linux許可權管理（chmod, chgrp, chown)

chgrp  :  change the group
chown:  change the owner
chmod:  change the permission

chgrp [-R] dirname/filename......

For example:
chgrp users install.log

chown [-R] username dirname/filename

For example:
chown root:root install.log

chmod [-R] xyz dirname/filename
r:4
w:2
x:1

For example:
chmod 777 ./bashrc

if you wanna change  the permission of .bashrc to -rw-r--r--
chmod 644 .bashrc

                u
                g       +        r
chmod              -       w        dirname
/ filename 
                o       =       x
                a

For example:
let's change the permission of a file to -rwxr-xr-x
chmod u=rwx,go=rx .bashrc (note: there is not space in statement:u=rwx,go=rx)

For example:
If there is a filename , I don't know it's permission, and we just like it have write permission. we can use the next statement.

chmod a+w .bashrc
Let's canceling the execute permission:
chmod a-x .bashrc