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. Often, it also contains the encrypted passwords for
each account. It should have general read permission (many utilities, like
ls(1) use it to map user IDs to user names), but write access only for
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. These days many people run some
version of the shadow password suite, where /etc/passwd has *'s instead
of encrypted passwords, and the encrypted passwords are in /etc/shadow
which is readable by the superuser only.
Regardless of whether shadow passwords are used, many sysadmins use a star in
the encrypted password field to make sure that this user can not authenticate
him- or herself using a password. (But see the Notes below.)
If you create a new login, first put a star in the password field, then use
passwd(1) to set it.
There is one entry per line, and each line has the format:
The field descriptions are:
the name of the user on the system. It should not contain
the encrypted user password or a star.
the numerical user ID.
the numerical primary group ID for this user.
This field is optional and only used for informational
purposes. Usually, it contains the full user name. GECOS means General
Electric Comprehensive Operating System, which has been renamed to GCOS
when GE's large systems division was sold to Honeywell. Dennis Ritchie has
reported: "Sometimes we sent printer output or batch jobs to the GCOS
machine. The gcos field in the password file was a place to stash the
information for the $IDENTcard. Not elegant."
the user's $HOME directory.
the program to run at login (if empty, use /bin/sh).
If set to a non-existing executable, the user will be unable to login
If you want to create user groups, their GIDs must be equal and there must be an
entry in /etc/group, or no group will exist.
If the encrypted password is set to a star, the user will be unable to login
using login(1), but may still login using rlogin(1), run
existing processes and initiate new ones through rsh(1),
cron(1), at(1), or mail filters, etc. Trying to lock an account
by simply changing the shell field yields the same result and additionally
allows the use of su(1).