File Permission | Basic Command in Linux | Devops Basic Part-8 | Techie Ansh

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In this Session we will learn about some basic command with example:
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Linux File Permissions
The basic Linux permissions model works by associating each system file with an owner and a group and assigning permission access rights for three different classes of users:

The file owner.
The group members.
Others (everybody else).
File ownership can be changed using the chown and chgrp commands.

Three file permissions types apply to each class of users:

The read permission.
The write permission.
The execute permission.

To view the file permissions, use the ls command:

ls -l file_name

Changing File permissions-:

The File permissions can be changed using the chmod command. Only root, the file owner, or user with sudo privileges can change the permissions of a file. Be extra careful when using chmod, especially when recursively changing the permissions. The command can accept one or more files and/or directories separated by space as arguments.

Permissions can be specified using a symbolic mode, numeric mode, or a reference file.

Symbolic (Text) Method
The syntax of the chmod command when using the symbolic mode has the following format:
chmod [OPTIONS] [ugoa…][-+=]perms…[,…] FILE...

The first set of flags ([ugoa…]), users flags, defines the users’ classes for which the permissions to the file are changed.

u - The file owner.
g - The users who are members of the group.
o - All other users.
a - All users, identical to ugo.
When the users’ flag is omitted, it defaults to a.

- - Removes the specified permissions.
+ - Adds specified permissions.
= - Changes the current permissions to the specified permissions. If no permissions are given after the = symbol, all permissions from the specified user class are removed.

The second set of flags ([-+=]), the operation flags, defines whether the permissions are to be removed, added, or set:

Here are some examples of how to use the chmod command in symbolic mode:

Give the members of the group permission to execute the file, but not to read and write to it:

chmod g=x filename
Remove the write permission for all users:

chmod a-w filename
Recursively remove the execute permission for other users:

chmod -R o-x dirname
Remove the read, write, and execute permission for all users except the file’s owner:

chmod og-rwx filename
The same thing can also be accomplished by using the following form:

chmod og= filename
Give read, write and execute permission to the file’s owner, read permissions to the file’s group, and no permissions to all other users:

chmod u=rwx,g=r,o= filename

Numeric Method:-

r (read) = 4
w (write) = 2
x (execute) = 1
no permissions = 0

Owner: rwx=4+2+1=7
Group: r-x=4+0+1=5
Others: r-x=4+0+0=4

To set up the setuid, setgid, and sticky bit flags, use four digits number.

When the 4 digits number is used, the first digit has the following meaning:

setuid=4
setgid=2
sticky=1
no changes = 0
The next three digits have the same meaning as when using 3 digits number.

If the first digit is 0 it can be omitted, and the mode can be represented with 3 digits. The numeric mode 0755 is the same as 755.