Understanding Unix Change Mode (chmod)

CHMOD is used to change permissions of a file, here’s how to calculate permission number


Number Meaning
400 read by owner
200 write by owner
100 execute by owner
040 read by group
020 write by group
010 execute by group
004 read by anyone
002 write by anyone
001 execute by anyone

To get a combination, just add them up.

For example, to grant read, write and execute permission for the owner of the file, read and execute for the group of the file and execute for anyone

400 + 200 + 100 + 040 + 010 + 001 = 751

Here’s how to read and understand permissions:

Let’s assume you ran ls -lah in a macOS directory

Owner  Group  Anyone
rwx    rwx    rwx    john staff foo
rwx    rwx    r-x    john staff bar
rw-    rw-    r-x    john staff baz

foo is 777
bar is 775
baz is 665


r Read
w Write
x Execute
- No permission

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