As subtle as a flying brick.

Be careful what you show the world.
You never know when the wolf is watching.

― Jennifer Donnelly, Revolution


You should date a girl who reads.

You should date a girl who reads.




Buy her another cup of coffee.



She has to give it a shot somehow.








Or better yet, date a girl who writes.

– Rosemarie Urquico


How to clean up dated folders

Suppose you have a backup directory with backup snapshots named by timestamp:

$ ls

You want to remove snapshots older than 3 days. The one-liner does it:

$ date
Tue May  7 13:50:57 KST 2013
$ ls | grep '....-..-..-......' | sort | xargs -I {} bash -c "[[ x{} < x$(date -d '3 days ago' +%Y-%m-%d-%H%M%S) ]] && rm -rfv {}"
removed directory: `2013-05-03-103022'
removed directory: `2013-05-04-103033'



Linux turn OFF password expiration / aging

/etc/shadow stores actual password in encrypted format for user’s account with additional properties related to user password.

The password expiration information for a user is contained in the last 6 fields. Password expiration for a select user can be disabled by editing the /etc/shadow file

However I recommend using chage command. The chage command changes the number of days between password changes and the date of the last password change.

This information is used by the system to determine when a user must change his/her password.

To list current aging type chage command as follows:

# chage -l vivek


Last password change                                    : August 23, 2013
Password expires                                        : never
Password inactive                                       : never
Account expires                                         : never
Minimum number of days between password change          : 0
Maximum number of days between password change          : 99999
Number of days of warning before password expires       : 7

To disable password aging / expiration for user foo, type command as follows and set:

Minimum Password Age to 0
Maximum Password Age to 99999
Password Inactive to -1
Account Expiration Date to -1

Interactive mode command:

# chage username


# chage -I -1 -m 0 -M 99999 -E -1 username

Understanding /etc/shadow file

Q. Can you explain /etc/shadow file used under Linux or UNIX?

A. /etc/shadow file stores actual password in encrypted format for user’s account with additional properties related to user password i.e. it stores secure user account information.

All fields are separated by a colon (:) symbol. It has one entry per line for each user listed in /etc/passwd file Generally, shadow file entry looks as follows:


(Fig.01: /etc/shadow file fields)

  1. User name : It is your login name
  2. Password: It your encrypted password. The password should be minimum 6-8 characters long including special characters/digits
  3. Last password change (lastchanged): Days since Jan 1, 1970 that password was last changed
  4. Minimum: The minimum number of days required between password changes i.e. the number of days left before the user is allowed to change his/her password
  5. Maximum: The maximum number of days the password is valid (after that user is forced to change his/her password)
  6. Warn : The number of days before password is to expire that user is warned that his/her password must be changed
  7. Inactive : The number of days after password expires that account is disabled
  8. Expire : days since Jan 1, 1970 that account is disabled i.e. an absolute date specifying when the login may no longer be used

The last 6 fields provides password aging and account lockout features (you need to use chage command to setup password aging). According to man page of shadow – the password field must be filled. The encrypted password consists of 13 to 24 characters from the 64 character alphabet a through z, A through Z, 0 through 9, \. and /. Optionally it can start with a “$” character. This means the encrypted password was generated using another (not DES) algorithm. For example if it starts with “$1$” it means the MD5-based algorithm was used.

Understanding /etc/passwd

Q. Can you explain /etc/passwd file format for Linux and UNIX operating systems?

A. /etc/passwd file stores essential information, which is required during login i.e. user account information. /etc/passwd is a text file, that contains a list of the system’s accounts, giving for each account some useful information like user ID, group ID, home directory, shell, etc. It should have general read permission as many utilities, like ls use it to map user IDs to user names, but write access only for the superuser (root).

Understanding fields in /etc/passwd

The /etc/passwd contains one entry per line for each user (or user account) of the system. All fields are separated by a colon (:) symbol. Total seven fields as follows.

Generally, passwd file entry looks as follows (click to enlarge image):


Fig.01: /etc/passwd file format

  1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
  2. Password: An x character indicates that encrypted password is stored in /etc/shadow file.
  3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
  4. Group ID (GID): The primary group ID (stored in /etc/group file)
  5. User ID Info: The comment field. It allow you to add extra information about the users such as user’s full name, phone number etc. This field use by finger command.
  6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
  7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell. Please note that it does not have to be a shell.

Task: See User List

/etc/passwd is only used for local users only. To see list of all users, enter:
$ cat /etc/passwd
To search for a username called tom, enter:
$ grep tom /etc/passwd

/etc/passwd file permission

The permission on the /etc/passwd file should be read only to users (-rw-r–r–) and the owner must be root:
$ ls -l /etc/passwd

-rw-r--r-- 1 root root 2659 Sep 17 01:46 /etc/passwd

Reading /etc/passwd file

You can read /etc/passwd file using the while loop and IFS separator as follows:

# seven fields from /etc/passwd stored in $f1,f2...,$f7
while IFS=: read -r f1 f2 f3 f4 f5 f6 f7
 echo "User $f1 use $f7 shell and stores files in $f6 directory."
done < /etc/passwd

Your password is stored in /etc/shadow file

Your encrypted password is not stored in /etc/passwd file. It is stored in /etc/shadow file. In the good old days there was no great problem with this general read permission. Everybody could read the encrypted passwords, but the hardware was too slow to crack a well-chosen password, and moreover, the basic assumption used to be that of a friendly user-community.

Almost, all modern Linux / UNIX line operating systems use some sort of the shadow password suite, where /etc/passwd has asterisks (*) instead of encrypted passwords, and the encrypted passwords are in /etc/shadow which is readable by the superuser only.

I’m not a fanboy, but aww.

it’s so much easier to say you’re anti social…


…or claim that you just don’t like people…


…or pretend that you just don’t care anymore…


…than to admit how lonely and damaged you truly feel.


Half-Life 2 For Steam on Linux

The first-person shooter Half-Life 2 released for Steam on Linux. I truly enjoyed Counter Strike, and I am going to install Half-Life 2 this weekend. From the release notes:

Half Life 2 logo

Half-Life 2, Half-Life 2: Episode 1, Half-Life 2: Episode 2 and Half-Life 2: LostCoast are now available as a Beta. This beta adds Linux support and converts the game to the new Steam Content delivery system.

Linux users can simply install the games access the beta. For Windows and OS X users right click the game in your Library, choose properties and then go to the Beta tab. Select the SteamPipe beta to start testing. Under Windows and OS X to opt out of testing simply deselect the beta option on this same page.

I think Valve has done a good job, and I’m hoping more game developers will release popular games on Linux.


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