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I'm using busybox with a limited passwd (I don't have --stdin option) and without chpasswd and I need to change the password of an user from bash. Here is my best result:

echo newpassword > pwdfile
echo newpassword > pwdfile
cat pwdfile | passwd myuser
Changing password for myuser
Enter the new password (minimum of 5, maximum of 8 characters)
Please use a combination of upper and lower case letters and numbers.
Enter new password:
Bad password: too simple.

Warning: weak password (continuing).
Re-enter new password:
passwd: The password for myuser is unchanged.
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up vote 2 down vote accepted

Busybox has chpasswd(8) which is a utility best used to create/update a lot of users very quickly and with one command. It accepts data from STDIN in username:password form. That means that you can do something like this:

$ cat pwdfile | chpasswd


$ < pwdfile chpasswd

Note that pwdfile must have username:new_password syntax.

Then again, you could always edit /etc/shadow yourself -- but please don't.

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There's no need to cat to a pipe, just use IO redirection; < pwdfile chpasswd. – Chris Down Dec 13 '11 at 15:04
Thanks @ChrisDown, added – n0pe Dec 13 '11 at 15:08
My busybox does not have chpasswd. – michelemarcon Dec 13 '11 at 15:13
What is the output of passwd --version? – n0pe Dec 13 '11 at 15:21
passwd: invalid option -- - BusyBox v1.00 (2011.04.28-16:58+0000) multi-call binary Usage: passwd [OPTION] [name] Change a user password. If no name is specified, changes the password for the current user. Options: -a Define which algorithm shall be used for the password (Choices: des, md5) -d Delete the password for the specified user account -l Locks (disables) the specified user account -u Unlocks (re-enables) the specified user account – michelemarcon Dec 14 '11 at 8:03

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