The previous article introduced linux file properties and permissions.
This article introduces the Linux Chmod command and chown command and their use.
linux chmod command
Chmod change file modes or Access Control Lists
Linux / Unix file call permission is divided into three levels: file owner, group and others.
Chmod can be used to control how files are called by others.
syntax
chmod [-fv] [-R [-H | -L | -P]] mode file ...options
- -v, –verbose
output a diagnostic for every file processed - -R, –recursive
change files and directories recursively
mode
The symbolic mode is described by the following grammar:
mode ::= clause [, clause ...]clause ::= [who ...] [action ...]action action ::= op [perm ...]who ::= a | u | g | oop ::= + | - | =perm ::= r | s | t | w | x | X | u | g | o
who
Reference Class Description
u owner file's owner
g group users who are members of
the file's group
o others users who are neither the
file's owner nor members of
the file's group
a all All three of the above, same as ugo
perm
r The read bits.
s The set-user-ID-on-execution
and set-group-ID-on-execution bits.
t The sticky bit.
w The write bits.
x The execute/search bits.
X The execute/search bits if the file
is a directory or any of the execute/search
bits are set in the original (unmodified)
mode. Operations with the perm symbol X''
are only meaningful in conjunction with the
op symbol+'', and are ignored in all other cases.
u The user permission bits in the original mode
of the file.
g The group permission bits in the original mode
of the file.
o The other permission bits in the original mode
of the file.
op
Operator Description
+ Adds the specified modes to the
specified classes
- Removes the specified modes from
the specified classes
= The modes specified are to be made
the exact modes for the specified
classes
examples
- test file gives execution permission to all users
$ sudo chmod ugo+x test 
- groups and others give file write permission
➜ chmod 666 test 
- chmod 777
Give all users access to read, write and execute files
➜ chmod 777 test
- chmod 755
File owner read and write execution permission, readable by group and other users, execution permission
- chmod a+r
a is equivalent to “ugo”
User, group and other users read permissions.
official example
- chmod 644
make a file readable by anyone and writable by the owner only. - chmod go-w
deny write permission to group and others. - chmod =rw,+X
set the read and write permissions to the usual defaults, but retain any execute permissions that are currently set. - chmod +X
make a directory or file searchable/executable by everyone if it is already searchable/executable by anyone. - chmod 755
chmod u=rwx,go=rx
chmod u=rwx,go=u-w
make a file readable/executable by everyone and writable by the owner only. - chmod go=
clear all mode bits for group and others. - chmod g=u-w
set the group bits equal to the user bits, but clear the group write bit.
linux chown command
Chown change file owner and group.
This command can only be used by root.
syntax
chown [-fhnv] [-R [-H | -L | -P]] owner[:group] file ...options
- -R, –recursive
operate on files and directories recursively - -v, –verbose
output a diagnostic for every file processed - -h, –no-dereference
affect symbolic links instead of any referenced file (useful only on systems that can change the ownership of a symlink)
examples
- change the user of the file
$ sudo chown nobody test
- change file group and user
$ sudo chown www-data:www-data test
- Change the owner of /folder and subfiles to “root”.
$ sudo chown -Rh root folder/
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