chmod

chmod is a Unix command for changing file permissions and ownership.

Getting started

Syntax 

$ chmod [options]

Example 

$ chmod 755 foo.txt
$ chmod +x main.py
$ chmod u-x main.py
$ chmod u=rwx,g=rx,o= run.sh

Change files and directories recursively 

$ chmod -R 755 my_directory

The chmod command stands for “change mode”

Chmod Generator

Chmod Generator allows you to quickly and visually generate permissions in numerical and symbolic.

Common Permissions

Command s Meaning
400 r——– Readable by owner only
500 r-x—— Avoid Changing
600 rw——- Changeable by user
644 rw-r–r– Read and change by user
660 rw-rw—- Changeable by user and group
700 rwx—— Only user has full access
755 rwxr-xr-x Only changeable by user
775 rwxrwxr-x Sharing mode for a group
777 rwxrwxrwx Everybody can do everything

Explains 

$ ls -l
-rw-r--r--  1 root root 3 Jun 29 15:35 a.log
drwxr-xr-x  2 root root 2 Jun 30 18:06 dir

Permission analysis of “dir” 

d  rwx  r-x  r-x
┬  ─┬─  ─┬─  ─┬─  
│   │    │    │  
│   │    │    └─ 4. Other｜5 (4+0+1)
│   │    └────── 3. Group｜5 (4+0+1)
│   └─────────── 2. User ｜7 (4+2+1)
└─────────────── 1. File Type | directory

Permission Modes

Permission Description Octal Decimal
--- No Permission 000 0 (0+0+0)
--x Execute 001 1 (0+0+1)
-w- Write 010 2 (0+2+0)
-wx Execute and Write 011 3 (0+2+1)
r-- Read 100 4 (4+0+0)
r-x Read and Execute 101 5 (4+0+1)
rw- Read and Write 110 6 (4+2+0)
rwx Read, Write and Execute 111 7 (4+2+1)

Objects

Who (abbr.) Meaning
u User
g Group
o Others
a All, same as ugo

Permissions

Abbreviation Permission Value
r Read 4
w Write 2
x Execute 1
- No permission 0

File Types

Abbreviation File Type
d Directory
- Regular file
l Symbolic Link

Examples

Operators

Symbol Description
+ Add
- Remove
= Set

chmod 600 

$ chmod 600 example.txt
$ chmod u=rw,g=,o= example.txt
$ chmod a+rwx,u-x,g-rwx,o-rwx example.txt

chmod 664 

$ chmod 664 example.txt
$ chmod u=rw,g=rw,o=r example.txt
$ chmod a+rwx,u-x,g-x,o-wx example.txt

chmod 777 

$ chmod 777 example.txt
$ chmod u=rwx,g=rwx,o=rwx example.txt
$ chmod a=rwx example.txt

Symbolic mode

Deny execute permission to everyone.

$ chmod a-x chmodExampleFile.txt

Allow read permission to everyone.

$ chmod a+r chmodExampleFile.txt

Make a file readable and writable by the group and others.

$ chmod go+rw chmodExampleFile.txt

Make a shell script executable by the user/owner.

$ chmod u+x chmodExampleScript.sh

Allow everyone to read, write, and execute the file and turn on the set group-ID.

$ chmod =rwx,g+s chmodExampleScript.sh

Removing Permissions

In order to remove read write permissions given to a file, use the following syntax:

$ chmod o-rw example.txt

For our file example.txt, we can remove read write permissions using chmod for group by running the following command:

$ chmod  g-rx example.txt

To remove chmod read write permissions from the group while adding read write permission to public/others, we can use the following command:

$ chmod g-rx, o+rx example.txt

But, if you wish to remove all permissions for group and others, you can do so using the go= instead:

$ chmod go= example.txt

Executable 

$ chmod +x ~/example.py
$ chmod u+x ~/example.py
$ chmod a+x ~/example.py

chmod 754 

$ chmod 754 foo.sh
$ chmod u=rwx,g=rx,o=r foo.sh

Practices

SSH Permissions 

$ chmod 700 ~/.ssh 
$ chmod 600 ~/.ssh/authorized_keys 
$ chmod 600 ~/.ssh/id_rsa 
$ chmod 600 ~/.ssh/id_rsa.pub 
$ chmod 400 /path/to/access_key.pem

Web Permissions 

$ chmod -R 644 /var/www/html/
$ chmod 644 .htaccess
$ chmod 644 robots.txt
$ chmod 755 /var/www/uploads/
$ chmod 755 `find /var/www/html -type d`

Batch Change 

$ chmod -R 644 /your_path 
$ find /path -type d -exec chmod 755 {} \; 
$ find /path -type f -exec chmod 644 {} \; 
$ chmod 644 $(find /your_path -type f)

See: Command Substitution

Also see