💡 Today I Learned

chmod - and the magic of file permissions

Allow everything

chmod 777 /opt/webapps

Causes these permissions:

drwxrwxrwx

How is this true? What does this mean?

777 -- three digits, each digit is the value of 3 flags, 3 bits.

Each digit refers to three different groups:

This

drwxrwxrwx

...can be broken out into 4 categories:

d  rwx   rwx   rwx

Concentrate on those last three, the repeated 'rwx's...

 +---------------  Permissions of the owner of the file,
 |     +---------  The group associated with the file, and
 |     |     +---  Everybody else.
 |     |     |
rwx   rwx   rwx
|||   |||   |||
|||   |||   ||+-- execute permission for everybody else
|||   |||   |+--- write permission for everybody else
|||   |||   +---- read permission for everybody else
|||   |||
|||   ||+-- execute permission for the group associated with the file
|||   |+--- write permission for the group associated with the file
|||   +---- read permission for the group associated with the file
|||
||+-- execute permission for the owner of the file
|+--- write permission for the owner of the file
+---- read permission for the owner of the file

If a permission was missing it would be represented by a dash '-'.

The Building Blocks of a permission

---   No read, no write, no execute = 0
--x   Execute (not read or write) = 1
-w-   Write (not read or execute) = 2
r--   Read (not execute or write) = 4

All permissions are made by adding some combination of 0,1,2,4.

Common patterns

600: Owner can read write but not execute, no one else can read write

sudo chmod 600 ~/.ssh/id_dsa

777: Everyone can do everything

sudo chmod 777 /public/publicfile

Common combos

r-x   Read Execute, Not write = 5
rw-   Read Write, Not execute = 6
rwx   Read Write and Execute = 7

Add read (or write, or execute)

If all you want to do is "add read permissions" (for example) you can use this syntax:

chmod +r /public/README.md

or add write permissions:

chmod +w /public/README.md

or add eXecute permissions:

chmod +x /public/README.md

Test file permissions with test

The test command is often used in scripts for testing the r,w or x flag of a file.

e.g.

test -x /public/deleteEverything

Means "does the file /public/deleteEverything exist? and if so, does it have the x (execute) bit set?

Similarly:

test -r /public/README.md test -w /public/README.md

And output success or fail, with:

test -x /home/user1/.templateengine && echo "The file has the execute flag"
test -x /home/user1/.templateengine || echo "The file does not exist or it does exist but does not have the execute flag"

See also