passwd Command

The use of passwords and their implementation is highly dependent upon your company's security policy and your system administrator. Some orgnization do not control password complexity and expiration,  while others require certain level of passwords (for example legnth and mixture of characters), perform full accounting on every user, and implement password aging.

The passwd command reads the /etc/passwd file to verify the the user name exists. The /etc/shadow file is used to store hash of the password. The password itself is never stored.

The external passwd command allows you to create or change the password associated with your user name. The super-user (system administrator) can change any user's password and is not prompted for the old password. As a user you can only change your password, if you know the current password. which creates difficulties if you longin  from your Pc terminal emulator using ssh without passord and now need to change passord for some reason.

the general format

     passwd [ user_name ]
     passwd [ -df ][ -n min ][ -x max ][ -w warn ]user_name
     passwd [ -fl ][ -n min ][ -x max ][ -w warn ]user_name
     passwd -s[-a ]
     passwd -s[ user_name ]

Options

The following list describes the options used to control how passwd functions.

Arguments

The following list describes the argument that may be passed to the passwd command.

Password aging

The passwd command is capable of checking the elapsed time since the password was last changed, referred to as password aging. If the specified perod expired, then the user is required to change the password. In other words password aging requires that a user must change passwords after a specified amount of time. The system administrator decides if password aging is activated and how long each password is aged.

The following is a list of requirements that a password must meet before passwd will allow it as your password.

1.  Must contain six characters (this is default; sen be set sysadmin to higher legth). The label PASSLEN specifies the maximum length for a password. It is in the file /etc/default/passwd.

2.  Must contain 2 alphabetic characters (upper or lowercase letters) and at least one special character or number.

3.  Must not be any circular shift combination or reversal of the user name. For example, if your user name is bill, your password could not be lbil, llbi, illb, or llib, blli, ibll, libl.

4.  New passwords must have at least 3 characters that differ from the old password.

The passwd command returns the following return codes upon completion:

The $? ($status-csh) shell variable contains the return code. So echo $? will display the return status.

Examples

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TIP:  Your password should be some combination of two preferably meaningless strings of intermixed numbers, characters, and symbols. Names of friends, wives, husbands, etc. are easily guessed by the criminal/jerk next door, down the street, or around the world via your profiles and  posts on social sites.

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If you change password on user level you will be prompted for old password. If you do not have a password, you will not see the following prompt, because UNIX knows you don't have an old password.

    cj> passwd
    Enter old passwd:

3.  After passwd verifies that you entered the correct old password it will prompt you for a new password.

    cj> passwd
    Enter old passwd:
    Enter new passwd:

4.  passwd will require you to repeat the new password so that you know you typed it correctly.  If you fail you will be asked to renter:

    cj> passwd
    Enter old passwd:
    Enter new passwd:
    Re-enter new passwd:

If the new password you typed does not match, your password will not be created or updated. Notice the following message that passwd returns in this situation.

    Mismatch - passwd not updated!

Linux passwd Command Tutorial for Beginners (8 Examples)

Following is some useful information that you need to know about passwd's operation:

       The user is first prompted for his/her old password, if one is present.
       This password is then encrypted and compared against the stored
       password. The user has only one chance to enter the correct password.
       The superuser is permitted to bypass this step so that forgotten
       passwords may be changed.

       After the password has been entered, password aging information is
       checked to see if the user is permitted to change the password at this
       time. If not, passwd refuses to change the password and exits.

       The user is then prompted twice for a replacement password. The second
       entry is compared against the first and both are required to match in
       order for the password to be changed.

       Then, the password is tested for complexity. As a general guideline,
       passwords should consist of 6 to 8 characters including one or more
       characters from each of the following sets:

       ·   lower case alphabetics

       ·   digits 0 thru 9

       ·   punctuation marks

       Care must be taken not to include the system default erase or kill
       characters.  passwd will reject any password which is not suitably
       complex.

For example, in my case, here's how I changed the passwd:

Please note that in case you have superuser privileges (or are a system admin), you can change any account's password by just mentioning the username of the account. For example:

passwd himanshu

Q2. How to make an account password-less?

The passwd command also lets you delete a password associated with a user account. For this, you need to use the -d command line option.

passwd -d

So this is basically a quick way to disable password for an account, essentially making it password-less.

Q3. How to forcibly expire a password?

Sometimes the system admin may want a user to change their password immediately. So in that case, they may use the -e command line option which results in immediate expiry of the password, forcing user to change their password on the very next login.

passwd -e

Q4. How to disable an account if password remains expired?

The passwd command also lets you deal with situations where-in user doesn't take any action even after their password is expired. Basically, the tool lets you disable the account altogether in case password remains expired for a set number of days. This you can do using the -i command line option, which requires a number as input.

For example:

passwd -i 5

So this command will make sure the account is disabled if the password remains expired for 5 days.

Q5. How to set a time gap between password changes?

The passwd command also allows you to set a minimum time gap between password changes. This is made possible through the -n command line option, which requires a numeric value that's treated as number of days.

For example:

passwd -n 10

This command makes sure there's a gap of 10 days between any two password changes. It's worth mentioning that a value of 0 indicates that the user may change his/her password at any time.

Q6. How to make passwd display account status info?

This you can do using the -S command line option.

Here's how the official documentation explains these fields:

           Display account status information. The status information consists
           of 7 fields. The first field is the user's login name. The second
           field indicates if the user account has a locked password (L), has
           no password (NP), or has a usable password (P). The third field
           gives the date of the last password change. The next four fields
           are the minimum age, maximum age, warning period, and inactivity
           period for the password. These ages are expressed in days.

Q7. How to set password validity period?

The passwd command allows you to do this through its -x option, which requires a number representing the maximum number of days a password remains valid.

For example:

passwd -x 100

Q8. How to make passwd warn users about due password change?

This can be done using the -w command line option, which requires a number as input, which is nothing but number of days prior to the password expiring that a user will be warned that his/her password is about to expire.

For example:

passwd -w 7 

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