How to Use the chmod Command on Linux

Learn how to Use the chmod Command on Linux

Stylized terminal prompt on a Linux laptop
Fatmawati Achmad Zaenuri/Shutterstock.com

Management who can entry information, search directories, and run scripts utilizing the Linux’s chmod command. This command modifies Linux file permissions, which look sophisticated at first look however are literally fairly easy as soon as you understand how they work.

chmod Modifies File Permissions

In Linux, who can do what to a file or listing is managed via units of permissions. There are three units of permissions. One set for the proprietor of the file, one other set for the members of the file’s group, and a last set for everybody else.

The permissions management the actions that may be carried out on the file or listing. They both allow, or forestall, a file from being learn, modified or, if it’s a script or program, executed. For a listing, the permissions govern who can cd into the listing and who can create, or modify information inside the listing.

You employ the chmod command to set every of those permissions. To see what permissions have been set on a file or listing, we are able to use ls.

Viewing and Understanding File Permissions

We will use the -l (lengthy format) choice to have ls checklist the file permissions for information and directories.

ls -l

On every line, the primary character identifies the kind of entry that’s being listed. If it’s a sprint (-) it’s a file. If it’s the letter d it’s a listing.

The subsequent 9 characters symbolize the settings for the three units of permissions.

  • The primary three characters present the permissions for the person who owns the file (person permissions).
  • The center three characters present the permissions for members of the file’s group (group permissions).
  • The final three characters present the permissions for anybody not within the first two classes (different permissions).

There are three characters in every set of permissions. The characters are indicators for the presence or absence of one of many permissions. They’re both a touch (-) or a letter. If the character is a touch, it implies that permission is just not granted. If the character is an r, w, or an x, that permission has been granted.

The letters symbolize:

  • r: Learn permissions. The file may be opened, and its content material considered.
  • w: Write permissions. The file may be edited, modified, and deleted.
  • x: Execute permissions. If the file is a script or a program, it may be run (executed).
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For instance:

  •  --- means no permissions have been granted in any respect.
  •  rwx means full permissions have been granted. The learn, write, and execute indicators are all current.

In our screenshot, the primary line begins with a d. This line refers to a listing known as “archive.” The proprietor of the listing is “dave,” and the identify of the group that the listing belongs to can also be known as “dave.”

The subsequent three characters are the person permissions for this listing. These present that the proprietor has full permissions. The r, w, and x characters are all current. This implies the person dave has learn, write and execute permissions for that listing.

The second set of three characters are the group permissions, these are r-x. These present that the members of the dave group have learn and execute permissions for this listing. Meaning they’ll checklist the information and their contents within the listing, they usually can cd (execute) into that listing. They don’t have write permissions, so they can’t create, edit, or delete information.

The ultimate set of three characters are additionally r-x.  These permissions apply to people who find themselves not ruled by the primary two units of permissions. These individuals (known as”others”) have learn and execute permissions on this listing.

So, to summarise, group members and others have learn and execute permissions. The proprietor, a person known as dave, additionally has write permissions.

For the entire different information (aside from the mh.sh script file) dave and members of the dave group have learn and write properties on the information, and the others have learn permissions solely.

For the particular case of the mh.sh script file, the proprietor dave and the group members have learn, write, and execute permissions, and the others have learn and execute permissions solely.

Understanding The Permission Syntax

To make use of chmod to set permissions, we have to inform it:

  • Who: Who we’re setting permissions for.
  • What: What change are we making? Are we including or eradicating the permission?
  • Which: Which of the permissions are we setting?

We use indicators to symbolize these values, and type brief “permissions statements” equivalent to u+x, the place “u” means ” person” (who), “+” means add (what), and “x” means the execute permission (which).

The “who” values we are able to use are:

  • u: Consumer, that means the proprietor of the file.
  • g: Group, that means members of the group the file belongs to.
  • o: Others, that means individuals not ruled by the u and g permissions.
  • a: All, that means the entire above.
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If none of those are used, chmod behaves as if “a” had been used.

The “what” values we are able to use are:

  • : Minus signal. Removes the permission.
  • +: Plus signal. Grants the permission. The permission is added to the prevailing permissions. If you wish to have this permission and solely this permission set, use the = possibility, described beneath.
  • =: Equals signal. Set a permission and take away others.

The “which ” values we are able to use are:

  • r:  The learn permission.
  • w: The write permission.
  • x: The execute permission.

Setting And Modifying Permissions

Let’s say now we have a file the place everybody has full permissions on it.

ls -l new_ file.txt

We wish the person dave to have learn and write permissions and the group and different customers to have learn permissions solely. We will do utilizing the next command:

chmod u=rw,og=r new_file.txt

Utilizing the “=” operator means we wipe out any present permissions after which set those specified.

let’s examine the brand new permission on this file:

ls -l new_file.txt

The present permissions have been eliminated, and the brand new permissions have been set, as we anticipated.

How about including a permission with out eradicating the prevailing permissions settings? We will try this simply too.

Let’s say now we have a script file that now we have completed modifying. We have to make it executable for all customers. Its present permissions appear to be this:

ls -l new_script.sh

We will add the execute permission for everybody with the next command:

chmod a+x new_script.sh

If we check out the permissions, we’ll see that the execute permission is now granted to everybody, and the prevailing permissions are nonetheless in place.

ls -l new_script.sh

We may have achieved the identical factor with out the “a” within the “a+x” assertion. The next command would have labored simply as nicely.

chmod +x new_script.sh

Setting Permissions for A number of Information

We will apply permissions to a number of information all of sudden.

These are the information within the present listing:

ls -l

Let’s say we need to take away the learn permissions for the “other” customers from information which have a “.page” extension. We will do that with the next command:

chmod o-r *.web page

Let’s examine what impact that has had:

ls -l

As we are able to see, the learn permission has been faraway from the “.page” information for the “other” class of customers. No different information have been affected.

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If we had needed to incorporate information in subdirectories, we may have used the -R (recursive) possibility.

chmod -R o-r *.web page

Numerical Shorthand

One other manner to make use of chmod is to offer the permissions you want to give to the proprietor, group, and others as a three-digit quantity. The leftmost digit represents the permissions for the proprietor. The center digit represents the permissions for the group members. The rightmost digit represents the permissions for the others.

The digits you need to use and what they symbolize are listed right here:

  • 0: (000) No permission.
  • 1: (001) Execute permission.
  • 2: (010) Write permission.
  • 3: (011) Write and execute permissions.
  • 4: (100) Learn permission.
  • 5: (101) Learn and execute permissions.
  • 6: (110) Learn and write permissions.
  • 7: (111) Learn, write, and execute permissions.

Every of the three permissions is represented by one of many bits within the binary equal of the decimal quantity. So 5, which is 101 in binary, means learn and execute. 2, which is 010 in binary, would imply the write permission.

Utilizing this technique, you set the permissions that you want; you don’t add these permissions to the prevailing permissions. So if learn and write permissions have been already in place you would need to use 7 (111) so as to add execute permissions. Utilizing 1 (001) would take away the learn and write permissions and add the execute permission.

Let’s add the learn permission again on the “.page” information for the others class of customers. We should set the person and group permissions as nicely, so we have to set them to what they’re already. These customers have already got learn and write permissions, which is 6 (110). We wish the “others” to have learn and permissions, in order that they must be set to 4 (100).

The next command will accomplish this:

chmod 664 *.web page

This units the permissions we require for the person, group members, and others to what we require. The customers and group members have their permissions reset to what they already have been, and the others have the learn permission restored.

ls -l

Superior Choices

In the event you learn the person web page for chmod you’ll see there are some superior choices associated to the SETUID and SETGID bits, and to the restricted deletion or “sticky” bit.

For 99% of the circumstances you’ll want chmod for, the choices described right here may have you lined.

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