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Linux is a clone of UNIX, the multi-user operating system which can be accessed by many users simultaneously. Linux can also be used in mainframes and servers without any modifications. But this raises security concerns as an unsolicited or malign user can corrupt, change or remove crucial data. For effective security, Linux divides authorization into 2 levels.

Ownership

Permission

In this Linux file commands tutorial, you will learn-

The concept of Linux File permission and ownership is crucial in Linux. Here, we will explain Linux permissions and ownership and will discuss both of them. Let us start with the Ownership.

Linux File Ownership

Every file and directory on your Unix/Linux system is assigned 3 types of owner, given below.

User

A user is the owner of the file. By default, the person who created a file becomes its owner. Hence, a user is also sometimes called an owner.

Group

A user- group can contain multiple users. All users belonging to a group will have the same Linux group permissions access to the file. Suppose you have a project where a number of people require access to a file. Instead of manually assigning permissions to each user, you could add all users to a group, and assign group permission to file such that only this group members and no one else can read or modify the files.

Other

Any other user who has access to a file. This person has neither created the file, nor he belongs to a usergroup who could own the file. Practically, it means everybody else. Hence, when you set the permission for others, it is also referred as set permissions for the world.

Now, the big question arises how does Linux distinguish between these three user types so that a user ‘A’ cannot affect a file which contains some other user ‘B’s’ vital information/data. It is like you do not want your colleague, who works on your Linux computer, to view your images. This is where Permissions set in, and they define user behavior.

Let us understand the Permission system on Linux.

Linux File Permissions

Every file and directory in your UNIX/Linux system has following 3 permissions defined for all the 3 owners discussed above.

Read: This permission give you the authority to open and read a file. Read permission on a directory gives you the ability to lists its content.

Write: The write permission gives you the authority to modify the contents of a file. The write permission on a directory gives you the authority to add, remove and rename files stored in the directory. Consider a scenario where you have to write permission on file but do not have write permission on the directory where the file is stored. You will be able to modify the file contents. But you will not be able to rename, move or remove the file from the directory.

Execute: In Windows, an executable program usually has an extension “.exe” and which you can easily run. In Unix/Linux, you cannot run a program unless the execute permission is set. If the execute permission is not set, you might still be able to see/modify the program code(provided read & write permissions are set), but not run it.

File Permissions in Linux/Unix

Let’s see file permissions in Linux with examples:

ls – l on terminal gives

ls - l

Here, we have highlighted ‘-rw-rw-r–‘and this weird looking code is the one that tells us about the Unix permissions given to the owner, user group and the world.

Else, if it were a directory, d would have been shown.

The characters are pretty easy to remember.

– = no permission

Let us look at it this way.

The first part of the code is ‘rw-‘. This suggests that the owner ‘Home’ can:

Read the file

Write or edit the file

He cannot execute the file since the execute bit is set to ‘-‘.

By design, many Linux distributions like Fedora, CentOS, Ubuntu, etc. will add users to a group of the same group name as the user name. Thus, a user ‘tom’ is added to a group named ‘tom’.

The second part is ‘rw-‘. It for the user group ‘Home’ and group-members can:

Read the file

Write or edit the file

The third part is for the world which means any user. It says ‘r–‘. This means the user can only:

Read the file

Changing file/directory permissions in Linux Using ‘chmod’ command

Say you do not want your colleague to see your personal images. This can be achieved by changing file permissions.

We can use the ‘chmod’ command which stands for ‘change mode’. Using the command, we can set permissions (read, write, execute) on a file/directory for the owner, group and the world.

Syntax:

chmod permissions filename

There are 2 ways to use the command –

Absolute mode

Symbolic mode

Absolute(Numeric) Mode in Linux

In this mode, file permissions are not represented as characters but a three-digit octal number.

The table below gives numbers for all for permissions types.

Number Permission Type Symbol

No Permission

—

Execute

–x

Write

-w-

Execute + Write

-wx

Read

r–

Read + Execute

r-x

Read +Write

rw-

Read + Write +Execute

rwx

Let’s see the chmod permissions command in action.

In the above-given terminal window, we have changed the permissions of the file ‘sample to ‘764’.

‘764’ absolute code says the following:

Owner can read, write and execute

Usergroup can read and write

World can only read

This is shown as ‘-rwxrw-r–

This is how you can change user permissions in Linux on file by assigning an absolute number.

Symbolic Mode in Linux

In the Absolute mode, you change permissions for all 3 owners. In the symbolic mode, you can modify permissions of a specific owner. It makes use of mathematical symbols to modify the Unix file permissions.

Operator Description

+ Adds a permission to a file or directory

– Removes the permission

= Sets the permission and overrides the permissions set earlier.

The various owners are represented as –

User Denotations

u user/owner

g group

o other

a all

We will not be using permissions in numbers like 755 but characters like rwx. Let’s look into an example

Changing Ownership and Group in Linux

For changing the ownership of a file/directory, you can use the following command:

chown user filename

In case you want to change the user as well as group for a file or directory use the command

chown user:group filename

Let’s see this in action

In case you want to change group-owner only, use the command

chgrp group_name filename

‘chgrp’ stands for change group.

Tip

The file /etc/group contains all the groups defined in the system

You can use the command “groups” to find all the groups you are a member of

You can use the command newgrp to work as a member a group other than your default group

You cannot have 2 groups owning the same file.

You do not have nested groups in Linux. One group cannot be sub-group of other

x- eXecuting a directory means Being allowed to “enter” a dir and gain possible access to sub-dirs

Summary:

Linux being a multi-user system uses permissions and ownership for security.

There are three user types on a Linux system viz. User, Group and Other

Linux divides the file permissions into read, write and execute denoted by r,w, and x

The permissions on a file can be changed by ‘chmod’ command which can be further divided into Absolute and Symbolic mode

The ‘chown’ command can change the ownership of a file/directory. Use the following commands: chown user file or chown user:group file

The ‘chgrp’ command can change the group ownership chrgrp group filename

What does x – eXecuting a directory mean? A: Being allowed to “enter” a dir and gain possible access to sub-dirs.

You're reading File Permissions In Linux / Unix: How To Read, Write & Change?

How To Find Out File Types In Linux

When it comes to Linux operating system, everything in general can be considered as a file. In UNIX, in total, there are seven standard file types −

FIFO special

Block special

Character special

Socket

Regular

How To Reverse Order Of Lines In A File In Linux

As a Linux user, it is common to come across situations where you need to reverse order of lines in a file. This could be necessary when you want to read a file in a reverse order or when you want to perform certain operations that require content of a file to be in reverse order. In this article, we will explore how to reverse order of lines in a file in Linux.

Introduction to Linux Commands for Reversing Order of Lines in a File

Linux provides several commands that can be used to reverse order of lines in a file. These commands are designed to manipulate text and provide powerful options to achieve various tasks. In this article, we will focus on three commonly used commands: tac, sed, and awk.

Using tac Command to Reverse Order of Lines in a File

In this example, tac command is used to reverse order of lines in “file.txt” file, and output is redirected to a new file called “reversed_file.txt”. This will create a new file with same content as original file but with order of lines reversed.

Using sed Command to Reverse Order of Lines in a File

The sed command is another Linux command that can be used to reverse order of lines in a file. sed command is a stream editor that is used to perform basic text transformations on an input stream. Here is an example of how to use sed command to reverse order of lines in a file −

In this example, sed command is used to reverse order of lines in “file.txt” file, and output is redirected to a new file called “reversed_file.txt”. sed command uses a regular expression to reverse order of lines in file.

Using awk Command to Reverse Order of Lines in a File

The awk command is a Linux command that is used to manipulate text files. awk command is a versatile command that can be used to perform various text processing tasks. Here is an example of how to use awk command to reverse order of lines in a file −

In this example, awk command is used to reverse order of lines in “file.txt” file, and output is redirected to a new file called “reversed_file.txt”. awk command uses an array to store lines of file and then prints lines in reverse order.

Using “-r” Option with tac Command

The tac command has an option called “-r” which can be used to reverse characters instead of lines. This option is useful when you want to reverse order of characters in each line. Here is an example −

In this example, “-r” option is added to tac command to reverse order of characters in each line of “file.txt” file. output is then redirected to a new file called “reversed_file.txt”.

Using “-n” Option with sed Command

The sed command has an option called “-n” which can be used to suppress automatic printing of pattern space. This option is useful when you want to specify which lines to print. Here is an example −

In this example, “-n” option is added to sed command to suppress automatic printing of pattern space. “$p” command is then used to print last line, which reverses order of lines in file. output is then redirected to a new file called “reversed_file.txt”.

Using “-F” and “-v” Options with awk Command

The awk command has two options “-F” and “-v” which can be used to set field separator and variables respectively. These options are useful when you want to specify how lines should be separated and manipulated. Here is an example −

Conclusion and Summary

In conclusion, reversing order of lines in a file is a common task in Linux, and it can be achieved using several commands, including tac, sed, and awk commands. tac command is simplest and most straightforward command to use, while sed and awk commands are more powerful and provide more options and customization. choice of which command to use depends on your specific requirements and complexity of text processing task at hand.

In addition to basic commands discussed in this article, there are other Linux commands that can be used to reverse order of lines in a file, including rev command and Perl command. These commands provide additional options and flexibility for manipulating text files.

10 7Zip (File Archive) Command Examples In Linux

If you’re a Linux user, you probably deal with file archives frequently. Whether you’re sending files to someone, backing up data, or just organizing your files, compressing them into a single file archive can save a lot of space and make things easier to manage. One tool you can use for this purpose is 7zip. In this article, we’ll cover 7zip command examples in Linux that you can use to compress and extract files in various ways.

What is 7zip? Installing 7zip on Linux

Before we dive into examples, you need to make sure that 7zip is installed on your Linux system. Most Linux distributions include 7zip in their default repositories, so you can use your package manager to install it. For example, on Ubuntu, you can use following command −

sudo apt-get install p7zip-full

On CentOS or Fedora, you can use −

sudo yum install p7zip

If 7zip is not available in your distribution’s repositories, you can download it from official website and compile it from source.

7zip Command Examples in Linux

Now that you have 7zip installed on your Linux system, let’s look at some command examples that you can use to compress and extract files.

Creating a 7z Archive

To create a 7z archive, you can use “7z” command followed by “a” option, which stands for “add.” Here’s an example −

7z a archive.7z chúng tôi chúng tôi file3.txt

This will create a new file called “archive.7z” that contains files “file1.txt”, “file2.txt”, and “file3.txt”. Note that you can add as many files as you want to archive.

Extracting a 7z Archive

To extract a 7z archive, you can use “7z” command followed by “x” option, which stands for “extract.” Here’s an example −

7z x archive.7z

This will extract contents of “archive.7z” file to current directory. If you want to extract files to a specific directory, you can use “-o” option followed by path to directory. For example −

7z x archive.7z -o /home/user/documents

This will extract files to “/home/user/documents” directory.

Compressing with Different Compression Levels

By default, 7zip compresses files with “normal” compression level, which is a good balance between compression ratio and speed. However, if you want to compress files more aggressively or less aggressively, you can use “-m” option followed by compression level. Here are some examples −

7z a -m0=lzma2 archive.7z chúng tôi chúng tôi file3.txt

This will use “lzma2” compression method with “fastest” compression level, which will compress files quickly but with a lower compression ratio.

7z a -m9=lzma2 archive.7z chúng tôi file2

This will use “lzma2” compression method with “ultra” compression level, which will compress files more aggressively but with a slower speed. You can experiment with different compression levels to find one that suits your needs.

Compressing Directories

In addition to compressing individual files, you can also compress entire directories using 7zip. To do this, simply specify path to directory instead of file names. For example −

7z a -m0=lzma2 archive.7z /home/user/documents/

This will compress entire “documents” directory using “lzma2” compression method with “fastest” compression level.

Using Encryption

If you need to encrypt your archives for security reasons, 7zip provides several encryption options. most secure option is AES-256 encryption, which is considered unbreakable by current standards. To use AES-256 encryption, you can use “-p” option followed by a password, and “-mhe=on” option to enable header encryption. For example −

7z a -pMyPassword -mhe=on archive.7z chúng tôi chúng tôi file3.txt

This will create a new file called “archive.7z” that is encrypted with AES-256 and protected by password “MyPassword”. Note that you should choose a strong password and keep it safe, as it cannot be recovered if you forget it.

Splitting Archives

If you need to split your archives into smaller parts for easier transfer or storage, you can use “-v” option followed by size of each part. For example −

7z a -v1m archive.7z chúng tôi chúng tôi file3.txt

This will create a new file called “archive.7z” that is split into 1 MB parts. To extract files from a split archive, you only need to extract first part (e.g. “archive.7z.001”), and 7zip will automatically detect other parts and extract files.

Adding Files to an Existing Archive

If you already have an existing archive and want to add more files to it, you can use “7z” command followed by “u” option, which stands for “update.” Here’s an example −

7z u archive.7z chúng tôi file5.txt

This will add files “file4.txt” and “file5.txt” to existing “archive.7z” file.

Creating a Compressed Tar Archive

7zip can also be used to create compressed tar archives, which are commonly used in Linux. To create a compressed tar archive, you can use “7z” command followed by “a” option and “-ttar” option to specify archive format. For example −

7z a -ttar archive.tar.7z chúng tôi chúng tôi file3.txt

This will create a compressed tar archive called “archive.tar.7z” that contains files “file1.txt”, “file2.txt”, and “file3.txt”.

Extracting a Specific File From an Archive

If you only need to extract a specific file from an archive, you can use “7z” command followed by “-o” option and path to file. For example −

7z x archive.7z -o /home/user/documents file2.txt

This will extract only file “file2.txt” from “archive.7z” file and save it to “/home/user/documents” directory.

Creating a Password-protected Archive

In addition to encryption, 7zip also allows you to create password-protected archives, which can be useful when you want to prevent unauthorized access to your files. To create a password-protected archive, you can use “-p” option followed by password. For example −

7z a -pMyPassword archive.7z chúng tôi chúng tôi file3.txt

This will create a new file called “archive.7z” that is password-protected with password “MyPassword”.

Listing Contents of an Archive

If you want to see contents of an archive without extracting them, you can use “7z” command followed by “l” option, which stands for “list.” For example −

7z l archive.7z

This will list contents of “archive.7z” file, including file names, sizes, and compression ratios.

Extracting an Archive with Progress Bar

If you want to see progress of an extraction operation, you can use “-bsp1” option to enable progress bar. For example −

7z x -bsp1 archive.7z

This will extract contents of “archive.7z” file with a progress bar that shows percentage of extraction progress.

Creating a Compressed Archive with Exclusion

If you want to create a compressed archive that excludes certain files or directories, you can use “-x” option followed by file or directory names. For example −

7z a archive.7z * -x!*.log -x!temp/

This will create a compressed archive called “archive.7z” that contains all files in current directory except for those with extension “.log” and directory “temp/”.

Extracting an Archive with Exclusion

If you want to extract an archive while excluding certain files or directories, you can use “-x” option followed by file or directory names. For example −

7z x archive.7z -x!*.log -x!temp/

This will extract contents of “archive.7z” file while excluding files with extension “.log” and directory “temp/”.

Creating a self-extracting Archive

If you want to create a self-extracting archive that can be run on any system without 7zip installed, you can use “-sfx” option followed by name of executable file to create. For example −

7z a -sfx chúng tôi chúng tôi chúng tôi file3.txt

This will create a self-extracting archive called “archive.exe” that contains files “file1.txt”, “file2.txt”, and “file3.txt”.

Extracting a self-extracting Archive

To extract a self-extracting archive, you simply need to run executable file. For example −

./archive.exe

This will extract contents of self-extracting archive to current directory.

These are just a few examples of many ways you can use 7zip commands in Linux. With its versatile features and powerful compression capabilities, 7zip is an essential tool for managing files and archives on your Linux system.

Conclusion

7zip is a powerful and versatile tool that can help you compress and extract files on your Linux system. With examples in this article, you should be able to use 7zip to manage your archives efficiently and securely. Whether you’re sending files to someone or backing up your data, 7zip is a reliable and easy-to-use tool that can save you time and disk space.

Golang Program To Read And Print All Files From Zip File

Golang has packages like os, io, and Archie/zip that can be used to read and print all the files from a zipped file. A zip file is a compressed collection of a set of files/folders. The OS package is used for performing copy operations, the io pacakge is for reading and writing operations whereas the archive/zip package is for unzipping and zipping the file. In this article, we are going to learn about using all these packages to perform the copy and unzipping of the files and printing the same.

Method 1: Using io and os package

This program uses the archive/zip package to read a zip file’s contents. It opens the zip file using the zip.OpenReader function, which returns a zip.ReadCloser type. Then, it uses a range loop to cycle over all of the files in the archive, uses the Open method to open each file, and print the file name and contents to the standard output.

Syntax zip.OpenReader

This function belongs to the archive/zip package. The primary focus of this function is to open and read the contents of a zip file. It takes only one input i.e. the zip file.

Algorithm

Step 1 − Create a package main and declare fmt(format package), os and io package in the program where main produces executable codes and fmt helps in formatting input and output.

Step 2 − Use the zip to open the zip file in the main menu. Use the OpenReader function, then put the output in the variable read. Print the error message and then retry if a problem exists. So that it closes automatically when the function completes, defer the Close method on read.

Step 3 − Use a range loop and the chúng tôi field to iterate through the files in the zip package.

Step 4 − Then, Use the Open method to open the current file at each iteration, then save the outcome to the variable v. Print the error message and then retry if a problem exists.

Step 5 − To ensure that v is automatically closed after the loop is finished, use the close statement and defer keyword on v.

Step 6 − Print the filename using printf, using the Name field of the chúng tôi type, and chúng tôi to copy the contents of v to the output.

Step 7 − If an error comes while printing the file name, print the error on the console.

Example

In this example, we will use io and os package functions to execute the program.

package main import ( "archive/zip" "fmt" "io" "os" ) func main() { read, err := zip.OpenReader("folder.zip") if err != nil { fmt.Println(err) return } defer read.Close() for _, f := range chúng tôi { v, err := f.Open() if err != nil { fmt.Println(err) return } defer v.Close() fmt.Printf("File Name: %sn", f.Name) _, err = io.Copy(os.Stdout, v) if err != nil { fmt.Println(err) return } fmt.Println() } } Output File1.txt File2.txt Method 2: Using archive/zip and io/ioutil package Syntax ioutil.ReadAll

This function belongs to the ioutil package. The main motive to use it is to read the contents of a file into a byte slice. It takes the filename as an input.

Algorithm

Step 1 − Create a package main and declare fmt(format package), archice/zip and io/ioutil package in the program where main produces executable codes and fmt helps in formatting input and output.

Step 2 − Use the zip to open the zip file in the main menu. Use the OpenReader function, then put the output in the variable read. Print the error message and then retry if a problem exists so that it closes automatically when the function completes, defer the Close method on read.

Step 3 − Use a range loop and the chúng tôi field to iterate through the files in the zip package.

Step 4 − Use the Open method to open the current file at each iteration, then save the outcome to the variable v. Print the error message and then retry if a problem exists.

Step 5 − To ensure that v is automatically closed after the loop is finished, postpone the Close function on v.

Step 6 − Use the ioutil.ReadAll function to read the file’s content and put the output of the into the variable b. If an error exists print it on the console.

Step 7 − Then, print the file name and its contents using printf, using the Name field of the chúng tôi type and print the contents of file stored in slice b.

Example

In this example we will use io/ioutil and archive/zip package to execute the program.

package main import ( "archive/zip" "fmt" "io/ioutil" ) func main() { read, err := zip.OpenReader("sample.zip") if err != nil { fmt.Println(err) return } defer read.Close() for _, f := range chúng tôi { v, err := f.Open() if err != nil { fmt.Println(err) return } defer v.Close() b, err := ioutil.ReadAll(v) if err != nil { fmt.Println(err) return } fmt.Printf("File Name: %sn", f.Name) fmt.Printf("%sn", string(b)) } } Output File1.txt File2.txt Conclusion

We executed the program of reading and printing the files from zip file using two methods. In the first method we used io and os package and in the second method we used archive/zip and io/ioutil package to execute the program.

Writing Text To File Using Linux Cat Command

Introduction

The Linux cat command is a powerful utility that allows users to concatenate, view and create files. One of its useful features is the ability to write text to a file, either by appending it to the end of the file or by overwriting the existing content of the file. In this article, we’ll explore how to use the cat command to write text to a file in Linux, covering the various options and flags that can be used to customize the behavior of the command.

Introduction to the Linux Cat Command

The cat command is a simple yet versatile command that is commonly used in Linux to perform various file operations. It’s short for “concatenate” and can be used to join multiple files together or to display the contents of a file on the screen.

The basic syntax of the cat command is as follows −

$ cat [options] [file]

The options argument is optional and specifies various flags that modify the behavior of the cat command. The file argument is the name of the file you want to manipulate. If you don’t specify a file, cat will read from standard input (for example, keyboard input). Here are some common examples of using the cat command −

To view the contents of a file on screen −

$ cat file.txt

To concatenate multiple files and display the resulting result on the screen −

$ cat chúng tôi chúng tôi file3.txt

To concatenate multiple files and save the resulting output to a new file −

As you can see, the cat command is a useful tool for viewing and manipulating text files in Linux. In the next section, we will look at how to use the cat command to write text to a file.

Write Text to a File Using the Cat Command

To write text to a file using the cat command, you can use the echo command to output the text and pipe it to the cat command. The echo command is a built-in shell command used to display a message on the screen or to write a message to a file.

Here’s an example of using the echo and cat commands to write “Hello world!” in a new file called chúng tôi −

For example, to add the text “This is a new line” to the end of the “hello.txt” file, you could use the following command −

Here is the output you would see if you ran this command −

Hello, World! This is a new line

You can also use the -n flag with the echo command to suppress the newline character at the end of the output. This can be useful if you want to write multiple lines to a file without adding extra blank lines between them.

For example, to write the following three lines to a file named “lines.txt” without adding any extra blank lines between them −

This is line 1. This is line 2. This is line 3.

You can use the following command −

This will write the three lines to the “lines.txt” file as follows −

This is line chúng tôi is line chúng tôi is line 3. This is line 1. This is line 2. This is line 3.

You can use the following command −

This is line 1. This is line 2. This is line 3. EOF

The << symbol indicates the start of input, and the string EOF (which can be any string you choose) indicates the end of input. The cat command will write everything between the << and EOF symbols to the “direct.txt” file.

Conclusion

In this article, we have seen how to use the cat command to write text to a file in Linux. The cat command is a powerful and versatile utility that can be used for a variety of tasks, including concatenating files, viewing file contents, and writing text to a file. Using the various switches and flags available with the cat command, you can customize its behavior to meet your specific needs.

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