Linux Permissions Explained

Linux file access permissions are used to control who is able to read, write and execute a certain file.

Linux Permissions

What are File Permissions ?

In linux, every file or folder has access permissions. Linux file permissions are 9 bits of information (3 types permissions x 3 type of users)

Permissions Types:

- (r)read access 
- (w)write access 
- (x)execute access

Types of users:

- (u)owner of the file 
- (g)group the owner belongs 
- (o)other users

u is the user’s permissions, g is the group’s permissions & o other, can be any number between 0-7

Permission Indicators:

View Permissions:

Syntax below

[~]$ls -l – List items in current directory and show permissions.

[~]$ls -l [path of file or folder] – Show permissions for the given path.

Setting Permissions with Chmod:

Syntax below

[~]$chmod [permission] [path of file or folder]

where [permission] can be either octal or symbolic notation e.g:

chmod 077 foo.sh # octal chmod +x bar.sh # symbolic

changes permission of file bar.sh the named file to executable.

• + - adds permissions
• x - executable rights

Chmod octal value binary value and meaning:

chmod stands for change mode

Dash (–) means no permission.

Octal Read Write Execute Binary Meaning 7 r w x 1 1 1 Full permissions 6 r w — 1 1 0 Read & write only 5 r — x 1 0 1 Read & execute only 4 r — — 1 0 0 Read only 3 — w x 0 1 1 Write & execute only 2 — w — 0 1 0 Write only 1 — — x 0 0 1 Execute only 0 — — — 0 0 0 No permissions

Chmod Helper

Code Examples:

Golang:

Format: os.Chmod(path, mode) click here for docs

package main 
import "os" 
err := os.Chmod("file.txt", 0777) 
if err != nil { fmt.Println(err) }

PHP:

Format: chmod( string $path, int $mode); click here for docs

<?php 
chmod("/somedir/somefile", 755); // decimal; probably incorrect chmod("/somedir/somefile", "u+rwx,go+rx"); // string; incorrect chmod("/somedir/somefile", 0755); // octal; correct value of mode 
?>

Python:

Format: os.chmod(path, mode) click here for docs

import os os.chmod("/anydir/anyfile", 0777)

Tags: linux, linux-permissions

Originally published at www.irfanbaigse.info on April 14, 2017.