Month: July 2012

The /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 lsuse it to map user IDs to user names, but write access only for the superuser (root).

The anatomy of /etc/passwd

The /etc/passwd contains one entry per line (row) for each user (or user account) of the system. All fields are separated by a colon (:) symbol. Total seven fields as follows. It is one of the many database text files in NIX systems. Generally, passwd file entry looks as follows :

 

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.

Viewing User List

/etc/passwdis only used for local users only. To see list of all users, enter:

$ less /etc/passwd

To search for a username called toro, enter:

$ grep toro /etc/passwd

/etc/passwd file permissions

The permissions on the /etc/passwd file should be read only to all users i.e. 644 (-rw-r–r–) and the owner must be root: $ ls -l /etc/passwdOutput:

-rw-r--r--. 1 root root 1563 Jul 13 11:03 /etc/passwd

Scanning through /etc/passwd file

One can read the /etc/passwdfile using the while loop and IFS separator as follows:

#!/bin/bash
# seven fields from /etc/passwd stored in $f1,f2...,$f7
#

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

Another way to list all entries in the passwd database is using the getent utility.  This will show all user accounts, regardless of the type of name service used. For example, if both local and LDAP name service are used for user accounts, the results will include all local and LDAP users:

$ getent passwd

The /etc/shadow file

Passwords are not stored in /etc/passwd file the. 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, both assumptions really wrong today. Almost, all modern Linux / UNIX line operating systems use the shadow password suite, where /etc/passwd has asterisks (*) instead of encrypted passwords, and the encrypted passwords are in /etc/shadow which is readable only by the superuser.