Changing Linux permissions
Authored by: John Abercrombie
This article describes how to change permissions on a Linux® cloud server by using the chmod command.
What is the chmod command?
Put simply, chmod stands for change mode and is used to set file or directory permissions in Linux. Root users use chmod to determine what users, groups, and others can access files or directories.
Typically, you use the chmod command in the following formats:
$ chmod (options) (permissions) (file name)
$ chmod (permissions) (file name)
The use of options with the chmod command is optional. The second example receives the most frequent use. Without an option present, chmod modifies the permissions of the file or directory designated in the command.
In a Linux system, you can use permissions to refer to the owner of the file or directory (user), the group that owns the file or directory (group), or anyone else who accesses the file or directory (others).
State these permissions by using the following notation:
alphanumeric characters (r, w, x)
octal numbers (0-7)
For example, you have a file called example.txt, and you want to set the following permissions:
The user can read, write, and execute the file.
The group members can read and execute, but not write the file.
Any others can only read the file.
The following command option sets your desired permissions for example.txt:
$ chmod u=rwx,g=rx,o=r example.txt
This option uses alphanumeric characters to denote users who can access the file as well as the permissions you want to set for these designated users.
The letter 'u' = user
The letter 'g' = group
The letter 'o' = other
The letter 'r' = read
The letter 'w' = write
The letter 'x' = execute
The following example shows how Linux portrays the permissions granted in the previous example:
-rwxr-xr--
The first three letters (rwx) are the user's permissions. The second three (r-x) represent the group's permissions, and the last three (r–) represent the others' permissions.
Next, we have the octal option for our example.txt exercise:
$ chmod 754 example.txt
In this example, the numbers 7, 5, and 4 each represent the user, group, and others permissions. The first number position determines the user's permission, the second number determines the group's permissions, and the third number assigns the permissions for others.
These numbers are not randomly selected. Adding together the numbers 4, 2, 1, and 0 determines the permissions. The numbers have the following assignments:
4 stands for 'read'
2 stands for 'write'
1 stands for 'execute'
0 stands for 'none'
Therefore, in our previous example, the 7 is determined from the combination of read (4), write (2), and execute (1): 4 + 2 + 1 = 7. The 5 is taken from adding read (4), none (0), and execute (1): 4 + 0 + 1 = 5. Lastly, the 4 is a combination of read (4), none (0), and none (0): 4 + 0 + 0 = 4.
You typically don't need to use options in the chmod command, but in case you do, you can use the following options with chmod:
-c, --changes: Gives a verbose output for changes made
-f, --silent, --quiet: Silences most error messages
-v, --verbose: Outputs a detailed message for every action processed
--no-preserve-root: Ignores the deference normally given by default to the / (root) directory
--preserve-root: Does not operate recursively on the / (root) directory
--reference=RFILE: Sets permissions to match those of RFILE, ignoring any specific MODE
-R, --recursive: Changes made to files or directories are applied recursively
--help: Displays the help message, then exits
--version: Displays version information, then exits
In addition to the permissions already discussed, the chmod command can set three other special permissions in Linux.
SUID (Set User ID)
SGID (Set Group ID)
SUID
An s in the user's rwx permission set, replacing the x, represents this permission, as shown in the following example:
$ chmod 4755 /usr/bin/passwd
$ ls -lh /usr/bin/passwd
-rwsr-xr-x.  1  root  root  28K  Mar  10  2020  /usr/bin/passwd
The first three letters in rwsr-xr-x represent the user's permissions. Notice that instead of an x there is an s in the user's three permission slots. Instead of showing x for execute, there is an s in the execute spot. Why is that?
Note: The 4 in front of the usual octal permission set represents this permission. So instead of 755, the permission is written as 4755.
You should use this option with caution because this particular permission allows a user to execute a binary program as though they were the owner of that program even though they are not. The most well-known example of this is the passwd command.
In the case of passwd, the user is able to execute the program even though the binary program is owned by root. However, because passwd is set as a SUID by default, it always executes as the root user.
If a regular user has SUID permissions on other binary programs, they could execute commands as root without having root permissions. It allows for an abnormal escalation of privileges that usually require sudo privileges to access.
SUID permissions can effect a precaution built into the Linux OS in the form of only binaries. It does not affect scripts.
SGID
An s in the group's rwx permission set, replacing the x in t r-x segment, represents the SGID permission, as shown in the following example:
$ chmod 2755 /usr/bin/screen
$ ls -lh /usr/bin/screen
-rwxr-sr-x.  1  root  screen  465K  Feb  10  2020  /usr/bin/screen
This s is in the second group of three permissions, which we discovered is the group's permissions earlier when written in alphanumeric style. In the octal rendering, a 2 in front of the other three octal permission numbers denotes this permission. Instead of 755, write this as 2755.
Like SUID, SGID permissions only work on binaries. They do not work on scripts. As to what SGID does, it allows unprivileged group members to execute a binary as if they were the root user.
Typically, you can use this permission on directories where members of the group need to have access within the directory shared by the group. Any files created in this directory have the same group owner, no matter which group member created the file.
A t in the others' rwx permission set, replacing the x, represents this permission, as shown in the following example:
$ chmod 1777 /tmp
$ ls -lhd /tmp
drwxrwxrwt.  8  root  root  4.0K  Nov  6  14:42  /tmp
Notice the t in the last set of three letters. The octal equivalent of the t is a 1 in front of the three permission octals. Instead of 777, write this as 1777.
What does this sticky bit do? Typically, you use this permission on a tmp directory, and its function is to prevent users from deleting files owned by other users. Usually, if a group has write access to a directory, any user within that group can delete any file within that directory. The sticky bit permission halts that. Only the creator of the file can delete it.
As a best practice, you should set the sticky bit permission on any directory whose others permission is octal 7 (read, write, execute). In our 754 example, you want to use the sticky bit permission if the octals are 757 where 7 is the third octal (representing the others permission).
Pemissions cheat sheet for files and directories
Following is a quick cheat sheet explaining each part of the example output:
-rwxrw-r--  rack  space  123G  Feb  03  15:36  example.txt
-: The - at the beginning tells you that this is a file. A d at the beginning indicates a directory.
rwx: The first three letters represent the file owner's permissions, and mean the owner can read, write, and execute the file example.txt.
rw-: The second three letters represent the group's permissions, and mean the group members can read and write to the file, but they cannot execute it.
r--: The third three letters represent the others' permissions, and mean that anyone who isn't the owner or in the group that owns the file can only read the file. They cannot write to the file or execute the file.
rack: The file's owner.
space: The group owner.
123G: The size of the file in gigabytes. An M would denote megabytes, and a K would denote kilobytes.
Feb 03 15:36: This denotes the date and time that the file was last modified.
example.txt: The name of the file. If you list this as '/example', it is the name of the directory.