Unix file permissions
I have used Unix for the major portion of my professional career, yet I have always failed to understand the intricacies surrounding the file permissioning system. This article is an attempt to dig deep and augment my understanding about them.
Feb 16, 2014 // 12 mins read
The file permissions on a typical Unix system looks like this,
$ ls -ld /usr/bin /usr/bin/cat drwxrwxr-x 3 root bin 8704 Sep 23 2004 /usr/bin -r-xr-xr-x 1 bin bin 9388 Jul 16 1997 /usr/bin/cat
The Unix file permissions can be visualized in two forms,
- Symbolic mode
- Octal mode
> SYMBOLIC MODE - --- --- --- | | | | | | | └── -- rw(x/t/T) for others (o) | | └── ------ rw(x/s/S) for groups (g) | └── ---------- rw(x/s/S) for a user (u) └── -------------- file type: regular (-) directory (d) character special (c) block special (b) fifo (p) symbolic link (l) socket (s)
In bit mode, the permissions are represented using the characters
T. The table below expands more on what each character stands for,
r -> Read w -> Write x -> Execute s -> Execute bit is ON and Set UID/GID bit is ON S -> Execute bit is OFF and Set UID/GID bit is ON t -> Execute bit is ON and Sticky bit is ON T -> Execute bit is OFF and Sticky bit is ON
When a specific permission for a file doesn’t apply to any of the user/group/other, Unix uses the default character
- to represent the lack of permission.
-r-xr-xrwx 1 joe bin 9388 Jul 16 1997 /usr/bin/cat
Joe has read/write/execute permissions on the file
/usr/bin/cat. However, all the other users of the group
bin to which Joe belongs have read and execute permissions only. All other users outside the group
bin can also read and execute that file.
rootuser is special.
rwxto all directories and
rwto all files. On a file, if any of the 3x bits are set, then root has execute permission. This special permission is often disabled on network mounted filesystems.
> OCTAL MODE 0 0 0 0 | | | | | | | └── -- r(4), w(2), x(1) for others (o) | | └── ------ r(4), w(2), x(1) for groups (g) | └── ---------- r(4), w(2), x(1) for a user (u) └── -------------- suid(4), sgid(2), sticky(1)
In the octal mode, the permissions are defined by 4-octal digits. The value for each of the last three digits can be computed as follows,
421 rwx 2^0 = 1 –> eXecute 2^1 = 2 –> Write 2^2 = 4 -> Read +-----+---+--------------------------+ | rwx | 7 | Read, write and execute | | rw- | 6 | Read, write | | r-x | 5 | Read, and execute | | r-- | 4 | Read, | | -wx | 3 | Write and execute | | -w- | 2 | Write | | --x | 1 | Execute | | --- | 0 | no permissions | +------------------------------------+ +------------+------+-------+ | Permission | Octal| Field | +------------+------+-------+ | rwx------ | 700 | User | | ---rwx--- | 070 | Group | | ------rwx | 007 | Other | +------------+------+-------+
The first digit also follows the same principle,
suid = 4 sgid = 2 sticky = 1
When a specific permission for a file doesn’t apply to any of the user/group/other, Unix uses
0 to represent the absence of all permissions for the corresponding user/group/other.
4755 ---- 4 = Set UID bit is ON. 7 = read(4) + write(2) + execute(1) permissions for the user. 5 = read(4) + execute(1) permissions for group members. 5 = read(4) + execute(1) permissions for other members outside the group.
If the first of the four digits is
0 then it means that the file doesn’t have any of the setuid, setgid or sticky bit present on it. Generally, the leading
0 can be omitted.
Set UID and Set GID bits
Apart from the usual
rwx bits, the UNIX permission system also has the
s bit which stands for SetUID for user or SetGID for group. For a file, if this bit is defined, then the
x is turned into
s for the corresponding user/group/others. For example,
$ ls -l /etc/passwd /etc/shadow /usr/bin/passwd -r--r--r-- 1 root sys 14006 Jan 14 11:17 /etc/passwd -r-------- 1 root sys 8281 Jan 14 11:18 /etc/shadow -r-sr-sr-x 3 root sys 96244 Sep 5 2001 /usr/bin/passwd
/etc/passwd file is writable only by
/etc/shadow file is where the passwords are stored and cannot be read by ordinary users.
joe wants to change his password. He can do that by running
/usr/bin/passwd. Notice those
r-s permissions. The passwd program has the suid and sgid bits set. This turns the x’s into s’s. The passwd program is owned by
joe runs it, it does not run as
joe. Instead, it runs as it owner which is
So the passwd program can change joe’s password for him. The sgid bit works the same way, except it causes the passwd program to run with the group
sys instead of joe’s group. The suid and sgid do not get their own position in the
ls. When the suid bit is set,
ls displays a
s rather than a
x for the owner execute permission.
The Sticky Bit
If the sticky bit is set on a directory, mere write permission on the directory is no longer enough to remove the files. You must additionally own the file or own the directory.
rootuser continues to be able to delete files from any directory regardless of the permissions.
The sticky bit affects the
other execute bit in the
ls display. Except that it uses
T rather than
S. For example,
drwxrwxrwt 5 root root 1024 Feb 11 20:43 /tmp
/tmp directory above, anyone can create new files. But because of the sticky bit, one user cannot delete another user’s files.
Some examples using special bits,
+-------------------+ | rwxrwxrwx 777 | all permissions granted | rwxr-xr-x 755 | group and others read & executable | rwx------ 700 | private file | rwsr-xr-x 4755 | set UID | rwxr-sr-x 2755 | set GID | rwxr-xr-t 1755 | sticky bit | rwSw-xr-x 4655 | setUID but not executable by user | rwxr-Sr-x 2745 | getGID, but not executable by group members | rwxr-xr-T 1754 | sticky bit, but not executable by others +-------------------+
rwxreally mean for a file?
For a file,
wpermissions are self-explanatory. The
xpermission means that the user can run the file which either is an output from a compiler or a shell script.
rwxreally mean for a directory? For a directory, things are a little more complex. A directory is a file too, and
rpermission means you can read it. With
wpermission you can create new files in the directory or remove old files.
It sometimes surprises people that you can remove a file which you cannot read. The unix
rmcommand will test for that and issue a warning, but you can suppress that warning with
rmdirwill not even bother to check at all.
But you really cannot do very much without
x permission. With directories, you usually have both read and execute permissions or neither. On a directory,
x is officially called “search permission”. You need
x to use a directory in a pathname. So if you try
cat /etc/passwd, you will need
/etc. You also need
x to cd into a directory.
Case 1 - Only `r` permission on a directory You can use `ls` to list the file names. But `ls -l` will not work. Case 2 - Only `w` permission on a directory This is completely useless and grants nothing at all. Case 3 - Only `x` permission on a directory When you have `x` permission but no `r` permission on a directory then you can open files in the directory if you happen to know the file's name. You can `cd` into the directory but cannot create a new file. Adding write permission will allow you to create files. And you can then delete files if you happen to know their name.
How to get octal permissions for a file?
File permissions in Linux can be displayed in octal format using Linux
$ stat -c '%A %a %n' * [Replace * with the relevant directory or the exact filename] $ man stat -c --format=FORMAT use the specified FORMAT instead of the default; output a newline after each use of FORMAT %A Access rights in human readable form %a Access rights in octal %n File name
The umask command
When a user creates a file or directory, it gets a default set of permissions. The user file-creation mode mask (
umask) is used to determine the file permissions for newly created files. It can be used to control the default file permission for new files. It is a four-digit octal number. A umask can be set or expressed using:
- Symbolic values
- Octal values
A umask set to
u=rwx,g=rwx,o= will result in new files having the modes
-rw-rw----, and new directories having the modes
$ umask u=rwx,g=rwx,o= $ umask 0007 $ mkdir foo $ touch bar $ ls -l drwxrwx--- 2 dave dave 512 Sep 1 20:59 foo -rw-rw---- 1 dave dave 0 Sep 1 20:59 bar
The octal notation for the permissions masked out are,
0 – Full permissions (Read, Write, Execute) 1 – Write and read 2 – Read and execute 3 – Read only 4 – Write and execute 5 – Write only 6 – Execute onlyadminadmin 7 – No permissions $ umask 022 $ mkdir foo $ touch bar $ ls -l drwxr-xr-x 2 dave dave 512 Aug 18 20:59 foo -rw-r--r-- 1 dave dave 0 Aug 18 20:59 bar
Though umask value is the same for files and folders, but the calculation of file base permissions and directory base permissions are different.
The minimum and maximum
umaskvalue for a folder is
777. The minimum and maximum
umaskvalue for a file is
666. This is because only scripts and binaries should have execute permissions, normal and regular files should have just read and write permissions. Directories require execute permissions for viewing the contents in it, so they can have
How the actual permissions are calculated from umask?
Suppose the umask value is
- For directories, the permission is derived by doing octal subtraction of
027from the maximum possible value,
777. So the directory permission is
750when its created. Owner will get full permission, group gets execute and write permissions and others no permissions.
- For files, the permission is derived by doing octal subtraction of
027from the maximum possible value,
666. So the file permission is
640when its created. Owner will get read & write permission, group gets read permission only, and others no permissions.
Find out the default umask
$ umask 0022
0 indicates there is no SUID/SGID/Sticky bit information set.
Some FAQ related to umask
1) How to set or change default
umask for all the new users? The
umask value can be set in
/etc/profile for all the new users. 2) How to set or change default
umask for existing users? For existing users you can edit
~/.bashrc file in their home directory.
Changing permissions with chmod
chmod command in Unix is abbreviated as CHange MODe. Chmod command is used to change permission for files and directories in Unix.
1) Symbolic method like
chmod +x filename.
The who part can be: u (user) g (group) o (other) a (all) (whatever is allowed by umask (subset of all)) The operator can be = or - or + = (set bits to bitlist) - (subtract bitlist from current bit + (add bitllist to current bits) The bitlist can be one of the following letters: r (read permission) w (write permission) x (execute permision) X (conditional execute permision) u (current permissions for user) g (current permissions for group) o (current permissions for others) s (set uid or set gid) t (sticky bit)
2) Octal method like
chmod 775 filename.
Let us review some examples in both symbolic and octal representaions for files.
+------------------------+-----------+ | chmod u=rwx,g=rwx,o=rx | chmod 775 | | chmod u=rwx,g=rx,o= | chmod 750 | | chmod u=rw,g=r,o=r | chmod 644 | | chmod u=rw,g=r,o= | chmod 640 | | chmod u=rw,go= | chmod 600 | | chmod u=rwx,go= | chmod 700 | +------------------------+-----------+