File changed as we read it linux

File Permissions in Linux / Unix: How to Read, Write & Change?

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.

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.

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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.

Let’s see file permissions in Linux with examples:

ls – l on terminal gives

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.

Here, the first ‘–‘ implies that we have selected a file.p>

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

The characters are pretty easy to remember.

r = read permission
w = write permission
x = execute permission
– = 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:

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

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.

chmod permissions filename

There are 2 ways to use the command –

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
0	No Permission	—
1	Execute	–x
2	Write	-w-
3	Execute + Write	-wx
4	Read	r–
5	Read + Execute	r-x
6	Read +Write	rw-
7	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.

Adds a permission to a file or directory

Sets the permission and overrides the permissions set earlier.

The various owners are represented as –

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How to Fix tar Command “Error: file changed as we read it” in Linux

The tar command is a commonly used tool for creating backups and archiving files in Linux systems. However, sometimes, when using the tar command, you may encounter the “error: file changed as we read it” error, which can be frustrating and hinder efficient system operation.

In this article, we will explore the “error: file changed as we read it” error, its causes, and solutions to fix it.

Understanding the “error: file changed as we read it” Error

The “error: file changed as we read it” error is a common error that occurs when using the tar command to create backups or archives in Linux systems. This error typically indicates that the tar command is trying to read a file that is being modified at the same time.

There are several scenarios that can trigger this error, such as the modification of a file during the backup process or the opening of a file by another program while the backup is in progress. This error can have a significant impact on the tar command operation, causing incomplete backups or archives.

Fixing the “error: file changed as we read it” Error

There are several solutions to fix the “error: file changed as we read it” error when using the tar command in Linux systems.

Solution 1: Using the –ignore-failed-read Parameters

One solution is to use the –ignore-failed-read parameters when using the tar command. This option instructs the tar command to ignore any errors encountered during the backup or archiving process, allowing the backup or archive process to continue despite the “error: file changed as we read it” error.

Note: This solution may result in incomplete backups or archives, leading to data loss.

Solution 2: Using the –exclude Option

Another solution is to use the –exclude option when using the tar command. This option instructs the tar command to exclude files that are being modified during the backup or archiving process, preventing the “error: file changed as we read it” error from occurring.

This solution is effective in preventing the error, but it requires manual identification and exclusion of files being modified during the backup process, which can be time-consuming.

Solution 3: Using the –warning=no-file-changed Option

A third solution is to use the –warning=no-file-changed option when using the tar command. This option instructs the tar command to treat the “error: file changed as we read it” error as a warning rather than an error, allowing the backup or archive process to continue.

This solution is effective in preventing the error from halting the backup or archiving process, but it can also result in incomplete backup.

Tips to Avoid the “error: file changed as we read it” Error

This error may be caused by files being archived by tar command itself being included in the tar archive file being generated.

Let’s take example to make it clearer:

The reason why the error is occurring is because the tar command is trying to create an archive file named “backup.tar“, which is also included in the list of files being archived. This causes the tar command to interpret “backup.tar” as both an input file and an output file, leading to an error because the file is being modified while it’s being archived.

To resolve the error, it is necessary to ensure that the output tar file is not included in the list of files being archived. This can be done by either changing the name or location of the output file, or by excluding it from the list of input files for the tar command. By doing so, the tar command will not interpret the output file as an input file, and the error will be avoided.

Conclusion

The “error: file changed as we read it” error is a common issue encountered when using the tar command in Linux systems. Understanding the causes of this error and implementing solutions such as using the –exclude option or making it sure that the output tar file is not being part of the files which are going to be archived.

About the author

Kashif

I am an Electrical Engineer. I love to write about electronics. I am passionate about writing and sharing new ideas related to emerging technologies in the field of electronics.

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tar: /: file changed as we read it. Does this mean that tar skipped one or more files?

I am running Ubuntu 16.0.4 LTS and I’m compressing a directory that contains 32Gb of (CSV) files, so that I can remove the 800k+ files in my directory and create some space.. I run the following command tar czxvf /path/to/archive.tar.gz /path/to/source/directory The last line before the command prompt returned (i.e. tar completed the task) was:

From the tar documentation, this is only a warning message. However, I need assurance that the 800K+ files have all been correctly archived and compressed by tar, before deleting the original files. Does the fact that the only message from tar did not contain an error message mean that no errors occured and that it is safe to delete the original files?

1 Answer 1

The tar documentation doesn’t specify, but from other sources (including this) it is apparent that tar does archive the file, but it may have captured a mixture where parts of the file were from before a change and parts were after.

For example, tar reads the first X bytes of a file. File changes. Tar reads the next Y bytes of the file. The archived version now contains X bytes from before the change and Y bytes from after.

In many cases, such as when the file is merely being appended to like a log file, this won’t really matter since the only changes occur after the end of the file and the existing file data isn’t touched. But it varies.

Note: there are some situations, like a file being truncated during reading, in which tar cannot continue and outputs an error. However, if all you received was the warning «file changed as we read it», then it did archive the file.

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