chmod 777 Launch!
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Linux chmod (full English: change mode) command is used to control user permissions to files.
In Linux/Unix, file access permissions are divided into three levels: file owner (Owner), user group (Group), and other users (Other Users).
Only the file owner and superuser can modify the permissions of files or directories. Permissions can be specified using absolute mode (octal numeric mode) or symbolic mode.
Usage permissions: all users
Syntax
chmod [-cfvR] [--help] [--version] mode file...
Parameter Description
mode: permission setting string, format as follows:
[ugoa...][[+-=][rwxX]...][,...]
Where:
- u represents the file owner, g represents the group to which the file owner belongs, o represents others besides these, a means all three.
-
- means add permission, - means remove permission, = means set permission exactly.
- r means readable, w means writable, x means executable, X means executable only if the file is a directory or already has execute permission set.
Other parameters:
- -c: only show changes if permission is actually modified
- -f: do not show error messages if permission cannot be changed
- -v: show detailed information about permission changes
- -R: apply changes recursively to all files and subdirectories in the current directory
- –help: display help information
- –version: display version
Symbolic Mode
Symbolic mode allows setting multiple items: who (user type), operator, and permission, where each item can be separated by commas. The chmod command modifies the access permissions of the specified user type(s) on a file. User types are represented by one or more letters in the ‘who’ position, as shown in the symbolic mode table for who:
| who | User Type | Description |
|---|---|---|
| u | user | file owner |
| g | group | file owner's group |
| o | others | all other users |
| a | all | all users, equivalent to ugo |
Operator symbols:
| Operator | Description |
|---|---|
| + | add permission for specified user type |
| - | remove permission for specified user type |
| = | set permission for specified user type, resetting all permissions |
Permission symbols:
| Symbol | Name | Description |
|---|---|---|
| r | read | set read permission |
| w | write | set write permission |
| x | execute | set execute permission |
| X | special execute | execute permission if file is directory or if other execute bit set |
| s | setuid/gid | set setuid or setgid bit depending on user type specified |
| t | sticky bit | set sticky bit, only superuser can set, only file owner (u) can use it |
Octal Syntax
The chmod command can use octal numbers to specify permissions. File or directory permissions are controlled by 9 permission bits grouped into three sets: user (owner) read, write, execute; group read, write, execute; others read, write, execute. Historically, file permissions were stored in bit masks where bits set to 1 indicate the corresponding permission.
| # | Permission | rwx | Binary |
|---|---|---|---|
| 7 | read + write + execute | rwx | 111 |
| 6 | read + write | rw- | 110 |
| 5 | read + execute | r-x | 101 |
| 4 | read only | r-- | 100 |
| 3 | write + execute | -wx | 011 |
| 2 | write only | -w- | 010 |
| 1 | execute only | --x | 001 |
| 0 | none | --- | 000 |
For example, 765 can be interpreted as:
- Owner permissions number: sum of owner’s three permission bits. For rwx, 4+2+1 = 7.
- Group permissions number: sum of group permission bits. For rw-, 4+2+0 = 6.
- Others permissions number: sum of others permission bits. For r-x, 4+0+1 = 5.
Examples
Set file file1.txt to be readable by everyone:
chmod ugo+r file1.txt
Set file file1.txt to be readable by everyone:
chmod a+r file1.txt
Set file1.txt and file2.txt so that owner and group have write permission, but others don’t:
chmod ug+w,o-w file1.txt file2.txt
Add execute permission to owner of ex1.py:
chmod u+x ex1.py
Set all files and subdirectories in current directory readable by everyone:
chmod -R a+r *
chmod can also use numbers to represent permissions, e.g.:
chmod 777 file
Syntax:
chmod abc file
Where a, b, c each represent a number for User, Group, and Other permissions respectively.
r=4, w=2, x=1
- For rwx attribute: 4+2+1=7;
- For rw- attribute: 4+2=6;
- For r-x attribute: 4+1=5.
chmod a=rwx file
and
chmod 777 file
have the same effect.
chmod ug=rwx,o=x file
and
chmod 771 file
have the same effect.
Using chmod 4755 filename can make the program have root permissions.
More Descriptions
| Command | Description |
|---|---|
| chmod a+r file | Add read permission for all users on file |
| chmod a-x file | Remove execute permission from all users on file |
| chmod a+rw file | Add read and write permissions for all users on file |
| chmod +rwx file | Add read, write, execute permissions for all users on file |
| chmod u=rw,go= file | Set read and write permissions for file owner, clear all permissions for group and others (space means no permissions) |
| chmod -R u+r,go-r docs | Add read permission for user and remove read permission for group and others on directory docs and all files/subdirectories recursively |
| chmod 664 file | Set read and write permission for file owner and group, and read permission for others |
| chmod 0755 file | Equivalent to u=rwx (4+2+1),go=rx (4+1 & 4+1). No special mode (0) |
| chmod 4755 file | 4 sets the setuid bit, remaining permissions are u=rwx (4+2+1),go=rx (4+1 & 4+1). |
| find path/ -type d -exec chmod a-x {} \; | Remove execute permission for all users on path/ and all its directories (not files). Use '-type f' to match files |
| find path/ -type d -exec chmod a+x {} \; | Allow all users to traverse or access the directory path/ |