File Permissions

The "umask" command can be used to read or set default file permissions for the current user.

root> umask 022

The umask value is subtracted from the default permissions (666) to give the final permission.

666 : Default permission
022 : - umask value
644 : final permission

The "chmod" command is used to alter file permissions after the file has been created.

root> chmod 777 *.log

Owner      Group      World      Permission
=========  =========  =========  ======================
7 (u+rwx)  7 (g+rwx)  7 (o+rwx)  read + write + execute
6 (u+wx)   6 (g+wx)   6 (o+wx)   write + execute
5 (u+Rx)   5 (g+Rx)   5 (o+Rx)   read + execute
4 (u+r)    4 (g+r)    4 (o+r)    read only
2 (u+w)    2 (g+w)    2 (o+w)    write only
1 (u+x)    1 (g+x)    1 (o+x)    execute only

Character eqivalents can be used in the chmod command.

root> chmod o+rwx *.log
root> chmod g+r   *.log
root> chmod -Rx   *.log

The "chown" command is used to reset the ownership of files after creation.

root> chown -R oinstall.dba *

The "-R" flag causes the command ro recurse through any subdirectories.

OS Users Management

See Linux Groups and Users.
The "useradd" command is used to add OS users.

root> useradd -G oinstall -g dba -d /usr/users/my_user -m -s /bin/ksh my_user

• The "-G" flag specifies the primary group.
• The "-g" flag specifies the secondary group.
• The "-d" flag specifies the default directory.
• The "-m" flag creates the default directory.
• The "-s" flag specifies the default shell.

The "usermod" command is used to modify the user settings after a user has been created.

root> usermod -s /bin/csh my_user

The "userdel" command is used to delete existing users.

root> userdel -r my_user

The "-r" flag removes the default directory.
The "passwd" command is used to set, or reset, the users login password.

root> passwd my_user

The "who" command can be used to list all users who have OS connections.

root> who
root> who | head -5
root> who | tail -5
root> who | grep -i ora
root> who | wc -l

• The "head -5" command restricts the output to the first 5 lines of the who command.
• The "tail -5" command restricts the output to the last 5 lines of the who command.
• The "grep -i ora" command restricts the output to lines containing "ora".
• The "wc -l" command returns the number of lines from "who", and hence the number of connected users