file permissions in linux

permission groups

every file/dir has 3 permission groups in linux:

• owner/user
• group
• others

each group has 3 permissions:

• read
• write
• execute

ex - drwxrwxr-x means the following:

splitting into parts: d rwx rwx r-x

• d - stands for dir, if its a file, it will be ”-”
• rwx - for the owner (rwx means the owner can r-read, w-write and x-execute)
• rwx - for the group (so the group can also read, write and execute)
• r-x - for other users (can read, can’t write but can execute)

How to change the permissions?

• chown - changing owner and group

sudo chown aniket <filename> # changes owner
sudo chown aniket:org <filename> # changes group and owner

• chmod - change mode

for changing a file using chmod we need to use the decimal equivalent numbers for “rwx” which are:-

• r= 4
• w= 2
• x= 1

so, lets say a group has all three permissions read, write and execute, it’s number will be:

r + w + x = 4 + 2 + 1 = 7

and if all groups want to have the same permissions it will be 777, so the command will be:

sudo chmod 777 <filename> # not the best way to give permissions

774 - owner (7) group (7) others (4) so the other people can only read the file

chmod table

so each group can have a number from 0 → 7 each having a permission combination!

some common permissions:

• 664 - File Baseline (rw → owner, group, and r → others)
• 755 - Directory Baseline (rwx → owner, rx→ group, others)

for a dir execute means ability to change into the directory!

• 400 - Key pair (r - owner, no permissions → others)

Links:

linux file permissions

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