Commands is used to absolutely assign all permissions to the owner, read and write permissions to the group and only executable permission to the others of the file note?
Answers
Symbolic method:
The first and probably easiest way is the relative (or symbolic) method, which lets you specify access classes and types with single letter abbreviations. A chmod command with this form of syntax consists of at least three parts from the following lists:
Access Class Operator Access Type
u (user) + (add access) r (read)
g (group) - (remove access) w (write)
o (other) = (set exact access) x (execute)
a (all: u, g, and o)
For example, to add permission for everyone to read a file in the current directory named myfile, at the Unix prompt, enter:
chmod a+r myfile
The a stands for "all", the + for "add", and the r for "read".
Note:
This assumes that everyone already has access to the directory where myfile is located and its parent directories; that is, you must set the directory permissions separately.
If you omit the access class, it's assumed to be all, so you could also enter the previous example as:
chmod +r myfile
You can also specify multiple classes and types with a single command. For example, to remove read and write permission for group and other users (leaving only yourself with read and write permission) on a file named myfile, you would enter:
chmod go-rw myfile
You can also specify that different permissions be added and removed in the same command. For example, to remove write permission and add execute for all users on myfile, you would enter:
chmod a-w+x myfile
In each of these examples, the access types that aren't specified are unchanged. The previous command, for example, doesn't change any existing settings specifying whether users besides yourself may have read (r) access to myfile. You could also use the exact form to explicitly state that group and other users' access is set only to read with the = operator:
chmod go=r myfile
The chmod command also operates on directories. For example, to remove write permission for other users on a subdirectory named mydir, you would enter:
chmod o-w mydir
To do the same for the current directory, you would enter:
chmod o-w
Be careful when setting the permissions of directories, particularly your home directory; you don't want to lock yourself out by removing your own access. Also, you must have execute permission on a directory to switch (cd) to it.
Absolute form
The other way to use the chmod command is the absolute form. In this case, you specify a set of three numbers that together determine all the access classes and types. Rather than being able to change only particular attributes, you must specify the entire state of the file's permissions.
The three numbers are specified in the order: user (or owner), group, other. Each number is the sum of values that specify read (4), write (2), and execute (1) access, with 0 (zero) meaning no access. For example, if you wanted to give yourself read, write, and execute permissions on myfile; give users in your group read and execute permissions; and give others only execute permission, the appropriate number would be calculated as (4+2+1)(4+0+1)(0+0+1) for the three digits 751. You would then enter the command as:
chmod 751 myfile
As another example, to give only yourself read, write, and execute permission on the current directory, you would calculate the digits as (4+2+1)(0+0+0)(0+0+0) for the sequence 700, and enter the command:
chmod 700
If it seems clearer to you, you can also think of the three digit sequence as the sum of attributes you select from the following table:
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 others
002 Write by others
001 Execute by others
To create an access mode, sum all the accesses you wish to permit. For example, to give read privileges to all, and write and execute privileges to the owner only for a file, you would sum: 400+200+100+040+004 = 744. Then, at the Unix prompt, you would enter:
chmod 744 myfile.ext
Some other frequently used examples are:
777
anyone can do anything (read, write, or execute)
755
you can do anything; others can only read and execute
711
you can do anything; others can only execute
644
you can read and write; others can only read
More
For more about chmod, consult the manual page. At the Unix prompt, enter:
man chmod
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