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Understanding Linux file permissions

directory, this attribute decides whether you have permission to enter,

run a search through that directory or execute some program from that


Take the permissions of tmp, which are drwxr-x---. The owner of this

directory is user mayank and the group owner of the directory is freeos.

The first 3 permission attributes are rwx. This permission allows full

read, write and execute access to the directory to user mayank. So, mayank

has full access here. The group permissions are r-x. There is no write

permission given here so while members of the group freeos can change into

the directory and list its contents, they cannot create new files or

sub-directories. They also cannot delete any files or make changes to the

directory content in any way. No one else has any access because the

access attributes for others are empty (---).

For foo the permissions are -rw-r--r--. Apply the above and you will see

that the owner of the file (mayank) can read and modify the file (Read and

Write access). Members of the group freeos can read the file but cannot

modify it. Everyone else can also read the file but not make any changes

to it.

Now that you can read file permissions, you should learn about how you can

set or modify permissions. You would use the chmod program for this. To

change file permissions, you need to be either the user or root. The

syntax of the chmod command is quite simple. File permissions may be

defined for users (u), groups (g) and others (o).

An example of the chmod command will be

chmod u-x,g+w,o+rw somefile

The chmod command here takes away execute permission from the user, sets

the write access bit for the group and also gives read and write access to

everyone else. The file permissions for the file before this command is

executed are -rwxr-xr-. After this command, the file permissions are

-rwxrwx---. First you choose to use 'u','g' or 'o' followed by '+' to add

a permission, '-' to take it away and '=' to wipe out any previous

permission bits and set the permission bits to what is specified. You can

also use 'a' to set a permission bit for all users.

Let's take permissions of -rwxrwxrwx for somefile and work on them.

chmod g-wx somefile

We're removing write and execute permission for members of the group. The

file will now have attributes of -rwxr-rwx.

You can also specify permissions for users, groups or others in the same

command, separated but commas.