The umask command
The umask command shows and sets the default permissions:
The umask value is just that, a mask. It masks out the permissions you don’t want to give.The umask value is subtracted from the full permission set for an object. The full permission for a file is mode 666 (read/write permission for all), but for a directory it’s 777 (read/write/execute permission for all).
Thus, in the example, the file starts out with permissions 666, and the umask of 022 is applied, leaving a file permission of 644. The umask value is normally set in the /etc/profile startup file.
The pam_time module is used as an account module-type. The pam_time module does not accept arguments. It instead uses the /etc/security/time.conf file to get information related to login time and location restrictions. There are two important points concerning the /etc/security/time.conf.
The /etc/security/time.conf file restricts access by time and location when used with pam_time.
Each line in /etc/security/time.conf file is called a rule.
Each rule uses the following syntax:
services;ttys;users;times
/etc/skel directory
The system will copy the contents of the /etc/skel directory to the user’s HOME directory.
The useradd command allows an administrator to create a default HOME directory configuration, then uses that as a template to create the new user’s HOME directory. This allows you to place default files for the system in every new user’s HOME directory automatically. On my Linux system, the /etc/skel directory has the following files.
root@dell:/etc/skel# ls -ld *
drwxr-xr-x 2 buddy buddy 4096 Oct 11 2008 Desktop
drwxr-xr-x 2 buddy buddy 4096 Mar 4 2009 Documents
drwxr-xr-x 2 buddy buddy 4096 Mar 4 2009 Download
drwxr-xr-x 2 buddy buddy 4096 Mar 4 2009 Music
drwxr-xr-x 2 buddy buddy 4096 Mar 4 2009 Pictures
drwxr-xr-x 2 buddy buddy 4096 Mar 4 2009 Public
drwxr-xr-x 2 buddy buddy 4096 Mar 4 2009 Templates
drwxr-xr-x 2 buddy buddy 4096 Mar 4 2009 Videos
System Default Values for useradd
The primary tool used to add new users to your Linux system is useradd. This command pro-
vides an easy way to create a new user account and set up the user’s HOME directory structure all at once. The useradd command uses a combination of system default values and command line parameters to define a user account.
To see the system default values used on your Linux distribution, enter the useradd command with the -D parameter.
root@dell:~# /usr/sbin/useradd -D
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/sh
SKEL=/etc/skel
CREATE_MAIL_SPOOL=no
The -D parameter shows what defaults the useradd command uses.
Shell Command Aliases
A command alias allows you to create an alias name for common commands (along with their
parameters) to help keep your typing to a minimum.
Most likely your Linux distribution has already set some common command aliases for you. To
see a list of the active aliases, use the alias command with the -p parameter:
Example:
$ alias -p
alias l.=’ls -d .* –color=tty’
alias ll=’ls -l –color=tty’
alias ls=’ls –color=tty’
alias vi=’vim’
alias which=’alias | /usr/bin/which –tty-only –read-
alias–show-dot –show-tilde’
