How to use "chmod" under linux

This "chmod" command is be used quite a lot. and I cry for understanding and being familiar with this command. While I am looking its helpful, it cannot satisfy me. And then I make a decision to collect it belows:

 

reference:

1, "man chmod" under liunx

2, https://help.ubuntu.com/community/FilePermissions

 

In linux and Unix, everything is a file. The super user "root" has the ability to access any file on the sytem, while it is unsafe for most users by using "root". Each file has access restriction with permissions, user restrictions with owner/group association. and permissions are referred to as bits.

 

This "chmod" command is used to change file mode bits, so this command is able to change the access to any file.

There are three types of access restrictions:

Permission

Action

chmod option

read

(view)

r or 4

write

(edit)

w or 2

execute

(execute)

x or 1

There are also three types of user restrictions:

User

 ls output

owner

-rwx------

group

----rwx---

other

-------rwx

 

here givt the functions use "chmod" command:

 

(1) permissions in action  

----how to get the file's permissions and "/etc/hosts" as a example

$ ls -l /etc/hosts

and u will get "-rw-r--r-- 1 root root 259 2011-01-01 08:37 /etc/hosts", this results means

   owner = Read & Write (rw-)   group = Read (r--)   other = Read (r--)

 

(2) changing permissions

Here give the usage about "chmod" command: chmod {options} filename

 

Options

Definition

u

owner

g

group

o

other

x

execute

w

write

r

read

+

add permission

-

remove permission

=

set permission

 

and then give some examples below:

chmod with Letters

First create some empty files:

$ touch file1 file2 file3 file4 $ ls -l   and get the results: -rw-r--r-- 1 user user 0 Nov 19 20:13 file1 -rw-r--r-- 1 user user 0 Nov 19 20:13 file2 -rw-r--r-- 1 user user 0 Nov 19 20:13 file3 -rw-r--r-- 1 user user 0 Nov 19 20:13 file4

Add owner execute bit:

$ chmod u+x file1 $ ls -l file1   and get the results: -rwxr--r-- 1 user user 0 Nov 19 20:13 file1

Add other write & execute bit:

$ chmod o+wx file2 $ ls -l file2 and get the results: -rw-r--rwx 1 user user 0 Nov 19 20:13 file2

Remove group read bit:

$ chmod g-r file3 $ ls -l file3 and get the results -rw----r-- 1 user user 0 Nov 19 20:13 file3

Add read, write and execute to everyone:

$ chmod ugo+rwx file4 $ ls -l file4   and get the results: -rwxrwxrwx 1 user user 0 Nov 19 20:13 file4

chmod with Numbers

Usage: chmod {options} filename

Options

Definition

#--

owner

-#-

group

--#

other

1

execute

2

write

4

read

Owner, Group and Other is represented by three numbers. To get the value for the options determine the type of access needed for the file then add. For example if you want a file that has -rw-rw-rwx permissions you will use the following:

Owner

Group

Other

read & write

read & write

read, write & execute

4+2=6

4+2=6

4+2+1=7

$ chmod 667 filename Another example if you want a file that has --w-r-x--x permissions you will use the following:

Owner

Group

Other

write

read & execute

execute

2

4+1=5

1

  (3) recursive permission changes  ----to change the permissions of multiple files and directions with one command Recursive chmod with -R and sudo

To change all the permissions of each file and folder under a specified directory at once, use sudo chmod with -R

$ sudo chmod 777 -R /path/to/someDirectory $ ls -l and gets the results: -rwxrwxrwx 1 user user 0 Nov 19 20:13 file1 drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder -rwxrwxrwx 1 user user 0 Nov 19 20:13 file2  

Recursive chmod using find, pipemill, and sudo

$ sudo find /path/to/someDirectory -type f -print0 | xargs -0 sudo chmod 644 $ ls -l and gets the results: -rw-r--r-- 1 user user 0 Nov 19 20:13 file1 drwxrwxrwx 2 user user 4096 Nov 19 20:13 folder -rw-r--r-- 1 user user 0 Nov 19 20:13 file2   To change permission of only directories under a specified directory (including that directory): $ sudo find /path/to/someDirectory -type d -print0 | xargs -0 sudo chmod 755 $ ls -l and gets the results: -rw-r--r-- 1 user user 0 Nov 19 20:13 file1 drwxr--r-- 2 user user 4096 Nov 19 20:13 folder -rw-r--r-- 1 user user 0 Nov 19 20:13 file2   (4) changing the file owner and group-----to change file's owner using the chown command. For example, to change the foobar file's owner to tux: $ sudo chown tux foobar To change the foobar file's group to penguins, you could use either chgrp or chown with special syntax: $ sudo chgrp penguins foobar $ sudo chown :penguins foobar Finally, to change the foobar file's owner to tux and the group to penguins with a single command, the syntax would be: $ sudo chown tux:penguins foobar