Understanding Linux File Permissions and Permission Calculator

In this tutorial we'll look at how to set Linux File Permissions and how ownership works. Also a handy Linux File Permissions Calculator

Linux File permissions specify three things you can do with a file - read, write and execute. They are referred to in Linux by a single-letter code.

• r - read - you may view the contents of the file.
• w - write - you may change the contents of the file.
• x - execute - you may execute or run the file if it is a program or script.

For every file, we define three sets of people for whom we may specify permissions.

• owner - a single person who owns the file. (typically the person who created the file, but ownership may be granted to someone else by certain users)
• group - every file belongs to a single group.
• others - everyone else who is not in the group or the owner.

Directory permissions are similar; they have the same letters, but their meanings are slightly different.

• r - you can read the contents of the directory (i.e. do an ls)
• w - you can write into the directory (i.e. create files and directories)
• x - you can enter that directory (ie cd)

You can view permissions using the ls command with long descriptions enabled.

ls -l
total 52
drwxr-xr-x 2 timmy timmy  4096 Jun 24 18:01 Desktop
drwxr-xr-x 2 timmy timmy  4096 Jul  8 21:08 Documents
drwxr-xr-x 2 timmy timmy 12288 Aug 17 17:48 Downloads
drwxr-xr-x 2 timmy timmy  4096 Sep  3  2016 Music
drwxr-xr-x 2 timmy timmy  4096 Aug 15 21:34 Pictures
drwxr-xr-x 2 timmy timmy  4096 Sep  3  2016 Public
drwxr-xr-x 3 timmy timmy  4096 May 14 16:38 Software
drwxr-xr-x 2 timmy timmy  4096 Sep  3  2016 Templates
drwxr-xr-x 2 timmy timmy  4096 May 21 21:49 Videos
drwx------ 2 timmy timmy  4096 May 28 11:28 VirtualBox VMs

On each line, we can see the file type (d in this example, for directory) followed by three sets of three letters. A hyphen is used when permission is not set, so r-- means read-only, rw- means read and write, and rwx means read, write, and execute. The permissions are listed for the owner, group and others. Following those permissions, we can see the owner's username and the group name.

Changing Linux File Permissions

To change Linux permissions on a file or directory, we use a command called chmod, which stands for change mode bits.

The command chmod has arguments

• Who are we changing the permission for? [ugoa] - user (or owner), group, others, all
• Are we granting or revoking the permission - indicated with either a plus ( + ) or minus ( - )
• Which permission are we setting? - read ( r ), write ( w ) or execute ( x )

Example Linux File Permissions

Here are a few examples of commands for setting Linux file permissions.

Granting Execute permission on testfile

chmod +x testfile

Removing Write and Execute permissions from testfile

chmod –wx testfile

You'll notice that this only changes the permissions for the owner of the file, not the group or others. To change group or other permissions, specify (g)roup or (o) there on the permission flag.

Set the write permission to the group on testfile

chmod g+w testfile

Remove the write and execute permission from the group on testfile

chmod g-wx testfile

Set the write permission for the others

chmod o+w testfile

Remove the read, write and execute permissions from others

chmod o-rwx workfolder

Changing Ownership of Files in Linux

Another helpful command is changing ownership of files and directories. The command is "chown" along with "name of new owner" & "name of file."

chown timmy testfile

We can also combine change group and ownership command by:

chown timmy:users testfile

You can also use the -R flag to change ownership and permissions recursively.

Shorthand Linux File Permissions

The method outlined above isn't hard for setting permissions, but it can be tedious if there are lots of permissions to set. There are shorthand codes which you can use to speed up the process. The codes are based on a decimal number converted to binary. Let's see how these shorthand permission numbers work.

These octal numbers can be combined into three to form owner, group, and others, so a shorthand permission of 700 will give the user read, write, and execute permission, but nothing will be given to everyone else. A value of 327 will give write and execute (3) permission for the user, w (2) for the group, and read, write, and execute for other users. A value of 777 will grant read, write and execute for the owner, group and others and is generally considered unsafe.

Some common file permission combinations include

• 644 - readable and writeable by the owner of the file and readable by users in the group owner of that file and readable by everyone else.
• 755 - used for directories and is the same as 644; however, it has the execute bit set for everyone. The execute bit is needed to be able to change into the directory.

Linux File Permissions Calculator

Finally, here is a handy Linux Permissions calculator. Tick the boxes to set permissions; the correct octal number will be shown in the text boxes.