Linux permissions help

1. What are file permissions

Every file or folder in Linux has access permissions. There are three types of permissions (what allowed to do with a file):

Permissions are defined for three types of users:

Thus, Linux file permissions are nine bits of information (3 types x 3 type of users), each of them may have just one of two values: allowed or denied.

Simply put, for each file it can be specified who can read or write from/to the file. For programs or scripts it also can be set if they are allowed to be executed.

2. File permissions notation

2.1. Textual representation like "-rwxr--r--"

It is used in Linux long directory listings. It consists of 10 characters. The first character shows the file type. Next 9 characters are permissions, consisting of three groups: owner, group, others. Each group consists of three symbols: rwx (in this order), if some permission is denied, then a dash "-" is used instead. Example:

-rwxr--r--
0123456789
r Read access is allowed
w Write access is allowed
x Execute access is allowed
- Replaces "r", "w" or "x" if according access type is denied

2.1.1. Examples

-rwxr-xr-x File,
owner has read, write, execute permissions,
group: only read and execute permissions,
others: only read and execute permissions. 
dr-x------ Directory,
owner has read and execute access,
group and others have no access

2.2. Numeric (octal) representation like "644"

If a numeric representation is used (like in chmod command, for example), then it is in the octal format (with the base of 8), and digits involved are 0 to 7. Octal format is used for the simplicity of understanding: every octal digit combines read, write and execute permissions together. Respective access rights for owner, group and others (in this order) are the last three digits of the numeric file permissions representation. Example: "0644". Here the second digit ("6" in the example) stands for rights of the owner, the third digit ("4" in the example) stands for rights of the group, the fourth digit ("4" in the example) stands for rights of others.

This table shows what numeric values mean:

Octal digit Text equivalent Binary value Meaning
0 --- 000 All types of access are denied
1 --x 001 Execute access is allowed only
2 -w- 010 Write access is allowed only
3 -wx 011 Write and execute access are allowed
4 r-- 100 Read access is allowed only
5 r-x 101 Read and execute access are allowed
6 rw- 110 Read and write access are allowed
7 rwx 111 Everything is allowed

We see that "1" stands for execute only, "2" stands for write only, "4" stands for read only. To combine the permissions you can simply add 1, 2 and 4 to get a needed combination. For instance, to get read and write permissions, you add 4 (read) and 2 (write), thus getting 6 (read and write). To get read and execute permissions, you add  4 (read) and 1 (execute), thus getting 5 (read and execute).

2.2.1. Examples

644 owner: read and write permissions,
group: only read permissions,
others: only read permissions. 
755 owner: read, write and execute permissions,
group: read and execute permissions,
others: read and execute permissions. 

2.2.2. Why there is a leading zero?

In programming, for instance, in C language, leading zero means that the value is in the octal format. Basically, it can be omitted. Owner, group and others rights are the last three digits of the permissions.

2.2.3. Four meaningful digits like "4755"

There are cases when you may come across four non-zero digits, in this case the first meaningful (non-zero) digit combines the following bits (in this order, high to low): SUID, SGID, sticky bit. We also know  that the last three are for owner, group and others.

See this table for more information about SUID and so on.

3. Difference in access permissions for files and folders

Access permissions for files and folders mean different things from the user standpoint. The table below shows the difference.

Access type File Folder
Read If the file contents can be read If the directory listing can be obtained
Write If user or process can write to the file (change its contents) If user or process can change directory contents somehow: create new or delete existing files in the directory or rename files.
Execute If the file can be executed If user or process can access the directory, that is, go to it (make it to be the current working directory)

4. Permissions required for web server

Web server assigns the rights of the web-server-specific user, typically user "nobody", to the connected web client, as if "nobody" is connected to the web server. "Nobody" doesn't belong to your group and thus it inherits permissions that "others" have to your files. 

5. Permissions set for FTP-uploaded files

When you upload files to your web hosting accounts, you become the owner of the files. Usually, by default files get 644 permissions, and depending on provider's FTP server configuration they may get different permissions in different situations. You also can change the file permissions with FTP client or by executing a chmod command in telnet.

6. Set user ID, set group ID, sticky bit

In addition to the basic permissions discussed above, there are also three bits of information defined for files in Linux:

6.1. Numeric representation

Octal digit Binary value Meaning
0 000 setuid, setgid, sticky bits are cleared
1 001 sticky bit is set
2 010 setgid bit is set
3 011 setgid and sticky bits are set
4 100 setuid bit is set
5 101 setuid and sticky bits are set
6 110 setuid and setgid bits are set
7 111 setuid, setgid, sticky bits are set

6.2. Textual representation

SUID If set, then replaces "x" in the owner permissions to "s", if owner has execute permissions, or to "S" otherwise. Examples:
-rws------ both owner execute and SUID are set
-r-S------ SUID is set, but owner execute is not set
SGID If set, then replaces "x" in the group permissions to "s", if group has execute permissions, or to "S" otherwise. Examples:
-rwxrws--- both group execute and SGID are set
-rwxr-S--- SGID is set, but group execute is not set
Sticky If set, then replaces "x" in the others permissions to "t", if others have execute permissions, or to "T" otherwise. Examples:
-rwxrwxrwt both others execute and sticky bit are set
-rwxrwxr-T sticky bit is set, but others execute is not set

7. Links

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