execute) + 2 (write) = 3       -wx
  4          Read permission                                                 r--
  5          Read and execute permission: 4 (read) + 1 (execute) = 5         r-x
  6          Read and write permission: 4 (read) + 2 (write) = 6             rw-
  7          All permissions: 4 (read) + 2 (write) + 1 (execute) = 7         rwx
                            
                        

Here's an example using the testfile. Running ls -1 on the testfile shows that the file's permissions are as follows:

                            
$ls -l testfile
-rwxrwxr-- 1 amrood users 1024 Nov 2 00:10 testfile
                            
                        

Then each example chmod command from the preceding table is run on the testfile, followed by ls –l, so you can see the permission changes

                                
$ chmod 755 testfile
$ls -l testfile
-rwxr-xr-x 1 amrood users 1024 Nov 2 00:10 testfile $chmod 743 testfile
$ls -l testfile
-rwxr---wx 1 amrood users 1024 Nov 2 00:10 testfile $chmod 043 testfile
$ls -l testfile
----r---wx 1 amrood users 1024 Nov 2 00:10 testfile
                                
                            

Example 1:
In this example, we will concatenate two strings using += operator. The input strings will be stored in two variables, a and b.
Code ↓

                    
#Read inputs a and b and store string variables in them.

read a b 
#append b to the string a 
a+=$b

#Output the resulting string 
echo $a
                    
                

Example 2:
In this example, we have two variables a and b, and we stored a string in each variable. We want to concatenate them one after the other. The result of concatenation will be stored in the variable c.
Code ↓

                    
#Read inputs a and b and store string variables in them. 

read a b

#concatenate the strings by writing one after the other. Store the result in c

c=$a$b

#output c
echo $c
                    
                

Problem solution:
Here, we will create a shell script program to create two variables and then print values of variables on the console screen.
Program/Source code:
The source code to create a Linux shell script program to create and print the value of variables is given below. The given program is compiled and executed successfully on Ubuntu 20.04.

                
#!/bin/bash

# Program name: "printVar.sh"

# Linux shell script program to create and print the value of variables.

country="India"
year=2021

echo "Country name: $country" 
echo "Year: $year"
                
            

Output ↓

                
$ sh printVar.sh
Country name: India
Year: 2021
                
            

Explanation:
Here, we created two variables country, year. which are initialized with "India", 2021 respectively. After that, we printed the value of variables on the console screen using echo command.