Introduction to the UNIX Operating System * Pima Community College * CSC137 -- 5.3 Password File Entry

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5.3 Password File Entry

The password file entry is used by many processes on a UNIX system. The most obvious is the login sequence. Others that use it are shells (for looking up user names and directories) and the ls command (for looking up user names). An example /etc/passwd file entry follows:

ltaber:IOVa/L/AmhI3U:505:10:Louis Taber:/home/ltaber:/bin/tcsh

I have shortened this entry so it fits on a screen. I will look at each field in turn. Note that the fields are delimited with ":", colons.

Login name ltaber This is the name that the user uses to login to the system with.
Encrypted Password IOVa/L/AmhI3U UNIX systems do not store the actual password. Only the encrypted password. If the UNIX platform is running a shadow password system then this field will be replaced by an asterisk "*". The actual encrypted password will be stored in a file that can only be read by root. This is much better for a secure system. Logins can be disabled by placing an asterisk as the first character in the password file. The encryption algorithm never generates an asterisk.
User number 505 This is the number that the system uses internally for the user. It is in the users process tables and in the i-nodes associated with the user. To convert from this number to the text name the system looks at the /etc/passwd file.
Group number 10 This is the number that the system uses internally for the user's group. It is in the users process tables and in the i-nodes associated with the user. To convert from this number to the text name for the group the system looks at the /etc/group file.
Users actual name Louis Taber This is the entry that has the "real" name of the user. This is also the field that I shortened. My real entry looks like this:
Louis Taber,BUS R5A,(520) 884-6039,(520) 327-4133
It is a comma separated into subfields; name, office, office number, home number.
Users login directory /home/ltaber This is the directory that the newly logged in users starts at. It is also the directory that the cd command will take a user to when given without arguments.
Login Shell [Optional] /bin/tcsh This optional field is used for the login shell program. It does not need to point to a shell at all. This is how turn-key systems are setup to run a specific program for specific users. Most systems let individuals users change this field with the the chsh command (change shell). Both the shell changed to and the shell changed from need to be in the /etc/shells file.

Instructor: ltaber@pima.edu** Red's Home page** The Pima College Site