In Linux and Unix, everything is a file. Directories are files, files are files and devices are files. Devices are usually referred to as a node; however, they are still files. All of the files on a system have permissions that allow or prevent others from viewing, modifying or executing. If the file is of type Directory then it restricts different actions than files and device nodes. The super user «root» has the ability to access any file on the system. Each file has access restrictions with permissions, user restrictions with owner/group association. Permissions are referred to as bits.
To change or edit files that are owned by root, sudo must be used — please see RootSudo for details.
If the owner read & execute bit are on, then the permissions are:
There are three types of access restrictions:
There are also three types of user restrictions:
Note: The restriction type scope is not inheritable: the file owner will be unaffected by restrictions set for his group or everybody else.
Folder/Directory Permissions
Directories have directory permissions. The directory permissions restrict different actions than with files or device nodes.
read restricts or allows viewing the directories contents, i.e. ls command
write restricts or allows creating new files or deleting files in the directory. (Caution: write access for a directory allows deleting of files in the directory even if the user does not have write permissions for the file!)
execute restricts or allows changing into the directory, i.e. cd command
First create some empty files:
user@host:/home/user$ touch file1 file2 file3 file4 user@host:/home/user$ ls -l total 0 -rw-r--r-- 1 user user 0 Nov 19 20:13 file1 -rw-r--r-- 1 user user 0 Nov 19 20:13 file2 -rw-r--r-- 1 user user 0 Nov 19 20:13 file3 -rw-r--r-- 1 user user 0 Nov 19 20:13 file4
user@host:/home/user$ chmod u+x file1 user@host:/home/user$ ls -l file1 -rwxr--r-- 1 user user 0 Nov 19 20:13 file1
Add other write & execute bit:
user@host:/home/user$ chmod o+wx file2 user@host:/home/user$ ls -l file2 -rw-r--rwx 1 user user 0 Nov 19 20:13 file2
user@host:/home/user$ chmod g-r file3 user@host:/home/user$ ls -l file3 -rw----r-- 1 user user 0 Nov 19 20:13 file3
Add read, write and execute to everyone:
user@host:/home/user$ chmod ugo+rwx file4 user@host:/home/user$ ls -l file4 -rwxrwxrwx 1 user user 0 Nov 19 20:13 file4 user@host:/home/user$
chmod with Numbers
Owner, Group and Other is represented by three numbers. To get the value for the options determine the type of access needed for the file then add.
For example if you want a file that has -rw-rw-rwx permissions you will use the following:
user@host:/home/user$ chmod 667 filename
Another example if you want a file that has —w-r-x—x permissions you will use the following:
user@host:/home/user$ chmod 251 filename
Here are a few examples of chmod usage with numbers (try these out on your system).
First create some empty files:
user@host:/home/user$ touch file1 file2 file3 file4 user@host:/home/user$ ls -l total 0 -rw-r--r-- 1 user user 0 Nov 19 20:13 file1 -rw-r--r-- 1 user user 0 Nov 19 20:13 file2 -rw-r--r-- 1 user user 0 Nov 19 20:13 file3 -rw-r--r-- 1 user user 0 Nov 19 20:13 file4
user@host:/home/user$ chmod 744 file1 user@host:/home/user$ ls -l file1 -rwxr--r-- 1 user user 0 Nov 19 20:13 file1
Add other write & execute bit:
user@host:/home/user$ chmod 647 file2 user@host:/home/user$ ls -l file2 -rw-r--rwx 1 user user 0 Nov 19 20:13 file2
user@host:/home/user$ chmod 604 file3 user@host:/home/user$ ls -l file3 -rw----r-- 1 user user 0 Nov 19 20:13 file3
Add read, write and execute to everyone:
user@host:/home/user$ chmod 777 file4 user@host:/home/user$ ls -l file4 -rwxrwxrwx 1 user user 0 Nov 19 20:13 file4 user@host:/home/user$
chmod with sudo
Changing permissions on files that you do not have ownership of: (Note that changing permissions the wrong way on the wrong files can quickly mess up your system a great deal! Please be careful when using sudo!)
To change the permissions of multiple files and directories with one command. Please note the warning in the chmod with sudo section and the Warning with Recursive chmod section.
To change all the permissions of each file and folder under a specified directory at once, use sudo chmod with -R
user@host:/home/user$ sudo chmod 777 -R /path/to/someDirectory user@host:/home/user$ ls -l total 3 -rwxrwxrwx 1 user user 0 Nov 19 20:13 file1 drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder -rwxrwxrwx 1 user user 0 Nov 19 20:13 file2
Recursive chmod using find, pipemill, and sudo
To assign reasonably secure permissions to files and folders/directories, it’s common to give files a permission of 644, and directories a 755 permission, since chmod -R assigns to both. Use sudo, the find command, and a pipemill to chmod as in the following examples.
To change permission of only files under a specified directory.
user@host:/home/user$ sudo find /path/to/someDirectory -type f -print0 | xargs -0 sudo chmod 644 user@host:/home/user$ ls -l total 3 -rw-r--r-- 1 user user 0 Nov 19 20:13 file1 drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder -rw-r--r-- 1 user user 0 Nov 19 20:13 file2
To change permission of only directories under a specified directory (including that directory):
user@host:/home/user$ sudo find /path/to/someDirectory -type d -print0 | xargs -0 sudo chmod 755 user@host:/home/user$ ls -l total 3 -rw-r--r-- 1 user user 0 Nov 19 20:13 file1 drwxr-xr-x 2 user user 4096 Nov 19 20:13 folder -rw-r--r-- 1 user user 0 Nov 19 20:13 file2
Warning with Recursive chmod
WARNING: Although it’s been said, it’s worth mentioning in context of a gotcha typo. Please note, Recursively deleting or chown-ing files are extremely dangerous. You will not be the first, nor the last, person to add one too many spaces into the command. This example will hose your system:
Posix ACLs are a way of achieving a finer granularity of permissions than is possible with the standard Unix file permissions. See the full page on ACLs FilePermissionsACLs
The commands, setfacl and getfacl, set and read ACLs on files and directories.
Example Usage
mkdir shared_dir chmod 777 shared_dir
Set the default ACL with ‘-d’ and modify with ‘-m’ the permissions for samba nobody user nogroup group which will apply to all newly created file/directories.
where filename is the name and path of the file to delete.
Nota bene: Be very careful when using the command rm with the -rf option since -r makes the file removal recursive (meaning it will remove files inside of folders) and -f will force the removal even for files which aren’t writable. To play it safe, please consider typing in the absolute path to the file
sudo rm -rf /path/to/file/filename
to prevent any mishaps that can/will occur. It takes longer to type but you can’t put a price on peace of mind. See the rm man page for details.
Sticky Bit
The sticky bit applies only to directories, and is typically used on publicly-writeable directories. Within a directory upon which the sticky bit is applied, users are prevented from deleting or renaming any files that they do not personally own.
To add or remove the sticky bit, use chmod with the «t» flag:
The status of the sticky bit is shown in the other execute field, when viewing the long output of ls. «t» or «T» in the other execute field indicates the sticky bit is set, anything else indicates it is not.
Making a public directory:
user@host:/home/user$ mkdir folder user@host:/home/user$ chmod 777 folder user@host:/home/user$ ls -l total 3 drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder
Adding the sticky bit (note the «t» in the other execute field):
user@host:/home/user$ chmod +t folder user@host:/home/user$ ls -l total 3 drwxrwxrwt 2 user user 4096 Nov 19 20:13 folder
See also
ToDo
umask (add file and directory umask section, with specific focus on security)
The User Private Group scheme. In other words, this page does the nuts and bolts ok, but we need to describe what the permissions should be. The default Ubuntu set up is not agnostic: Every user has their default private group. Directories for collaboration need to have special group and permission set for correct functioning.
* Suggestion: I often use find instead of chmod -R, because it’s easier to differentiate between files and directories that way. Yes, I know about the ‘X’ permission, but I don’t trust it.
The sticky bit. It’s needed for «other» in shared directories like /tmp. It’s needed for «group» in shared directories where write permission is given to a group, like /var/www
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