| 0<\/td> | No permission<\/td><\/tr><\/tbody><\/table><\/figure> To set multiple permissions, simply add the numbers. For example, 3 (1+2) <\/strong>makes an item executable and writable, while 7 (1+2+4) <\/strong>will grant full privileges over the item.<\/p>For example, this command will set the script.sh<\/strong> file’s permission to -rwxrw-r–<\/strong>:<\/p>chmod 764 script.sh<\/pre>Numerical notation is more straightforward than symbolic. However, it is less flexible as you can’t add or remove a certain permission, making it suitable for applying specific settings quickly.<\/p> Let’s say an owner can read and write an item, but you want to make it executable. With symbolic, you can add the permission using u+x<\/strong>. Using numerical, you must add the existing privilege again using 7 <\/strong>instead of only 1<\/strong>.<\/p>Recursive permission changes<\/h3>The chmod’s -R<\/strong> option enables the recursive mode, which lets you set new permissions to a folder and all its content using a single command.<\/p>For example, we have a script <\/strong>folder containing the subscript1<\/strong> and subscript2<\/strong> subdirectories, as well as several .sh<\/strong> files. The structure looks as follows:<\/p>script\/\n│\n├── subscript\/\n│ └── script1.sh\n│\n├──subscript2\/\n│\n├── script2.sh\n│\n└── script3.sh<\/pre>Using recursive chmod, we can change permissions of all items above by simply modifying the script<\/strong> folder like so:<\/p>chmod -R 777 script<\/pre>Changing ownership with chown and chgrp<\/h2>In Linux, if you create an item, you will be its owner by default. If you belong to a group, all other members will inherit the same permissions. You can change the ownership using the chown<\/strong> or chgrp<\/strong> command.<\/p>Chown is the more common command, which lets you change the ownership to both users and groups. The syntax looks as follows:<\/p> chown [options] [user:group] [item]<\/pre>For example, if you want to change the ownership of script.sh<\/strong> to johndoe<\/strong>, who belongs to the admins<\/strong> group, use the following command:<\/p>chown johndoe:admins script.sh<\/pre>To grant ownership to a new user while keeping the current group, simply omit the group name like so:<\/p> chown johndoe script.sh<\/pre>Similarly, omit the user if you wish to change an item’s ownership to another group. However, ensure to start the group name with a colon (:<\/strong>) similar to this:<\/p>chown :admins script.sh<\/pre>The chown command also lets you add the -R<\/strong> option to change the item ownership recursively:<\/p>chmod -R johndoe:admins scriptfolder<\/pre>Unlike chown, chgrp only lets you change an item’s ownership to another group. Here’s an example command:<\/p> chgrp admins script.sh<\/pre>Special permission bits<\/h2>In addition to read, write, and execute, Linux has additional permissions that give you more control over items on your system. This section will explain what they are and how to set them up.<\/p> Setuid and setgid<\/h3>Set user ID<\/strong> (setuid) permission lets you execute a file as the owner instead of the current user. You can add it using the s <\/strong>character of the symbolic notation like so:<\/p>chmod u+s script.sh<\/pre>For example, if root <\/strong>is the owner of script.sh<\/strong>, executing the file will run it as root<\/strong> even if you are logged in as another user. This behavior is useful for launching a program as a specific user, especially during automation.<\/p>\n\n\n Important!<\/strong>
\nYour current user who will run the program must have the execute permission over the file. You can grant it by modifying the group or other user classes.<\/p><\/div>\n\n\n\n<\/p>After adding the s<\/strong> permission, the file owner’s privilege should show as rws<\/strong>. It is essentially the same as rwx<\/strong>, but indicates that setuid is active. The correct setuid permission might look like this:<\/p>-rwsr-xr-x<\/pre>Set group ID <\/strong>(setgid) is similar to setuid, except it runs files using the associated group’s permissions. To grant it, use the following command:<\/p>chmod g+s script.sh<\/pre>Like setuid, the account that runs the file should have the execute permission as the owner or other users. Setgid permissions might look like this:<\/p> -rwxr-sr-x<\/pre>When you create a new file inside a folder with setgid, the associated group will have ownership permission over it. Typically, your group will own the new file.<\/p> Sticky bit<\/h3>By default, all users with write permissions over a folder can rename and delete files inside it. This behavior is risky since they might accidentally remove important items.<\/p> Sticky bit<\/strong> prevents this by disallowing users to delete or rename files unless they are the owner. Setting it up won’t revoke the write permission, meaning users can modify the file’s content.<\/p>To enable sticky bit on a folder, add the t<\/strong> permission using the chmod symbolic mode like so:<\/p>chmod +t folder<\/pre>The t<\/strong> permission will replace other users’ execute privileges. Here’s an example of a folder’s permission with sticky bit enabled:<\/p>drwxrwxrwt<\/pre>You can also add sticky bit recursively to all subfolders and files inside a folder by adding the -R<\/strong> option like so:<\/p>chmod -R +t folder<\/pre>Conclusion<\/h2>Setting the proper permissions and ownership of items is crucial for server security and functionality. In Linux, you can grant read,<\/strong> write<\/strong>, and execute<\/strong> permissions to three user classes: owner<\/strong>, group<\/strong>, and others<\/strong>.<\/p>To change permissions in Linux, use chmod <\/strong>followed by the settings and items you want to modify. For the setting, use symbolic notation if you want more flexibility in changing the permissions or numerical if you want a simpler command.<\/p>Meanwhile, use chown<\/strong> to change an item’s ownership to another user or group. To modify the group ownership, you can also use the chgrp<\/strong> command.<\/p>In addition to the three permissions, Linux also has special ones. For example, setuid <\/strong>and setgid <\/strong>let you run a file as the owner or associated group. You can also add them using the chmod<\/strong> command.<\/p> |
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