Save file permissions 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.

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 –

Источник

Back up and restore file permissions

Is there a way to back up and restore file ownership and permissions (the things that can be changed with chown and chmod )? You can do this in Windows using icacls. What about access control lists?

It would help if you stated which distro you are using, as different distros use different package managers.

3 Answers 3

You can do this with the commands from the acl package (which should be available on all mainstream distributions, but might not be part of the base installation). They back up and restore ACL when ACL are present, but they also work for basic permissions even on systems that don’t support ACL.

To back up permissions in the current directory and its subdirectories recursively:

getfacl -R . >permissions.facl 

setfacl --restore=permissions.facl 

@Gilles,based on unix.stackexchange.com/questions/364517/… setfacl files then cannot chmod again,maybe will cause conflict?

@kittygirl I have no idea what you’re asking. What does “setfacl files then cannot chmod again” mean? What does this have to do with unix.stackexchange.com/questions/364517/… ? What conflict?

I’m not aware of anything «off the shelf» that would do this. Here’s a starter script for you, though, that will handle basic permissions. It does not handle ACLs of any description — but your Question explicitly excludes those. (It will also fail on pathological filenames — those starting with whitespace, or containing non-printable characters.)

Save the permissions

find * -depth -exec stat --format '%a %u %g %n' <> + >/tmp/save-the-list 

Restore the permissions

while read PERMS OWNER GROUP FILE do chmod "$PERMS" "$FILE" chown "$:$" "$FILE" done 

#!/bin/bash # prepare files home="/home/exchange" cd $home >acl echo "#!/bin/bash">recovery_acl.sh echo "cd $home">>recovery_acl.sh f='./' # create acl file sorted by dir_level for i in `seq 0 15`;do find . -mindepth $i -maxdepth $i -type d -exec getfacl <> +|grep -E '*UTS|file:'>>acl done sed -i 's/default\:user/\-dm\ u/g' acl sed -i 's/default\:group/\-dm\ g/g' acl sed -i 's/user/\-m\ u/g' acl sed -i 's/group/\-m\ g/g' acl sed -i 's/\#\ file\:\ /\.\//g' acl sed -i 's,\\,\\\\,g' acl while IFS='' read -r line ; do # grep dir name if echo "$line" | grep -q "$f" ; then dir="$line" continue fi echo setfacl $line '"'$dir'"'>>recovery_acl.sh # grep non def acl (for files) if echo "$line" | grep -q '\-m' ; then echo setfacl $line '"'$dir'"'/*>>recovery_acl.sh fi done < "acl" sed -i "s/\\\134/\\\\\\\134/g" recovery_acl.sh sed -i "s/\\\040/\\\\ /g" recovery_acl.sh 

$ chmod --reference=reference_file file

$ ls -l users.list $ ls -l keys.list $ sudo chmod --reference=users.list keys.list $ ls -l keys.list

$ chown --reference=reference_file file

$ ls -l keys.list $ touch api.list $ ls -l keys.list $ sudo chown --reference=keys.list api.list $ ls -l api.list

$ sudo chmod --reference=users.list users1.list users2.list users3.list $ sudo chown --reference=users.list users1.list users2.list users3.list