Linux quick notes

chmod

to change access mode of a file (file permissions)

u = the user who owns the file

g = the group the file belongs to

o = others

a = all of the above (ab abbreviation for ugo)

example: to prevent outsiders from executing archive.sh before : -rwxr-xr-x archive.sh

command: chmod o=r archive.sh

after : -rwxr-xr-- archive.sh

example : to take away all permissions for the group for topsecret.inf, leave the permissions part of the command empty.

Before : -rw-r----- topsecret.inf

command : chmod g= topsecret.inf

after : -rw------- topsecret.inf

example:

chmod go=rx wordmatic.txt

chmod go-w wordmatic.txt

chmod a+wx calcmatic.bak

To kill non responding application:

ctrl-alt-Esc

move or rename files:

mv

whatis

information on a topic

makewhatis

creates an updated whatis database

umask

the default permission given to new files created

suid and sgid

special permissions given to program so when it runs by a user, it inherits the rights of the owner of the pprogram, not the user running it.

eg. /etc/passwd

setting & removing suid & sgid

for eg.:

# chmod u+s /usr/bin/myapp

# chmod g-s /home/drobbins

Directory permissions:

Directory permissions are a bit different that file permissions.

For a directory, if the “read” (r)flag is set, you may list the contents of the directory.

“write” (w) means you may create files in the directory; and “execute” (x) means you may enter the directory and access any sub-directory inside.

Without the “execute” (x) flag, the filesystem objects inside the directory aren't accessible.

Without the “read” ( r ) flag, the filesystem objects inside the directory aren't viewable but can still be accessible as long as someone knows the full path to the object.

If a directory has the “sgid” flan enabled, any filesystem objects created inside it will inherit the group of the directory. This particular feature come in handy when you need to create a directory tree to be used by a group of people that all belong to the same group.

simply do this:

# mkdir /home/groupspace

# chgrp mygroup /home/groupspace

#chmod g+s /home/groupspace

now any users in the group mygroup can creat files or directories inside /home/groupspace, and they will be automatically assigned a group ownership if mygroup as well. Depending on the users' umask settings, new filesystem objects may or may not be readable, writable, or executable by other members of th mygroup group.

after installing Windows server 2003 on an available partition, the previous grub was destroyed from from MBR by Windows.

solution:

restart from rescue cd

reinstall grub: /sbin/grub-install /dev/hda

symbolic link:

ln -s [target] [link_name]

rsync

eg.:

rsync -av –delete /home/user/ username@server:/backup/home/user

If a directory is being backed up, remember to use / at the end of the directory name.

rsync uses ssh

Users & Groups

newgrp

useradd

groupadd

usermod

userdel -r (-r to remove user home directory too)

groupdel

ACL

acl shou be in the /etc/fstab

# getfacl

# setfacl

VNC

start vnc server:

vncserver

change password:

vncpasswd

connect to server from client by specifying the ip address/hostname and desktop

eg.:

192.168.123.10:1

configuration file:

.vnc/xstartup

default window manager is TWM, you can change it

eg.:

startkde or startgnome

to kill vnc:

vncserver -kill :1

default vnc viewer port: 5901

Java based viewer: 5801