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Adriano Galello
Adriano Galello

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Linux Files Permissions and Those chmod Numbers we Use 🀔

One thing to know about Linux is that you have file permissions for 3 categories.

  1. User: The user who owns the file.
  2. Group: Users belong to groups. Users in the same group also own the file.
  3. Others: Permissions for users that are not the owners of the file and don't belong to the group.

Example

Let's assume that your username is mary and you run an ls on your home folder

mary:/home# ls -lah
-rwxr--r--      1 mary   accounting    8 Jan 190 06:47 my_super_script.sh
 ^^^^^^^^^        ^^^^   ^^^^^^^^^^
 Permissions      User     Group
.....
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The first dash (-) represents the file type and it's not related to permissions.

Permissions are composed of 9 characters. In our case, those 9 characters are rwxr--r--

Any permission not granted will be represented by a dash (-).

Now, it might seem confusing reading rwxr--r--. This is because permissions for the 3 categories are put all together without spaces or delimiters of any kind.

But you can look at it like this to better understand it:

mary   accounting   Others    8 Jan 190 06:47 my_super_script.sh
rwx       r--        r--
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Here we broke down the 9 character string rwxr--r-- into 3 groups. Each one representing the three categories we mentioned earlier (user, group, others).

User Permission (mary) Group Permissions (accounting) Permissions for others
rwx r-- r--
  • User Permission: The user mary can read, write and execute the file (rwx).
  • Group Permissions: Users in the group accounting can read the file but they can't write it or execute it (r--)
  • Permission for others: Users that are not mary or don't belong to the accounting group can read the file but they can't write it or execute it (r--)

Changing permissions and those numbers on the chmod command

You've probably copied and paste things like chmod 644 myfile.html or raised hell with chmod -R 777 folder when something wasn't working.

Remember that chmod -R 777 folder will do this:

chmod777

But where do those numbers come from?. From this table:

Code Meaning
0 Permission denied
4 Read access
2 Write permission
1 Execute permission

Let's see some examples and use the table above to better understand it

command user group others
chmod 664 6 4 4
4+2 (read + write) 4 (read) 4 (read)
chmod 600 6 0 0
4+2 (read + write) No read, write or execute No read, write or execute
chmod 777 7 7 7
4+2+1 (read + write + execute) 4+2+1 (read + write + execute) 4+2+1 (read + write + execute)

Remember that you set permissions for 3 categories: user, group and others

What you're doing it's just adding up the code for each permission in each category.

Read Official Docs, it's always helpful

For more detailed information read the manual https://linux.die.net/Intro-Linux/sect_03_04.html

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